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Chapter 3 Notes
Forces and Motion
Newton’s 3 Laws of Motion
1. An object at rest will stay at rest
unless acted on by an unbalanced
force. The law of inertia.
2. F = m x a
3. For every action, there is an equal
and opposite reaction.
Force:
q
Things tend to keep
doing what they are doing
until something comes
along to change it.
q
A force has the ability to
cause a change. It does not
mean a force MUST cause
motion just that it can.
q
2 units used for force.
English system-pounds (lb),
metric system-newtons (N)
4.48 lb = 1N
Mass and Force
q 1N = 1 kgm/s2
q Force (weight) and
mass have different
units. Mass is in
grams (g) or kilograms
(kg),
q Mass is the amount of
“stuff” (matter) in the
object.
qOn Earth 2.2 lb = 1 kg. Mass does not depend on
gravity. The mass of a person is the same
anywhere. Weight is different depending on the
location of the object. It does depend on gravity.
Mass and Inertia
 Inertia-when an object
resists the change in
motion. It wants to keep
doing what it is doing.
Depends on mass. The
more mass = the more
inertia. Think of which is
harder to start moving?
 The only way to change
mass is to physically
change the object. Ex:
cutting off your arm
Mass and Inertia
F = m x a F =
Force (N), m =
mass (kg), a =
acceleration
(m/s2)
 Forces cause
acceleration.
Balanced and Unbalanced Forces
 The motion of an object
depends on the total of
all forces on the objectnet force
 One direction is + and
the opposite is –
 Balanced Force-net
force = zero, no change
in motion.
 Unbalanced force-net
force is NOT zero.
Gravity
 All object have gravity between them.
You feel the pull toward the center of
the Earth because it is the largest. It
depends on how much mass the objects
have. The more mass = the more
gravity.
 Gravity also depends on how close
together the 2 objects are.
This shows
a feather
and an
apple falling
to Earth in a
non-air
chamber.
Both fall at
the same
rate.
Mass and Weight
Weight changes
depending on your
location. Your
acceleration toward
the Earth is 9.8
m/s2. Weight is a
force so we can plug
it into Newton’s 2nd
law.
Fw = m x a Where the
a = 9.8 m/s2.
Newton’s 3rd Law
 Deals with a pair
of objects. All
forces come in
pairs.
 Action/reaction
forces act on 2
different objects,
NOT on the same
object.
Friction
 Friction always works AGAINST motion. (In





the opposite direction.)
Friction comes from 2 surfaces moving
against each other.
Air friction-also known as air resistance, air
moving around an object.
Viscous friction-objects in water or other
fluid create this.
Sliding friction-2 surfaces rub together.
Rolling friction-one object rolls over another.
Friction and motion
 A car rolling down a
hill has gravity pulling
it down and friction
pushing back toward
the top of the hill.
 All true (real)
Gravity
machine have friction
and eventually
everything will stop.
Friction
 If you are on a skateboard and throw a ball





forward, you will go backward.
The heavier the ball or the faster you throw
it, the quicker you will go backward.
P = m x v, P = momentum (kgm/s), m =
mass (kg), v = velocity (m/s)
Law of Conservation of Momentummomentum cannot be created or destroyed
in a group of objects NOT effected by an
outside force.
Momentum usually has + and – to indicate
direction.
This is how a jet moves. Air is pushed back
through the engine and it propels the plane
forward.
Air Out
Air in
Jet moves forward