Notes: Mechanics
The Nature of Force, Motion & Energy
I.
Force – A push or pull.
a) A force is needed to change an object’s
state of motion.
b) Net force- The sum (addition) of all the
forces acting on an object.
• Net Force not equal to zero = Unbalanced Force.
• Unbalanced Force = acceleration!
c) Measured in Newtons (metric) and pounds
(English)
d) Examples of types
of forces.
weight
drag
friction
buoyant force
thrust
lift
centripetal
support
applied
normal
compression
tension
many more…
II. Mechanical Equilibrium – An object in mechanical
equilibrium is stable (no changes in motion).
a) Static Equilibrium – object at rest.
b) Dynamic Equilibrium – object moving at a constant
speed in a straight line.
c) An object in equilibrium, net force = zero.
S (sigma) means “sum of”
III. Vector: An arrow used to illustrate the
magnitude (size) and direction of some quantity.
a) Force, momentum and velocity are vector
quantities.
b) Parallelogram Rule - “Tip to Tail” Method
Vectors
Combining Vectors = Resultant Vector
No Change in Motion = 0 Net Forces = Equilibiurm
That means if an object……
Mythbusters:
•Is at rest, it will stay
at rest.
Killer Tissue Box
Or
•If an object is in motion, it will stay in motion at
CONSTANT VELOCITY (NO Acceleration).
IV. Newton’s First Law of Motion
(Law of Inertia): Every object remains at rest or
remains in motion at a constant velocity (in a
straight line at a constant speed ), unless
external unbalanced forces cause the object to
change its motion.
a) Examples. Golf balls /cups Demo, table cloth
trick, seatbelts, Mythbusters – Tissue Box.
b) 1st Law ***Main Ideas:
1) Inertia- the resistance for an object to
changes in motion. Depends on mass.
2) An object won’t move, stop moving or turn
unless something (Net Force > 0,
Unbalanced Forces) makes it move, stop
moving or turn.
V.
Mass – The measure of the amount of inertia
an object has.
a) The more mass an object has, the greater
its inertia and the more force is needed to
change its state of motion.
b) Mass is not Weight!!!
•
Weight is the force of gravity
an object.
on
c) One kilogram of mass weighs 10 Newtons.
VI. Frame of reference: An object or objects
assumed to be stationary, used to compare
the motion of other objects. Most common –
the ground (Earth). Motion is Relative!
a) An object is moving if its position relative to a fixed
point is changing.
b) Examples: Spinning Earth, Solar System,
Universe, Virtual reality rides .
Motion Formulas
1)
2)
3)
Speed = distance/time
S = d/t
Velocity = distance/time
V = d/t
Acceleration = change in velocity/time
a = (v final – v initial)/ t
VII. Speed - distance traveled in an interval of
time.
a) Example units:
meters/second (m/s)
kilometers/hour (km/h)
miles/hour (mph), etc.
b) Average speed – The total distance
covered divided by the total time.
Speed = distance/time
S = d/t
c) Instantaneous Speed – The speed at any
instant in time.
VIII. Velocity - the speed of an object and the
direction it is traveling. Velocity is a vector
quantity.
a) Example units:
meters/second east (m/s east)
km/hour north
b) Calculated by:
Velocity = distance/time
V = d/t
IX.
Acceleration - A measure of how fast velocity
changes.
a) Three ways for velocity to change:
speed up
slow down
turn (change direction)
b) Accelerations are caused by unbalanced forces.
c) Example unit:
meters/second/second (m/s2)
d) Calculated by:
acceleration = change in velocity/time
a = (v final – v initial)/ t
Examples of Acceleration
X. Falling Objects - The ‘acceleration due to
gravity’ (“g”) on earth is 9.8 m/s2.
a)
All objects fall at this rate when air resistance
(friction - drag) is ignored.
XI. Projectile Motion – Any object that
travels though air or space, acted on
only by gravity (and friction).
a) Examples- thrown ball, orbiting
spacecraft, jumping, shot bullet.
b) Projectiles have both a vertical
component and horizontal component
that are independent of one another.
XII. Newton’s Second Law of Motion - The
acceleration caused by a force on an object
is dependent upon the size of the force and
the mass of the object.
a) acceleration = Force/mass
•
a = F/m
b) Force = mass x acceleration
•
F=ma
c) 2nd Law ***Main Ideas:
• Forces cause accelerations!!!
a F
=
m
Examples:
Empty shopping cart
Regular dart
a = F
m
Full shopping cart
Weighted dart
Baird
XIII. Momentum - Inertia in motion!
Refers mainly to how hard
something is to stop.
a) Ex. Running back
b) Calculated by:
momentum (p) = mass x velocity
XV.Impulse-Momentum – relationship
between force, time and
momentum. The size of a force and
the amount of time that force acts
on an object effects the
momentum of the object.
a) Ex. Athletic movements, airbags
b) Calculated by:
Force x Time = Mass x velocity
Ft = mv
c) Law of Conservation of Momentum –
Momentum cannot be created or destroyed,
it can only be transferred from one object to
another.
XIV. Newton’s Third Law of Motion
a) “Action-Reaction”
b) Whenever one object exerts a
force on a second object, the
second object exerts an equal
and opposite force on the first.

When identifying equal and opposite
forces it is important to remember that
forces work in pairs.
• IMPACT - DEFINE