ap physics c mechanics review lecture notes - all

advertisement
Flipping Physics Lecture Notes:
AP Physics C: Kinematics Review (Mechanics)
•
Dimensions are your friends!!:
o Be careful with your conversions and give all your answers units (when they have them!).
3
o
ρ Krypton
g ⎛ 100cm ⎞ ⎛ 1kg ⎞
kg
= 3.75
=
0.375
cm3 ⎜⎝ 1m ⎟⎠ ⎜⎝ 1000g ⎟⎠
m3
•
Vector vs. Scalar:
o Vectors have both magnitude and direction.
o Scalars have magnitude only (no direction) but can be positive or negative.
•
Instantaneous velocity is the derivative of position as a function of time:
o
•
•
•
!
Δx
!
Not to be confused with average velocity: v average =
Δt
!
dv
!
Instantaneous acceleration is the derivative of velocity as a function of time: ainstantaneous =
dt
!
Δv
!
o Not to be confused with average acceleration: aaverage =
Δt
The derivative represents the slope of the line.
Uniformly Accelerated Motion or UAM.
®
AP Physics C Equation Sheet
Flipping Physics
v f = v i + aΔt
1
x = x 0 + v xot + ax t 2
2
v x 2 = v x 0 2 + 2ax x − x 0
1
Δx = v i Δt + aΔt 2
2
2
2
v f = v i + 2aΔx
)
(
)
o
1
v + v i Δt
2 f
The AP Physics C UAM Equations assume ti = 0; Δt = t f − t i = t f − 0 = t
o
x 0 means the initial position.
Δx =
Free fall is when the only force acting on an object is the Force of Gravity. (No air resistance)
o
•
®
v x = v x 0 + ax t
(
•
!
dx
!
v instantaneous =
dt
ay = −g = −9.81
m
m
& g Earth = +9.81 2
2
s
s
o This is Uniformly Accelerated Motion where you already know the acceleration!
Free fall graphs of an object dropped from a height of 2.0 meters:
0194 Lecture Notes - AP Physics C- Kinematics Review (Mechanics).docx
page 1 of 2
•
Component vectors are the vectors in the x, y (and possibly z) directions that make up a vector.
•
m
!
a = 2.5 2 @26° E of N
s
O a
m
sin θ = = x ⇒ ax = asin θ = 2.5sin 26 = 1.09593 ≈1.1 2
§
H a
s
A a
m
cosθ = = y ⇒ ay = acosθ = 2.5cos 26 = 2.24699 ≈ 2.2 2
§
H a
s
Unit Vectors î, ĵ,and k̂ are vectors with a value of 1 in the x, y, and z directions respectively.
o
( )
( )
o
•
•
In other words the acceleration in the above example in unit vector form is:
m
m
!
!
a = ⎡⎣1.1î + 2.2 ĵ ⎤⎦ 2 This is the same as a = 2.5 2 @26° E of N
s
s
Vector addition is much easier using Unit Vectors. Example:
!
!
!
A = ⎡⎣1.00 î + 2.00 ĵ ⎤⎦ m; B = ⎡⎣2.50 î −1.50 ĵ ⎤⎦ m; C = ⎡⎣3.00 î + 3.50 ĵ ⎤⎦ m
! ! ! !
R = A + B + C = ? = ⎡⎣1.00 î + 2.00 ĵ ⎤⎦ + ⎡⎣2.50 î −1.50 ĵ ⎤⎦ + ⎡⎣3.00 î + 3.50 ĵ ⎤⎦
!
⇒ R = ⎡⎣1+ 2.5 + 3 ⎤⎦ î + ⎡⎣2 −1.5 + 3.5 ⎤⎦ ĵ = ⎡⎣6.50 î + 4.00 ĵ ⎤⎦ m
!
r is generally used as the position vector symbol. This is so you can give the position of an
object in 2 (or even 3) dimensions. For example if an object is located -2.0 meters in the xdirection, 7.4 meters in the y-direction and -3.7 meters in the z-direction, we can illustrate its
!
position as r = ⎡⎣ −2.0 î + 7.4 ĵ + −3.7 k̂ ⎤⎦ m .
!
o Note: The position vector equation r is much shorter than the word description.
•
•
Relative velocity is simply vector addition.
!
!
!
v pE = v pw + v wE
o
The velocity of the plane with respect to the Earth equals the velocity
of the plane with respect to the wind plus velocity of the wind with
respect to the Earth.
o
Don’t forget it’s tip-to-tail vector addition. So
!
!
v pw + v wE are drawn
“tip-to-tail”.
Projectile Motion is when the only force acting on an object is the Force of Gravity and the object
is moving in both the x and y directions. (No air resistance)
x direction
y direction
Free-Fall
a =0
x
Constant Velocity
vx =
Δx
Δt
ay = −g = −9.81
m
s2
Uniformly Accelerated Motion
Δt (or t) is the same in both directions because it is a scalar and has magnitude only (no direction).
o Break the initial velocity into its components.
0194 Lecture Notes - AP Physics C- Kinematics Review (Mechanics).docx
page 2 of 2
Flipping Physics Lecture Notes:
AP Physics C: Dynamics Review (Mechanics)
•
st
Newton’s 1 Law: When viewed from an inertial reference frame, an object at rest will remain at
rest and an object in motion will remain at a constant velocity unless acted upon by a net external
force.
o An inertial reference frame is where the acceleration of the reference frame zero.
o A non-inertial reference frame is where the acceleration of the reference frame is not zero.
o Also called the “Law of Inertia”.
§ Inertia is the tendency of an object to resist acceleration.
•
! !
!
F
F
!
!
∑
nd
= net .
Newton’s 2 Law: ∑ F = ma on the equation sheet it is a =
m
m
!
!
rd
Newton’s 3 Law: F12 = − F21
•
∑ F = ma ⇒ newtons, N =
•
The basic forces with which we begin dynamics:
•
!
o
!
Force of Gravity also called Weight.
§
§
§
§
§
o
FN : A pushing force caused by a surface.
The force normal is normal to (perpendicular to) the surface.
The force normal is always a push. (Never a pull. A surface can’t “pull”.)
The force normal acts on the contact point between the two surfaces.
Force of Tension,
§
§
§
Fg = mg
The force of gravity is caused by the interaction between the object and the planet.
The force of gravity is always down.
2
The acceleration due to gravity, g, her on planet Earth is +9.81 m/s .
Sometimes the symbol is W.
The force of gravity acts on the center of gravity of the object. (Which is the same
as the center of mass in a constant gravitational field like the one we live in.)
Force Normal,
§
§
§
o
kg ⋅ m
s2
FT : The force caused by a rope, cable, wire, string, etc.
Always in the direction of the rope, cable, wire, string, etc.
Always a pull. (Never a push. You can’t “push” with a rope.)
Sometimes the symbol is T.
o
Force Applied, Fa : The force of one object pushing or pulling on another object.
o
Force of Friction,
§
Ff : The force caused by the interaction between two surfaces.
With regards to the direction of the Force of Friction.
•
•
•
Ff always:
is parallel to the surface.
opposes motion (opposes sliding between the two surfaces)
is independent of the direction of the Force Applied.
!
!
µ FN
§
General formula on the equation sheet: Ff ≤
§
Static friction is when the two surfaces do NOT slide relative to one another.
•
§
max
!
= µ s FN
Kinetic friction is when the two surfaces DO slide relative to one another.
•
§
!
!
!
Fsf ≤ µ s FN & Fsf
!
!
Fkf = µ k FN
The coefficient of friction, µ , is an experimentally determined, dimensionless
number which depends on the materials of the two interacting surfaces.
0195 Lecture Notes - AP Physics C- Dynamics Review (Mechanics).docx
page 1 of 3
•
•
General range is 0 – 2:
o However, µ can get up to 4 in extreme circumstances.
•
µs > µk
(For the same two interacting materials.)
Free Body Diagrams or Force Diagrams. The five steps are …
1. Draw the Free Body Diagram(s).
2. Break forces in to components.
3. Redraw the Free Body Diagram(s).
4. Sum the forces.
5. Sum the forces (in a direction perpendicular to the direction in step 4).
§ Only forces are drawn in Free Body Diagrams.
§ When on an incline we will often break the force of gravity in to it’s parallel and
perpendicular components and sum the forces in the parallel and perpendicular
directions.
Fg = mg cosθ & Fg = mg sin θ
⊥
!
Always draw the Free Body Diagram without braking forces into components first
and then redraw the Free Body Diagram. These are specific instructions from
The AP CollegeBoard!
§ When summing the forces you must identify:
• Positive directions, especially for pulleys!
• Which object(s) you are summing the forces on.
• Which direction you are summing the forces in.
§ You can only sum the forces on multiple objects at the same time if they all have
the same acceleration.
Translational equilibrium.
o Translational motion simply means moving from one location to another.
§
•
o
Translational Equilibrium means the net force acting on the object is zero,
o
An object in translational equilibrium is not accelerating.
§
!
!
∑ F = 0 = ma ⇒ a = 0 .
•
•
!
!
∑F = 0.
The object moves with a constant velocity or is at rest.
The Drag Force or the Resistive Force,
FR : The force caused by the interaction of an object and
the fluid the object is moving through.
FD .
o
Sometimes the symbol is R or
o
Opposite the direction of motion of the object.
o
For “small” objects moving at “slow” speeds,
§
o
!
!
FR = −bv .
The resistive force equals the negative of, b, the proportionality constant times
the velocity of the object.
!
For all other objects (and more generally applicable), FR
§
§
§
§
=
1
Dρ Av 2 .
2
D is the Drag Coefficient of the object, has no dimensions, is experimentally
determined, and depends on the shape and surface texture of the object.
ρ is the density of the medium through which the object is moving.
A is the cross sectional area of the object normal to the direction of motion.
v is the velocity of the object.
0195 Lecture Notes - AP Physics C- Dynamics Review (Mechanics).docx
page 2 of 3
•
Terminal velocity is when an object moving through a fluid has reached translational equilibrium.
Force example an object which is falling downward in the Earth’s atmosphere has a free body
diagram with the force of gravity down and the resistive force up.
o
1
Dρ Av 2 − mg Dρ Av 2
1
2
2
=
−g
∑ Fy = FR − Fg = may ⇒ 2 Dρ Av − mg = may ⇒ ay =
m
2m
§
o
o
In other words, in the absence of air resistance,
ay = −g !!!
Dρ Av 2
Dρ Av 2
2mg
With ay = 0 ⇒ 0 =
−g⇒
= g ⇒ v terminal =
2m
2m
Dρ A
Note: This equation is only true for “an object which is falling downward in the Earth’s
atmosphere”. A rocket moving upward will have a different equation for terminal velocity
because the free body diagram is different.
0195 Lecture Notes - AP Physics C- Dynamics Review (Mechanics).docx
page 3 of 3
Flipping Physics Lecture Notes:
AP Physics C: Work, Energy, and Power Review (Mechanics)
•
!
!
Work done by a constant force: W = F iΔr = F Δr cos θ
o Work is the dot product of Force and the displacement of the object.
§ The dot product is also called the scalar product, because it is a scalar.
⎛ kg ⋅ m ⎞
W ⇒ joules, J = N ⋅ m = ⎜
m
⎝ s 2 ⎟⎠
!
!
Example: F = ⎡⎣ 2.7 î − 3.1 ĵ ⎤⎦ N and Δr = 4.6 î m then (include drawing)
! !
W = F i Δr = ⎡⎣2.7 î − 3.1 ĵ ⎤⎦ i ⎡⎣ 4.6 î + 0 ĵ ⎤⎦ = 2.7 4.6 + −3.1 0 = 12.42 ≈12 J
§
o
( )( ) (
•
Work done by a non-constant force:
)( )
xf
W = ∫ Fx dx
xi
This is a definite integral.
§ “Definite” simply means it has limits xi and xf.
§ Integral, or anti-derivative is the area “under” the curve.
• Area “under” the curve specifically means the area between the curve
and the horizontal axis where area above the horizontal axis is positive
and area below the horizontal axis is negative.
Notice we now have two different equations for work, one for work done by a constant force and
one fore work done by a force that varies. This will happen very often in AP Physics C and you
need to be careful to identify the difference.
o
•
•
!
o
o
o
•
!
The force caused by a spring: Fs = −k Δx
k is the “spring constant” and is a measure of how much force is takes to compress or
expand a spring per meter.
Δx is the displacement of the spring from equilibrium position (or rest position).
The negative means the force of the spring is opposite the direction of the displacement
of the spring.
xf
xf
xi
xi
WF = ∫ Fs dx = ∫
s
(
x
f
⎛ kx 2 ⎞ ⎛ kx 2 ⎞
⎛ kx 2 ⎞ ⎤
f
i
−kx dx = ⎜ −
⎟⎥ = ⎜− 2 ⎟ −⎜− 2 ⎟
⎝ 2 ⎠ ⎥⎦ x ⎝
⎠ ⎝
⎠
)
i
⎡⎛ kx 2 ⎞ ⎛ kx 2 ⎞ ⎤
⇒ WF = − ⎢⎜ f ⎟ − ⎜ i ⎟ ⎥ = − ⎡⎣U ef − U ei ⎤⎦ = −ΔU e ⇒ WF = −ΔU e
s
s
⎢⎣⎝ 2 ⎠ ⎝ 2 ⎠ ⎥⎦
1 2
o We have defined elastic potential energy: U e = kx .
2
o
•
xf
∑W = ∫ ∑ F dx ⇒ ∑W = ΔKE
xi
o
o
The Net Work – Kinetic Energy Theorem.
Derivation is here: http://www.flippingphysics.com/wnet-ke.html
This equation is always true.
1
mv 2
2
Gravitational Potential Energy in a constant gravitational field is: U g = mgh
o
•
The above example shows the work done by the force of the spring equals the negative
of the change in elastic potential energy of the spring.
This equation is where kinetic energy is defined:
KE =
0196 Lecture Notes - AP Physics C- Work, Energy, and Power Review (Mechanics).docx
page 1 of 3
•
o
h is the “vertical height above the horizontal zero line” and you have to always identify the
horizontal zero line.
o
If you prefer the equation from the AP sheet, it is:
ΔU g = mgΔh
§ The AP equation is the “change in” gravitational potential energy.
Energy can be neither created nor destroyed, so in a non-isolated system the change in energy of
the system equals the sum of the energy transferred to or from the system:
o
ΔE system = ∑ T
If the system is isolated, no energy is transferred into or out of the system:
§
ΔE system = 0
The change in energy of the system is the change in mechanical energy of the
system plus the change in internal energy of the system. ΔME + ΔE internal = 0
§
The change in internal energy of the system is done by nonconservative forces
or friction. In other words, the energy which is dissipated by friction goes into the
system as internal energy. ΔE internal = −Wnc
•
§
A nonconservative force is a force where the work done by the force is
dependent on the path taken by the object. Conservative forces are
where the work done by the force is not dependent on the path taken by
the object.
In other words: ΔME − Wnc = 0 ⇒ Wnc = ΔME and because I don’t know of
any forces which are nonconservative which are not friction:
§
o
Wfriction = ΔME (is only true when there is no energy transferred into our out of
the system.)
If the system is isolated and there is no work done by friction:
§
Wfriction = ΔME ⇒ 0 = ΔME = ME f − ME i ⇒ ME i = ME f
§
§
We have conservation of mechanical energy.
Which is only true when the system is isolated and no work is done by friction.
•
Whenever you use Wfriction = ΔME or ME i = ME f you have to identify the initial point, the final
•
point and the horizontal zero line.
All forms of Mechanical Energy are in terms of joules, just like Work.
•
Power is the rate at which work is done:
o
Pinstantaneous
§
•
(
)
Note: Force must be constant to use this equation.
o
The equations for power on the AP sheet are:
o
P ⇒ Watts =
P=
! !
dE
& P = F iv
dt
J
& 746watts = 1hp
s
Remember, every derivative is also an antiderivative (or an integral). For example:
o
•
W
dW
& Pinstantaneous =
Δt
dt
!
dW d ! !
dΔr ! !
=
=
F i Δr = F i
= F iv
dt
dt
dt
Paverage =
P=
Wf
tf
tf
dW
⇒ dW = P dt ⇒ ∫ dW = ∫ P dt ⇒ ΔW = ∫ P dt
Wi
ti
ti
dt
The equation which relates conservative forces and potential energy is:
Fx = −
dU
(and it is not
dx
on the AP equation sheet.)
0196 Lecture Notes - AP Physics C- Work, Energy, and Power Review (Mechanics).docx
page 2 of 3
•
•
•
o
Aside: Much of the time when the phrase “conservative force” is used on the AP Exam,
you need to use this equation.
o
For a spring:
o
For gravity:
Fs = −
Fg = −
dU e
dx
dU g
dy
=−
=−
d ⎛ 1 2⎞
kx ⎟ = −kx
dx ⎜⎝ 2
⎠
(
)
d
mgy = −mg (Force of gravity is always down)
dy
Neutral Equilibrium is where the Potential Energy of the object remains constant regardless of
position. For example, a ball rolling on a level surface.
Stable Equilibrium is where the Potential Energy of the object increases as the position of the
object moves away from the equilibrium position and therefore the object naturally returns to the
equilibrium position. For example, a water bottle being tipped to the side.
Unstable Equilibrium is where the Potential Energy of the object decreases as the position of the
object moves away from the equilibrium position and therefore the object naturally moves away
from the equilibrium position. For example, a marker being tipped to the side.
0196 Lecture Notes - AP Physics C- Work, Energy, and Power Review (Mechanics).docx
page 3 of 3
Flipping Physics Lecture Notes:
AP Physics C: Integrals in Kinematics Review (Mechanics)
To be reviewed after students learn about integrals!!
• FYI: I do not teach integrals until we get to Work. By then the students who are taking calculus
concurrently with AP Physics C Mechanics have had enough experience with derivatives that
they only freak out a little bit when I teach them integrals. J
•
Remember, every derivative is also an antiderivative (or an integral). For example:
o
a=
vf
tf
tf
vf
dv
⇒ dv = adt ⇒ ∫ dv = ∫ adt ⇒ v ⎤⎦ = v f − v i = Δv = ∫ adt
v
vi
ti
ti
i
dt
The area “under” an acceleration as a function of time graph is the change in velocity of
the object.
§ Remember the area “under” the curve specifically means the area between the
curve and the horizontal axis where area above the horizontal axis is positive and
area below the horizontal axis is negative.
Another Example:
o
•
o
v=
xf
tf
tf
xf
dx
⇒ dx = v dt ⇒ ∫ dx = ∫ v dt ⇒ x ⎤⎦ = x f − x i = Δx = ∫ v dt
xi
xi
ti
ti
dt
The area “under” an velocity as a function of time graph is the change in position of the
object or the displacement of the object.
Graphs of throwing a ball upward with a positive velocity initial.
o
•
o
o
o
dx
à Velocity is the slope of a position vs. time graph.
dt
dv
a=
à Acceleration is the slope of a velocity vs. time graph.
dt
v=
tf
Δv = ∫ adt à Change in velocity is the area “under” an acceleration as a function of
ti
time graph.
o
tf
Δx = ∫ v dt à Change in position or displacement, is the area “under” a velocity as a
ti
function of time graph.
0197 Lecture Notes - AP Physics C- Integrals in Kinematics Review (Mechanics).docx
page 1 of 2
•
Assuming the acceleration is constant, we can derive two of the Uniformly Accelerated Motion
equations. For example:
o
o
•
()
()
dv
⇒ dv = adt ⇒ ∫ dv = ∫ adt ⇒ v t = at + C
dt
⇒ v 0 = a 0 + C ⇒ v 0 = C = v i ⇒ v t = at + v i ⇒ v f = v i + at
a=
() ()
()
Another example:
o
o
() (
)
dx
1
⇒ dx = v dt ⇒ ∫ dx = ∫ v dt ⇒ x t = ∫ v i + at dt = v it + at 2 + C
dt
2
2
1
1
⇒ x 0 = v i 0 + a 0 + C ⇒ x 0 = C = x i ⇒ x t = x i + v it + at 2
2
2
v=
()
()
()
()
0197 Lecture Notes - AP Physics C- Integrals in Kinematics Review (Mechanics).docx
()
page 2 of 2
Flipping Physics Lecture Notes:
AP Physics C: Momentum, Impulse, Collisions and Center of Mass Review
(Mechanics)
•
The symbol for momentum is a lowercase p.
o Momentum is a vector!
!
!
p = mv
kg ⋅ m
kg ⋅ m
!
!
p = mv ⇒
(not to be confused with Newtons which are
)
s
s2
!
! dp
nd
Newton’s 2 law in terms of momentum is: ∑ F =
dt
!
!
! dp d
dv
! dm !
∑ F = dt = dt mv = dt v + m dt (the product rule)
! dm !
!
v + ma (Usually we assume the mass of the object does not change.)
o ⇒ ∑F =
dt
!
!
dp
!
!
∑ Fexternal = dt = 0 ⇒ ∑ pi =∑ pf : When all the forces are internal to the system, the net
o
•
•
•
( )
force equals zero, the derivative of momentum as a function of time is zero, therefore the
momentum does not change, therefore momentum is conserved.
o Momentum is conserved during collisions and explosions.
•
Impulse derivation:
tf
pf
t
!
! dp
!
!
!
!
!
! !
!
! f
F
=
⇒
F
dt
=
d
p
⇒
F
dt
=
d
p
=
p
−
p
⇒
Δ
p
=
F
dt
=
J
∑ dt ∑
∑
∫t ∑
∫p
∫
f
i
t
i
i
i
o
Symbol for Impulse is J and it is a vector.
o
! tf
!
kg ⋅ m
Units for Impulse: J = ∫ ∑ F dt ⇒ N ⋅ s (yes, this is the same as momentum:
)
s
t
i
§
o
Impulse is the area “under” a force as a function of time curve.
§
o
o
⎛ kg ⋅ m ⎞
kg ⋅ m
N ⋅s = ⎜
s=
2
⎟
s
⎝ s ⎠
Not to be confused with the equation for work:
Impulse approximation says
! !
∑ F ≈ Fimpact
xf
W = ∫ Fx dx
xi
§
!
!
dp
Therefore, the impulse approximation says: Fimpact =
dt
§
Impulse, J, and Impact Force often get confused. Please note they are different!
Can also use the average force and change in time to determine Impulse:
§
! !
J = Faverage Δt
! tf
!
This creates a rectangle with the same area as J = ∫ ∑ F dt
ti
Type of Collision
Elastic
Inelastic
Is Momentum Conserved?
Yes
Yes
Is Kinetic Energy Conserved?
Yes
No
0198 Lecture Notes - AP Physics C- Momentum, Impulse, Collisions and Center of Mass Review (Mechanics).docx
page 1 of 2
o
o
o
•
Collisions between hard spheres are “nearly” elastic and therefore are generally
considered to be elastic in physics classes.
“Perfectly Inelastic” Collisions are where the objects stick to one another. Sometimes
they are called “Completely Inelastic” or “Totally Inelastic”. These terms all mean the
same thing.
Most collisions are actually Inelastic.
∑m x
∑m
x cm =
Center of mass of a system of particles:
i
i
=
m1 x 1 + m2 x 2 + ...
i
o
x is the distance from a zero reference line; usually the origin.
o
Velocity of a system of particles:
o
•
m1 + m2 + ...
v cm =
Do the same thing with acceleration:
dx cm
dt
acm =
Center of mass of an object with shape: rcm =
=
d ⎛ ∑ mi x i ⎞
⎜
⎟=
dt ⎝ ∑ mi ⎠
dv cm
dt
=
d ⎛ ∑ mi v i
⎜
dt ⎝ ∑ mi
∑ m dx = ∑ m v
∑ m dt ∑ m
⎞ ∑ma
⎟=
⎠
∑m
i
i
i
i
i
i
i i
i
1
r dm (not on AP equation sheet)
mtotal ∫
o
The position of the center of mass of an object with shape equals one over the total mass
of the object times the integral with respect to mass of the posotion of all of the
infinitesimally small pieces of the object, which are called dm, relative to a zero-reference
line.
o
If you prefer:
x cm =
1
mtotal
∫ x dm
§
Volumetric Mass Density:
§
Surface Mass Density:
§
ρ=
m
(not on AP equation sheet)
∀
m
(not on AP equation sheet)
A
m
Linear Mass Density: λ =
(not on AP equation sheet)
L
σ=
0198 Lecture Notes - AP Physics C- Momentum, Impulse, Collisions and Center of Mass Review (Mechanics).docx
page 2 of 2
Flipping Physics Lecture Notes:
AP Physics C: Rotational Kinematics Review (Mechanics)
•
•
•
•
!
!
rev ⎞
!
!
Δθ
dθ ⎛ rad
Angular velocity: ω average =
or
& ω instantaneous =
min ⎟⎠
Δt
dt ⎜⎝ s
!
!
!
!
Δω
dω ⎛ rad ⎞
Angular acceleration: α average =
& α instantaneous =
Δt
dt ⎜⎝ s 2 ⎟⎠
Uniformly Angularly Accelerated Motion: U α M (when α = constant = # )
o
5 variables, 4 equations, If you know 3 variables, you can find the other 2, which leaves
you with 1 …
o
1
1
ω f = ω i + αΔt ; Δθ = ω i Δt + αΔt 2 ; ω f2 = ω i2 + 2αΔθ ; Δθ = ω i + ω f Δt
2
2
(
)
Arc length, s, is the linear distance travelled when moving along a circle or part of a circle. In
other words it is the linear length when traveling along an arc.
o s = rΔθ : arc length equals the radius of the object times the angular displacement of
the object.
§ Must use radians when using this equation.
§ 1 revolution = 360° = 2π radians
§ The equation for circumference is an example of this equation where the angular
( )
displacement is one revolution or 2π radians: C = r 2π
§
§
§
•
•
Arc length is a linear dimension, so its units are linear: meters, etc.
Not on equation sheet
I use a lowercase cursive ! for arc length, because my s looks like a 5. Sorry.
d
ds
dθ
s = rΔθ ⇒
=r
⇒ v t = rω (is on the AP equation sheet)
dt
dt
dt
dv
d
dω
v t = rω ⇒
v t = rω ⇒ t = r
⇒ at = rα (not on the AP equation sheet)
dt
dt
dt
s = rΔθ ⇒
(
(
)
)
o
o
Both of these equations assume the radius stays constant.
Must use radians when using both of these equations.
o
⎛ m⎞
v t is tangential velocity, or the linear velocity of an object moving in a circle. ⎜ ⎟
⎝ s⎠
o
⎛ m⎞
at is tangential acceleration, or the linear acceleration of an object moving in a circle. ⎜ 2 ⎟
⎝s ⎠
Both tangential quantities are tangent to the circle the object is moving along. This
also means they are perpendicular to the radius of the circle the object is moving
along.
Uniform Circular Motion is where objects move in a circle with an angular acceleration of zero.
o α = 0 (The symbol for angular acceleration is alpha, α .)
o Even though the magnitude of the object’s velocity does not change, the direction of the
velocity does, that means the velocity is not constant, therefore there must be an
acceleration. The acceleration responsible for this change in the direction of the velocity
is called centripetal acceleration, ac.
§
•
0199 Lecture Notes - AP Physics C- Rotational Kinematics Review (Mechanics).docx
page 1 of 2
o
ac =
⎛ m⎞
= rω 2 in ⎜ 2 ⎟
r
⎝s ⎠
v t2
Centripetal means “center seeking” because the centripetal acceleration is always
toward the center of the circle the objects path describes.
nd
According to Newton’s 2 law, where there is an acceleration, there must be a net force.
Therefore, if an object is moving in a circle, there is a centripetal acceleration and there must be a
centripetal force.
o
•
o
Centripetal force:
∑F
in
= mac
Centripetal force is the net force in the in direction.
§ It is not a new force.
§ It is never in a free body diagram
§ The “in” direction is positive. (The “out” direction is negative.)
o See “conical pendulum” example from AP Physics 1 Kinematics Review.
Non-Uniform Circular Motion will have an angular acceleration which is nonzero. α ≠ 0
o This means there will also be a tangential acceleration, at, which is parallel to the
tangential velocity and normal to the centripetal acceleration.
o
•
o
•
The net acceleration of an object in Non-Uniform Circular Motion is:
!
! !
anet = ac + at
The period, T, of an object moving in a circle is the time it takes for one revolution. Therefore:
o
ω=
Δθ 2π
2π
: Period is in seconds
=
⇒T =
Δt
T
ω
0199 Lecture Notes - AP Physics C- Rotational Kinematics Review (Mechanics).docx
page 2 of 2
Flipping Physics Lecture Notes:
AP Physics C: Rotational Dynamics Review – 1 of 2 (Mechanics)
•
A rigid object with shape is rotating. Every piece of this object has kinetic energy. The total kinetic
energy is the sum of all of the kinetic energies of every small piece of the object:
( )
(
( )
)
2
2
1
1
1
1
1
KEt = ∑ i KE i = ∑ i mi v i = ∑ i mi riω i = ∑ i mi ri2ω i2 = ∑ i mi ri2 ω 2 = Iω 2
2
2
2
2
2
o This uses v t = rω and that every part of the object has the same angular velocity, ω
o
KE rotational =
1 2
Iω : Rotational Kinetic Energy of a rigid object with shape or a system of
2
particles that is not changing shape.
•
•
I = ∑ i mi ri2 where I is called the Moment of Inertia or “Rotational Mass”.
o
This is the Moment of Inertia for a system of particles.
o
Units for Moment of Inertia: I
Moment of Inertia for a rigid object with shape:
o
1
r dm
mtotal ∫
Deriving the Moment of Inertia of a Uniform Thin Hoop about its Cylindrical Axis
o
I z = ∫ r 2 dm = R 2 ∫ dm = R 2 m ⇒ I cm = mR 2
“Thin” means all of the dm’s are located a distance R from the center of mass.
“Uniform” means the hoop is of a constant density.
“Cylindrical Axis” means the line through the center of the hoop and normal to the plane
of the hoop.
Deriving the Moment of Inertia of a Uniform Rigid Rod about its Center of Mass
o
o
o
•
I = lim Δm→0 ∑ ri 2 Δmi ⇒ I = ∫ r 2 dm
Not to be confused with the equation for the center of mass of a rigid object with shape:
rcm =
•
= ∑ i mi ri 2 ⇒ kg ⋅ m2
o
m dm
m
=
⇒ dm = λ dx ⇒ dm = dx
L dx
L
m is the total mass of the rod
§
L is the total length of the rod
§
§
λ is the linear mass density of the rod, which is
λ=
constant in this “uniform” rod.
L
2
L
o
m
m ⎡ x3 ⎤2
2
2 m
2
I y = ∫ r dm = ∫ r
dx = ∫ x dx = ⎢ ⎥
L
L L
L ⎣ 3 ⎦− L
−
o
⎡⎛ L ⎞
⎛ L⎞
⎢
⎜⎝ − 2 ⎟⎠
m ⎢ ⎜⎝ 2 ⎟⎠
⇒ Iy = ⎢
−
L⎢ 3
3
⎢
⎣
2
3
3
2
⎤
⎥
⎥ m ⎡ L3 L3 ⎤ m ⎡ 2L3 ⎤ 1
2
⎥ = L ⎢ 24 + 24 ⎥ = L ⎢ 24 ⎥ = 12 mL
⎣
⎦
⎣
⎦
⎥
⎥
⎦
0200 Lecture Notes - AP Physics C- Rotational Dynamics Review - 1 of 2 (Mechanics).docx
page 1 of 3
•
Deriving the Moment of Inertia of a Uniform Rigid Rod about one end
o This is the same as before, only with different limits …
L
•
L
m 2
m ⎡ x3 ⎤
m L3 1
o Iy =
x
dx
⇒
=
= mL2
⎢
⎥
L ∫0
L ⎣ 3 ⎦0 L 3 3
2
The Parallel-Axis Theorem: I = I cm + mD
•
o Only true for objects with constant density.
o m is the total mass of the rigid, constant density object.
o D is the distance from the center of mass of the object to the new axis of rotation.
o Not on the AP equation sheet.
Example: Moment of Inertia of a Uniform Rigid Rod about its end.
o
Known for Uniform Rigid Rod:
I cm =
1
mL2
12
2
I end
•
•
⎛ L⎞
⎛ 1 3⎞
1
1
1
4
1
= I cm + mD = mL2 + m ⎜ ⎟ = mL2 + mL2 = ⎜ + ⎟ mL2 = mL2 = mL2
12
4
12
3
⎝ 2 ⎠ 12
⎝ 12 12 ⎠
2
Example: Moment of Inertia of a Uniform Thin Hoop about its Rim.
o
Known for Uniform Thin Hoop about its Center of Mass: I cm = mR
o
I rim = I cm + mD 2 = mR 2 + mR 2 = 2mR 2
Torque: τ = rF sin θ
o This is the magnitude of the torque. Torque is a vector.
o r is the distance from the axis of rotation to the location
on the object the force is applied.
o F is the magnitude of the force.
o θ is the angle between r and F.
o
•
•
2
sin θ =
O d
= ⇒ d = r sin θ is the “moment arm” or
H r
“lever arm” or “effective distance”
o Units for torque are N ⋅ m
§ Not to be confused with the units for energy, joules, even though joules are also
N ⋅ m.
But “What is torque?” Torque is the rotational equivalent of force. Force is the ability to cause an
acceleration of an object. Torque is the ability of a force to cause an angular acceleration of an
object.
The rotational form of Newton’s Second Law:
!
!
!
!
∑ F = ma ⇒ ∑ τ = Iα
Must identify axis of rotation when summing the torques.
Must identify what objects you are summing the torque on.
§ Note: The angular acceleration of each object around the axis of rotation must be
the same.
o Must identify the direction of positive rotation.
o Now that we have defined Moment of Inertia, pulleys can have mass. When pulleys have
mass the force of tension on either side of a pulley are not the same!
Right Hand Rule for direction of torque
o Don’t be too cool for the right hand rule. Limber Up!
o Use your right hand.
o Fingers start at the axis of rotation.
o Point fingers along direction of “r”.
o Curl fingers along the direction of “F”.
o Thumb points in the direction of the torque.
o
o
•
0200 Lecture Notes - AP Physics C- Rotational Dynamics Review - 1 of 2 (Mechanics).docx
page 2 of 3
•
Rolling Without Slipping: v cm = Rω & acm = Rα
o
Just like v t = rω & at = rα only …
o
§ R is the radius of the solid object
§ These are for the center of mass of the object, not the tangential quantities.
Neither of these are on the AP equation sheet.
o
FYI: Rolling With Slipping: v cm ≠ Rω & acm ≠ Rα
o
When an object is rolling without slipping it has both translational and rotational kinetic
energies!!
0200 Lecture Notes - AP Physics C- Rotational Dynamics Review - 1 of 2 (Mechanics).docx
page 3 of 3
Flipping Physics Lecture Notes:
AP Physics C: Rotational Dynamics Review – 2 of 2 (Mechanics)
•
! ! ! ! !
τ =r×F ≠F×r
o
Torque is the cross product (also called the vector product) of
§ Torque is a vector!
!
!
r & F.
!
!
r is the position vector from the axis of rotation to the location of the force, F .
Magnitude of torque à τ = rF sin θ
! !
! !
o The order does matter! ( A × B = − B × A )
!
!
o Cross product is the area of the parallelogram with sides r & F .
!
!
In case you forgot how to do the cross product. Example: A = − î + ĵ − 2 k̂ & B = 2 î − 3 ĵ + 4 k̂
o
o
•
î
ĵ
k̂
! !
A × B = −1 1 −2 = 1 −2 î − −1 −2 ĵ + −1 1 k̂
−3 4
2 4
2 −3
2 −3 4
! !
⇒ A × B = ⎡⎣ 1 4 − −2 −3 ⎤⎦ î − ⎡⎣ −1 4 − −2 2 ⎤⎦ ĵ + ⎡⎣ −1 −3 − 1 2 ⎤⎦ k̂
! !
⇒ A × B = ⎡⎣ 4 − 6 ⎤⎦ î − ⎡⎣ −4 + 4 ⎤⎦ ĵ + ⎡⎣3 − 2 ⎤⎦ k̂ = −2 î + k̂
( )( ) ( )( )
•
!
!
An object is in Rotational Equilibrium if the net torque acting on it equals zero, which means the
o
o
•
!
∑ F = 0 = ma ⇒ a = 0
object is not angularly accelerating:
•
( )( ) ( )( )
An object is in Translational Equilibrium if the net force acting on it equals zero, which means the
object is not accelerating:
•
( )( ) ( )( )
!
!
!
∑ τ = 0 = Iα ⇒ α = 0 (must identify axis of rotation)
This means the object is either not rotating or has a constant angular velocity.
If an object is in translational equilibrium and in rotational equilibrium about one axis of
rotation, then the object is in rotational equilibrium about any axis of rotation.
!
L is Angular Momentum and it is a vector!
!
!
!
! dp
!
!
! dL
!
⇒ ∑τ =
o ∑ F = ma ⇒ ∑ τ = Iα & ∑ F =
dt
dt
For a particle or any object which is not rotating:
o
o
!
!
!
Just like torque, we have a cross product equation for angular momentum: L = r × p
§ r is the position vector from the axis of rotation to the location of the center of
mass of the moving object.
§ And a magnitude equation for angular momentum: L = rmv sin θ
• With this equation, need to use Right Hand Rule to find direction.
Yes, a particle or a rigid object which is not rotating can have an angular momentum!
!
!
L = Iω
•
For a rigid object with shape:
•
!
⎛ rad ⎞ kg ⋅ m2 ⋅ rad kg ⋅ m2
!
L = Iω ⇒ kg ⋅ m2 ⎜
=
=
s
s
⎝ s ⎟⎠
!
!
dp
!
!
= 0 ⇒ ∑ pi = ∑ pf
Derivation of conservation of linear momentum: ∑ Fexternal =
dt
!
!
!
!
dL
= 0 ⇒ ∑ Li = ∑ Lf
Derivation of conservation of angular momentum: ∑ τ external =
dt
o
•
Units for angular momentum:
(
)
0201 Lecture Notes - AP Physics C- Rotational Dynamics Review - 2 of 2 (Mechanics).docx
page 1 of 2
Note the similarities between the two, please.
Remember net torque requires the axis of rotation to be identified, which means the axis
of rotation needs to be identified for conservation of angular momentum
Conservation of Angular Momentum Example: A piece of gum with mass, m, and velocity, v, is
o
o
•
spat at a solid cylinder of mass, M, radius, R, and moment of inertia
1
MR 2 . The cylinder is on a
2
horizontal axis through its center of mass and is initially at rest. The line of action of the gum is
located horizontally a height, y, above the axis of the cylinder. If the gum sticks to the cylinder,
what is the final angular velocity of the gum/cylinder system? The Drawing!!
o
Gum knowns: m =
o
Solving for
o
sin θ gi =
ωf = ?
mg ,v = v gi
Cylinder knowns:
1
M = mc ,ω ic = 0,R,I c = mc R 2
2
(will be the same for both gum and cylinder)
!
!
dL
=0
o Know angular momentum is conserved because: ∑ τ external =
dt
!
!
!
!
!
!
o ∑ L i = ∑ L f ⇒ L gi + Lci = L gf + Lcf ⇒ rgi mg v gi sin θ gi + 0 = rgf mg v gf sin θ gf + I cω f
O y
= ⇒ y = rgi sin θ gi ⇒ ymg v gi = rgf mg v gf sin θ gf + I cω f
H rgi
1
v gf = v t = Rω f ⇒ ymg v gi = Rmg Rω f sin 90 + mc R 2ω f
2
ymg v gi
⎛
m ⎞
⇒ ymg v gi = R 2ω f ⎜ mg + c ⎟ ⇒ ω f =
⎛
2 ⎠
m ⎞
⎝
R 2 ⎜ mg + c ⎟
2 ⎠
⎝
o
o
FYI: Sawdog, one of my Quality Control Team members, pointed out that, after colliding with the cylinder,
the gum is moving in a circle, so it’s angular momentum can be described using
(
)
I gω f . More specifically:
!
Lgf = I gω f = mg rg 2 ω f = mg R 2ω f It’s a slightly different solution that results in the same answer.
0201 Lecture Notes - AP Physics C- Rotational Dynamics Review - 2 of 2 (Mechanics).docx
page 2 of 2
Flipping Physics Lecture Notes:
AP Physics C: Rotational vs. Linear Review (Mechanics)
Name:
Displacement
Velocity
Acceleration
Uniformly Accelerated Motion
(UAM)
or
Uniformly Angularly Accelerated Motion
(UαM)
!
Δx = x f − x i
!
!
Δx
dx
!
!
v avg =
& v inst =
Δt
dt
!
!
Δv !
dv
!
aavg =
& ainst =
Δt
dt
v f = v i + at
Linear:
!
Δθ = θ f − θ i
!
!
!
!
Δθ
dθ
ω avg =
& ω inst =
Δt
dt
!
!
!
!
Δω
dω
α avg =
& α inst =
Δt
dt
ω f = ω i + αt
1
x f = x i + v it + at 2
2
2
2
v f = v i + 2a x f − x i
1
θ f = θ i + ω it + α t 2
2
ω f = ω i + 2α θ f − θ i
(
xf − xi =
Mass
Kinetic Energy
(
)
)
1
v + vi t
2 f
1
mv 2
2
Newton’s Second Law
Force / Torque
Force
Ptranslational =
dW ! !
= F iv
dt
!
!
p = mv
Momentum
(
)
)
1
ω + ωi t
2 f
= ∑ i mi ri 2
θf − θi =
I object with shape = ∫ r 2 dm
!
!
! dp
!
∑ F = ma & ∑ F = dt
Power
(
I particles
Mass
KE translational =
Rotational:
1 2
Iω
2
!
!
!
! dL
∑ τ = Iα & ∑ τ = dt
! ! !
τ =r×F
KE rotational =
dW ! !
Protational =
= τ iω
dt
!
! !
L particle = r × p
!
!
Lobject with shape = Iω
Thank you to Aarti Sangwan for pointing out that I didn’t include a rotational form of work in the video.
Name:
Linear:
Rotational:
! !
! !
Work (constant force)
W = F ⋅ Δr = F Δr cos θ
W = τ ⋅ Δθ
Work (non-constant force)
Net Work-Kinetic Energy
Theorem
xf
W = ∫ Fx dx
xi
Wnet = ΔKE =
1
1
mv f 2 − mv i 2
2
2
θf
W = ∫ τ dθ
θi
Wnet = ΔKE =
1
1
Iω f 2 − Iω i 2
2
2
A little bonus: Look what happens when we combine a couple of the above formulas:
!
1
1
!
Wnet = τ net ⋅ Δθ = IαΔθ = Iω f 2 − Iω i 2 ⇒ 2αΔθ = ω f 2 − ω i 2 ⇒ ω f 2 = ω i 2 + 2αΔθ (UαM!)
2
2
0202 Lecture Notes - AP Physics C- Rotational vs. Linear Review (Mechanics).docx
page 1 of 1
Flipping Physics Lecture Notes:
AP Physics C: Universal Gravitation Review (Mechanics)
•
The Force of Gravity or Weight of an object:
Fg = mo g where “o” stands for “object”.
o
A subscript is missing:
o
All forces require two objects. This equation is the force of gravitational attraction which
exists between the object and a planet.
§
•
Fg = mg
For us, the planet usually is the Earth.
g Earth = 9.81
m
s2
Any two objects which have mass have a force of gravitational attraction between them. This
force is determined using Newton’s Universal Law of Gravitation. (The Big G Equation)
Gm1m2
N ⋅ m2
kg 2
o
Fg =
o
Requires two objects: mass 1 and mass 2.
r is not the radius by definition. r is the distance between the centers of mass of
the two objects. Yes, sometimes r is the radius.
§ This equation is the magnitude of the force of gravitational attraction. The force
is always directed toward the other object.
Thank you to one of my Quality Control team members, Frank Geshwind, for pointing out
r2
: G is the Universal Gravitational Constant.
G = 6.67 ×10−11
§
§
that I should have talked about the vector form of this equation:
•
Note:
§
Some textbooks even use this equation:
Note the subtle change here.
!
Gm1m2 !
Fg = −
r12
r3
!
r12 is no longer the unit vector and
therefore the cube of r needs to be in the denominator. J
Setting the two equations for the Force of Gravity acting on an object on the surface of the Earth
equal to one another yields this:
o
•
r̂12 is the unit vector from object 1 to object 2.
§
•
!
Gm1m2
Fg = −
r̂12
r2
Fg = Fg ⇒ mo g =
As long as
Gmo mE
r2
⇒ g Earth =
(R
GmE
Earth
+ Altitude
)
2
REarth ≫ ΔAltitude the g Earth ≈ constant .
o
In other words, close to the surface of the planet, the acceleration due to gravity can be
considered to be constant. We live in a constant gravitational field.
o
The gravitational potential energy is then constant:
0203 Lecture Notes - AP Physics C- Universal Gravitation Review (Mechanics).docx
U g = mgh
page 1 of 3
•
When viewed from a frame of reference which is not on the surface of the planet, the
acceleration due to gravity is not constant and we need a different equation:
o
Universal Gravitational Potential Energy:
r
§
§
Note: This equation is not
§
Fg =
Gm1m2
r2
r is not squared.
The gravitational potential energy which exists between an object and the Earth in terms of the
objects distance, r, from the center of the Earth looks like this:
o
o
Assumes constant density Earth. (Which is not true, however, we often assume it is.)
Be aware the graph as shown above left has an incorrect part:
§ This cannot be correct because this implies an object would experience a
change in gravitational potential energy of zero.
•
§
o
ΔU g = U f − U i = 0 − 0 = 0 (This makes no sense.)
This implies it takes zero energy to move an object from the center of a planet to
infinitely far from the planet.
•
WF = ΔME = ΔU g = 0 (Again, this makes no sense.)
a
§ This would mean the planet has a binding energy of zero; still makes no sense.
§ I will be releasing a video with the correct solution soon.
The Gravitational Potential Energy which exists between the two objects when the object
is on the surface of the Earth is:
•
Gm1m2
This equation assumes a zero line which is infinitely far
away. ( r ≈ ∞ )
This means Universal Gravitational Potential Energy can
never be positive.
This equation requires two objects.
§
•
Ug = −
Ug = −
Gmo mE
RE
What is the minimum amount of work necessary to completely remove an object from a planet if
the object is resting on the surface of the planet? This is called the Binding Energy. Assume the
object is moved infinitely far away, has zero velocity when it gets there, and there is no friction.
ΔE system = ∑ T ⇒ ΔME + ΔE internal = WF
a
⎛ Gm1m2 ⎞ Gmo mp
(Binding Energy)
⇒ WF = ME f − ME i + 0 = 0 − U gi = 0 − ⎜ −
=
a
r ⎟⎠
Rp
⎝
•
What is the minimum velocity to launch an object off the Earth and have it never return? This is called
Escape Velocity. Assume no atmosphere and no Earth rotation. Note: Mechanical Energy is conserved:
ME i = ME f ⇒ U gi + KE i = 0 ⇒ −
Gmo mE
RE
GmE
2GmE
1
1
+ mo v i 2 = 0 ⇒ v i 2 =
⇒ v escape =
2
2
RE
RE
0203 Lecture Notes - AP Physics C- Universal Gravitation Review (Mechanics).docx
page 2 of 3
•
What is the total mechanical energy of an object in circular orbit?
ME total = U g + KE = −
Gmo mplanet
r
1
+ mo v o2
2
The only force acting on the object moving in circular orbit is the force of gravity which acts
inward. So we can sum the forces on the object in the in direction:
ME total
•
•
•
Gmo mp
v o2
Gmo mp
1
mo v o2 = KE o
r
2r
2
r
Gmo mp Gmo mp Gmo mp ⎛
Gmo mp
1 ⎞ Gmo mp ⎛ 1 ⎞
=−
+
=
−1+
=
−
=
−
⎜⎝
⎜⎝ 2 ⎟⎠
r
2r
r
2 ⎟⎠
r
2r
∑ Fin = Fg = mac ⇒
2
= mo
⇒
=
Kepler’s 3 laws. I find having a basic understanding of the his first two laws is adequate,
rd
however, you need to know how to derive Kepler’s 3 law.
st
Kepler’s 1 Law is that orbits are elliptical and defined as such:
o Two foci at F1 and F2.
o Each focus is located a distance c from the center of the ellipse.
o Semimajor axis of length a. (Major axis of length 2a)
o Semiminor axis of length b. (Minor axis of length 2b)
o r1 + r2 = 2a
2
2
2
o a =b +c
o Eccentricity of an ellipse is defined as c/a. For a circle c = 0 therefore eccentricity = 0.
o The eccentricity of the Earth’s orbit is 0.017, which means its orbit is nearly circular.
nd
Kepler’s 2 Law states that a line between the sun and the planet sweeps out an equal area
over an equal time interval.
The result of this is that the closer an object is to the planet during an orbit, the faster its
orbital speed will be.
rd
Let’s derive Kepler’s 3 law: We assume circular orbit. The only force acting on the orbital object
is the force of gravity acting inward. Sum the forces in the in direction:
o
•
∑F
in
= Fg = mac ⇒
Gmo mp
r2
= mo rω ⇒
2
Gmp
r3
2
2
⎛ 4π 2 ⎞
⎛ Δθ ⎞
⎛ 2π ⎞
4π 2
2
3
=ω =⎜
=⎜ ⎟ = 2 ⇒T =⎜
⎟r
⎟
T
⎝ Δt ⎠
⎝ T ⎠
⎝ Gmp ⎠
2
0203 Lecture Notes - AP Physics C- Universal Gravitation Review (Mechanics).docx
page 3 of 3
Flipping Physics Lecture Notes:
AP Physics C: Simple Harmonic Motion Review (Mechanics)
•
•
An object is in Simple Harmonic Motion if the acceleration of the object is proportional to the
object’s displacement from an equilibrium position and that acceleration is directed toward the
equilibrium position. a ∝ Δx
For example: A horizontal mass-spring
system on a frictionless surface has the
following free body diagram:
k
x
m
o
∑F
o
Amplitude, A, is defined as the maximum distance from equilibrium position. Therefore:
x
§
= −Fs = max ⇒ −kx = max ⇒ ax = −
amax =
a=
o
Note:
o
Therefore:
k
A
m
dv
dx
d ⎛ dx ⎞ d 2 x
&v=
⇒a= ⎜
=
dt
dt
dt ⎝ dt ⎟⎠ dt 2
d2 x
k
=
−
x
m
dt 2
k
= ω 2 where ω is called the angular frequency
m
d2x
= −ω 2 x
o Therefore:
2
dt
o
Let
§
§
This is the condition for simple harmonic motion.
This equation is not on the AP equation sheet. Memorize It!!
ω=
k Δθ 2π
m
(The period of a mass-spring system)
=
=
⇒ T = 2π
m Δt
T
k
o
Note:
o
Period of a pendulum:
o
T=
T = 2π
L
(know how to derive)
g
1
2π
&ω =
= 2π f ⇒ ω = 2π f
f
T
§
§
Frequency, f, is the number of cycles an object goes through per second.
Angular frequency and frequency are related, ω = 2π f , however, they are not
the same.
0204 Lecture Notes - AP Physics C- Simple Harmonic Motion Review (Mechanics).docx
page 1 of 3
•
One equation that satisfies the condition for Simple Harmonic Motion is:
o
o
o
•
•
a=
)
⎛
π⎞
y = cos ⎜ θ + ⎟ is “phase shifted” to the
2⎠
⎝
y = cosθ by
left from
v=
(
This equation is on the AP physics equation sheet, however, the equations for velocity
and acceleration in simple harmonic motion are not.
Have to use angles in radians in this equation.
φ or “phi” is the “phase constant” or the “phase shift”
of the wave. For example:
§
•
()
x t = Acos ω t + φ
(
))
(
(
π
2
radians.
)) ( )
(
dx d
=
Acos ω t + φ = A − sin ω t + φ ω
dt dt
⎡d
⎤
d
o ⇒v t = A
cos ω t + φ = A − sin ω t + φ ⎢
ωt + φ ⎥
dt
⎣ dt
⎦
o
()
( ( ))
⇒ v (t ) = − Aω sin (ω t + φ )
o
&
(
(
)) (
)
))
(
v max = Aω
(
))
( (
⎡d
⎤
dv d
d
=
− Aω sin ω t + φ = − Aω
sin ω t + φ = − Aω cos ω t + φ ⎢
ωt + φ ⎥
dt dt
dt
⎣ dt
⎦
(
( (
o
⇒ a = − Aω cos ω t + φ
o
&
()
))(ω ) ⇒ a(t ) = − Aω
2
(
cos ω t + φ
) (
)
)
amax = Aω 2
(
(
))
()
⇒ a t = −ω 2 Acos ω t + φ = −ω 2 x t ⇒
d2x
= −ω 2 x
2
dt
0204 Lecture Notes - AP Physics C- Simple Harmonic Motion Review (Mechanics).docx
page 2 of 3
•
Simple Harmonic Motion is NOT Uniformly Accelerated Motion
•
Total mechanical energy in Simple Harmonic Motion:
o
ME total =
(
1 2 1
kA = m v max
2
2
)
2
0204 Lecture Notes - AP Physics C- Simple Harmonic Motion Review (Mechanics).docx
page 3 of 3
Flipping Physics Lecture Notes:
AP Physics C: Equations to Memorize (Mechanics)
While I am not a fan of memorization and do my best to avoid having my students memorize, there are a
few items which are not on the equation sheet which I do suggest you memorize.
(
)
•
1
v + v i Δt (The fourth Uniformly Accelerated Motion equation)
2 f
The Force of Gravity or Weight of an object: Fg = mg
•
Fg = mg cosθ & Fg = mg sin θ (The components of the force of gravity parallel and
•
perpendicular on an incline where θ is the incline angle)
General range for coefficients of friction: 0 – 2
•
•
Δx =
⊥
!
ΔE system = ∑ T (General equation relating the change in energy of the system to the net energy
transferred into or out of the system.)
•
∑W = ΔKE
•
Wfriction = ΔME (only true when there is no energy added to or removed from the system via a
(always true)
force.)
•
ME i = ME f (only true when there is no energy added to or removed from the system via a force
and there is no work done by a nonconservative force.)
dU
(The equation which relates a conservative force and the potential energy
dx
•
Fx = −
•
associated with that force.)
That every derivative is an integral and every integral is a derivative. For Example:
x
o
U
f
f
dU
Fx = −
⇒ Fx dx = −dU ⇒ ∫ Fx dx = − ∫ dU ⇒ W = −ΔU
dx
x
U
i
( )
(
i
)
WF = Fg Δr cos θ = mg Δhcos 180° = −mgΔh ⇒ WF = −ΔU g
•
Book Example:
•
!
!
∑ pi =∑ pf (Conservation of Momentum. It may seem obvious,
g
g
however, you need to remember when it is valid.)
•
!
!
dp
!
!
= 0 ⇒ ∑ pi = ∑ pf
o ∑ Fexternal =
dt
!
!
∑ Li = ∑ Lf (Conservation of Angular Momentum. Again, it may
seem obvious, however, you need to remember when it is valid.)
•
•
•
!
!
!
!
dL
= 0 ⇒ ∑ Li = ∑ Lf
o ∑ τ external =
dt
1
rcm =
r dm (The center of mass of a rigid object with shape)
mtotal ∫
m
m
& λ=
(Volumetric Mass Density and Linear Mass Density)
∀
L
s = rΔθ & at = rα (arc length and tangential acceleration)
ρ=
0205 Lecture Notes - AP Physics C- Equations to Memorize (Mechanics).docx
page 1 of 3
o
•
Although
v t = rω is on the equation sheet, so it is easy to get to the other two.
v cm = Rω & acm = Rα (The velocity and acceleration of the center of mass of a rigid object
which is rolling without slipping. Easy to remember from the previous equations.)
•
1 revolution = 360° = 2π radians
•
ω f2 = ω i2 + 2αΔθ ; Δθ =
•
I = I cm + mD 2 (The parallel axis theorem)
(
)
1
ω + ω f Δt (Uniformly Angularly Accelerated Motion equations)
2 i
•
d2x
= −ω 2 x (The condition for simple harmonic motion)
dt 2
v max = Aω (The maximum velocity during simple harmonic motion)
•
amax = Aω 2 (the maximum acceleration during simple harmonic motion)
•
There are equations which many of you will want to memorize, however, I strongly discourage.
•
!
! 1
!
FR = −bv & FR = Dρ Av 2 (Resistive force equations)
2
o
Neither of these equations are on the equation sheet. Don’t memorize these equations.
§
! 1
FR = Dρ Av 2 and give you that equation or tell
2
!
!
you the drag force is “proportional to” the velocity, which means FR = −bv .
The problem will specify to use
•
746 watts = 1 hp (will be provided if you need it)
•
Do not memorize:
G = 6.67 ×10
−11
N ⋅ m2
kg 2
Instead, be familiar with the “Table of Information” and the page of general math formulas
on the AP Physics equation sheet.
Do not memorize the following equations; instead know how to derive them. This will be of much
more use to you during the AP exam because they want you to understand where these
equations come from and therefore will generally ask you a question that relates to their
derivations. I did all of these derivations during the review.
o
•
o
v cm =
∑mv
∑m
i
i
&
acm =
i
∑ma
∑m
i i
(velocity and acceleration of the center of mass of a
i
system of particles. Simply take the derivative with respect to time once or twice of the
position of the center of mass of a system of particles to get these equations.)
o
Terminal velocity:
o
Binding Energy:
o
Escape Velocity:
2mg
Dρ A
v terminal =
WF =
Gmo mp
a
v escape =
Rp
2GmE
RE
0205 Lecture Notes - AP Physics C- Equations to Memorize (Mechanics).docx
page 2 of 3
o
Total Mechanical Energy of Orbital Object:
⎛ 4π ⎞
3
T2 = ⎜
⎟r
⎝ Gmp ⎠
ME total = −
Gmo mp
2r
2
o
Kepler’s Third Law:
o
Velocity in simple harmonic motion:
o
Acceleration in simple harmonic motion:
o
Moments of Inertia of …
()
(
v t = − Aω sin ω t + φ
()
(
)
a t = − Aω 2 cos ω t + φ
)
§
Uniform Hoop or thin cylindrical shell about its cylindrical axis:
§
Uniform rigid rod about its center of mass:
§
Uniform Solid cylinder or disk about its cylindrical axis:
§
§
I cm =
I cm = mR 2
1
mL2
12
I cm =
1
mR 2
2
• Okay, I didn’t do this one during the review.
Use the parallel axis theorem to find the moment of inertia of any of these about
any other axis.
A quick note about moments of inertia and the AP Exam. If you need the
equation for a Moment of Inertia to solve a problem, it will be provided. And,
while you do not need to memorize the equations for moments of inertia of
various objects, you do need to be able to determine relative relationships
between various moments of inertia of objects and those are just based on the
basic equation for moment of inertia:
I = ∑ i mi ri 2
0205 Lecture Notes - AP Physics C- Equations to Memorize (Mechanics).docx
page 3 of 3
Download
Random flashcards
State Flags

50 Cards Education

Countries of Europe

44 Cards Education

Art History

20 Cards StudyJedi

Sign language alphabet

26 Cards StudyJedi

Create flashcards