Unit 3 - Dynamics
How Forces Affect Motion
Unit 3 Part 1 –
Newton’s Laws of Motion
Physics Book Chapter 4
Conceptual Physics Book Chapters 4-6
Motion Theories

The old way looked at the what

The new way looked and looks at the why
 4th
century BC - Aristotle
 Did
not experiment
 Used
 Two
only logic and observation
states of motion natural and violent
 Earth
at rest everything else moves in complete circles
Motion Theories
 1600
Galileo
 Used
experiments with ramps
 Discussed
friction came up with the idea of inertia:
property of all matter
 1670
Newton
 Built
off of Galileo’s idea
1st Law of Motion: The Law of Inertia
 Newton
said, “An object at rest wants to stay
at rest unless acted on by a net (unbalanced)
force, and an object in motion wants to stay
in motion in a straight line at a constant
speed unless acted on by an net (unbalanced)
force.
 Forces
do not cause motion they change
motion
 Inertia is a property of all matter
1st Law of Motion: The Law of Inertia
 Inertia
is related to the mass of an
object
Mass
is the measure of inertia or how easy
or hard it is to change an objects position
Mass
can also be how much stuff makes up an
object.
Units
1
of mass kilograms or slugs
slug = 14.59 kg
1st Law of Motion: The Law of Inertia
Which one has more inertia?
1st Law of Motion: The Law of Inertia
http://science360.gov/obj/video/70fadaa8c3d4-4132-ba1f-c98be5caeb14/science-nflfootball-newtons-first-law-motion
Forces and Free Body Diagrams
A
force is defined as any
push or pull
 To
display forces free body
diagrams are used.
 Include
only the forces
acting on the object.
 Force
is a vector! Use an
arrow to indicate a force.
Forces and Free Body Diagrams
 Fit
in two main categories
 Contact
forces between 2 objects
Shoot
a basketball
Push a shopping cart
Friction
 Non-contact
GRAVITY
Electricity
Magnetism
or at a distance
Forces and Free Body Diagrams
 Equilibrium
 All
 If
forces are equal up = down , lefts = rights
any one force is not equal then there is a nonzero net force on the object
Forces and Free Body Diagrams
 Equilibrium
involving gravity
Proportions
Directly
Proportional
Means
two quantities are on opposite
sides of the equal sign, either both in
the numerator or both in the
denominator
In
𝑑
𝑡
the equation s = , s and d are
directly proportional
What happens to s when d increases?
When d decreases?
Proportions
Indirectly
Proportional
Means
two quantities are on opposite
sides of the equal sign, one in the
numerator and the other in the
denominator
In
𝑑
𝑡
the equation 𝑠 = , s and t are
indirectly proportional
What happens to s when t increases?
When t decreases?
Newton’s 2nd Law: Law of Acceleration
 Newton
came up with is in 1680
 Law of acceleration is a proportion
Deals
with the sum of the forces,
acceleration and mass


𝑎=
𝐹
𝑚
=
𝐹𝑛𝑒𝑡
𝑚
𝐹 = 𝑚𝑎 is the equation rearranged.
Newton’s 2nd Law: Law of Acceleration
 “When
a net external force acts on an object of
mass m, there is an acceleration that results.”
 The
acceleration is directly proportional to the
force applied and inversely proportional to the
mass.
Newton’s 2nd Law: Law of Acceleration
Units
of force are Newtons and pounds
A Newton is a kg m/s2
A pound is a slug ft/s2
Newton’s 2nd Law: Law of Acceleration
https://www.youtube.com/watch?v=iwP4he
WDhvw
Example Problem
 Two
people are pushing a stalled car. The mass
of the car is 1850 kg. One person applies a
force of 275 N to the car, while the other person
applies a force of 395 N in the same direction.
A third force of 560 N also acts on the car, but
in the opposite direction. (This force is due to
the friction of the tires on the pavement.) Find
the acceleration of the car.
Newton’s 3rd Law of Action/Reaction
 Whenever
one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object.
 “for
every action there is an equal and opposite
reaction”
 Hammer
hits nail and nail stops hammer
 Swimming:
you push the water backward and the
water pushes you forward
Newton’s 3rd Law of Action/Reaction
 If
this is true, then why doesn’t the cannon
move as far as the cannonball?
 Remember
𝐹
𝑚
that 𝑎 = and that a and m are
indirectly proportional.
 The
cannon has larger m, smaller a.
 The
cannonball has smaller m, larger a.
Weight and Normal Force
 Weight
is the force of gravity on an object.
It
depends on where an object is. (On the
moon, in space, at Earth’s sea level, at high
altitude…)
It
is directly proportional to mass, but not the
same thing!
𝑊𝑒𝑖𝑔ℎ𝑡
= 𝑚𝑎𝑠𝑠 ∗ 𝑔𝑟𝑎𝑣𝑖𝑡𝑦
Weight and Normal Force
 More
About Weight
= 𝑚𝑔 (g=9.8 on Earth)
 This is really still the 𝐹 = 𝑚𝑎 equation!
The weight vector always points DOWN in
a free-body diagram.
The SI unit for weight is the Newton.
𝑊
Weight and Normal Force
 The
normal force is the perpendicular
component of the force that a surface exerts on
an object with which it is in contact.
 Does
NOT necessarily point directly upwards!
 Equal
and opposite of the weight IF no other forces
are acting and the object is on a horizontal surface.
 Think
of “atomic springs” in the surface compressing
to produce the normal force.
Newton’s 3rd Law of Action/Reaction
https://www.youtube.com/watch?v=91QYouih4bQ
Friction

Friction is the parallel component of the force that a
surface exerts on an object with which it is in contact.

Two types:
 If
the object IS NOT moving, then the force is called
static friction.
 If
the object IS moving, then the force is called kinetic
friction.
Friction

The FUN equation to calculate the frictional force:
𝑓 = 𝜇𝑁

µ is the coefficient of friction and depends on the materials
involved and whether it is static or kinetic friction.
 Rubber
 Teflon
 The
on Dry Concrete: 𝜇𝑠 = 1.0, 𝜇𝑘 = 0.8
on Teflon: 𝜇𝑠 = 0.04, 𝜇𝑘 = 0.04
higher the coefficient, the greater the friction.
Friction

Static friction: 𝑓𝑠𝑀𝐴𝑋 = 𝜇𝑠 𝑁 calculates the maximum static
frictional force.
 If
a force exerted is greater than this, the object will start
moving!
 If
the force exerted is less than this, the object remains
stationary and the frictional force is equal to the force exerted.

Kinetic friction: 𝑓𝑘 = 𝜇𝑘 𝑁
True Weight vs. Apparent Weight
 The
apparent weight is the force that an object
exerts on a scale.
 If
the scale and the object are either at rest or
moving at a constant speed (a=0), then
True=Apparent.
 If
the scale and object are accelerating, then
apparent weight will be different!
True Weight vs. Apparent Weight

If the elevator is accelerating
upwards, the apparent weight will be
more than the true weight. (You get
a heavier feeling as the elevator
starts to move up.)

If the elevator is accelerating
downwards, the apparent weight is
less than the true weight. (You feel
lighter as the elevator starts to move
down.)

If the elevator is in free fall (𝑎 = 𝑔)
then apparent weight is 0.
(weightlessness!)