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CHAPTER SIX: LAWS OF MOTION
6.1 Newton’s First Law
6.2 Newton’s Second Law
6.3 Newton’s Third Law and
Momentum
Ch. 6.1
Force changes motion
 A force is a push or pull, or any action that is
able to change motion.
Galileo’s Inertia
Inertia is the tendency of things to resist changes
in motion
Example: Removing the tablecloth from a table: Too
little force, too little time to overcome "inertia" of
tableware.
Newton’s First Law of motion
Often Called the Law of Inertia
 Newton’s first law says that objects continue
the motion they already have unless they are
acted on by a net force.

If the net force is zero, an object at rest will stay at
rest.

If an object is acted upon by unbalanced forces, its
motion will change.
Newton’s First Law of motion
Motion tends to
continue unchanged.
 Example: The elephant at
rest tends to remain at rest.
MASS AND INERTIA
Inertia is also called mass
Mass – measure of the quantity of matter in an object
 Mass is measured in kilograms
 One kilogram is the amount of mass in a 2.2 pound weight
 1 Kg = 9.8 Newton's on Earth
NET FORCE
Newton’s first law is often
written in terms of the net
force.
“An object at rest will stay at
rest and an object in motion
will continue in motion at
constant velocity UNLESS
there is a net force.”
According to these vectors, in
what direction is the net force?
FORCE CHANGES MOTION
 Forces can be used to increase or decrease
the speed of an object, or to change the
direction an object is moving.
LAW OF INERTIA
Inertia is the property of
an object that resists
changes in motion.
 Objects with more mass
have more inertia and are
more resistant to changes
in their motion.

Which ball has more inertia?
SOLVING PROBLEMS
A car drives along the highway at constant velocity.
Find the car’s weight and the friction force if the engine
produces a force of 2,000 newtons between the tires
and the road and the normal force on the car is
12,000 N.
SOLVING PROBLEMS
Looking for:
1.

weight of car in newtons, force due to friction
Given:
2.
ForceN = 12,000N (up);
 ForceE = 2,000N (forward)

Relationships:
3.

Newton’s 1st Law: net force = zero at constant velocity; so
ForceN = ForceW and ForceE = ForceF
Solution
4.
FE = 200 N
FN = 12,000N
Draw a free body diagram.
 There is no net force upward,
so the weight of the car is an
equal downward force of
FF = -200 N
−12,000 N.
 The forward engine force
balances the friction force so
the friction force is −2,000 N.

FW = -12,000N
SOLVING PROBLEMS
THREE EXAMPLES OF INERTIA
Ch. 6.2
Newton’s Second Law of Motion
The force of a moving object is directly proportional to
the object’s mass and acceleration.
ACCELERATION
Equals
Force / Mass
Inversely
Proportional
to Mass
EFFECTS OF A FORCE ON ACCELERATION
Newton’s second law
 Newton’s first law tells us that motion cannot
change without a net force.
 According to Newton’s second law, the
amount of acceleration depends on both the
force and the mass.
Newton’s second law
There are three main ideas related to Newton’s
Second Law:
1.Acceleration is the result of unbalanced forces.
2.A larger force makes a proportionally larger acceleration.
3.Acceleration is inversely proportional to mass.
 Which means that when mass increases, acceleration decreases.
Newton’s second law

Unbalanced forces cause changes in speed, direction, or both.
ACCELERATION AND DIRECTION
 Another important factor of the second law is that the
acceleration is always in the same direction as the net force.
ACCELERATION AND FORCE
The second law says that
acceleration is
proportional to force.
If force is increased or
decreased, acceleration will
be increased or decreased
by the same factor.
ACCELERATION AND MASS
The greater the mass, the smaller the acceleration for a
given force.
This means acceleration is inversely proportional to
mass.
ACCELERATION, FORCE AND MASS
 The acceleration caused by a force is proportional to force and
inversely proportional to mass.
F
a m
The stronger the force on an
object, the greater its
acceleration.
Force is directly
proportional to acceleration.
If twice the force is
applied, the acceleration is
twice as great.
The greater the mass,
the smaller the
acceleration for a given
force.
Mass is inversely related
to force.
An object with twice
the mass will have half
the acceleration if the
same force is applied.
APPLYING THE SECOND LAW

Keep the following important
ideas in mind:
1. The net force is what causes
acceleration.
2. If there is no acceleration,
the net force must be zero.
3. If there is acceleration,
there must also be a net
force.
4. The force unit of newtons is
based on kilograms,
meters, and seconds.
SOLVING PROBLEMS
A car has a mass of 1,000 kilograms. If a net
force of 2,000 N is exerted on the car, what is its
acceleration?
1.Looking for: car’s acceleration
2.Given: mass = 1,000 kg; net force = 2,000 N
3.Relationships: a = F / m
4.Solution: 2,000 N ÷ 1,000 kg = 2 N/kg = 2 m/s2
Newton’s Third Law of Motion
 Third Law – Whenever an object exerts a force on
a second object, the second object exerts an
equal and opposite force on the first.
 For every action there is an equal and opposite
reaction, or All forces occur in pairs.
The Third Law: Action/Reaction

Newton pushes on the elephant and the elephant pushes back!
The Third Law: Action/Reaction


Newton’s Third Law (action-reaction) applies when a
force is placed on any object, such as a basketball.
There can never be a single force, alone, without its
action-reaction partner.
6.3 ACTION AND REACTION

When sorting out action and
reaction forces it is helpful to
examine or draw diagrams.
Here the action force is on the ________________, and
the reaction force is on the _______________.
ACTION-REACTION
 Note: each of the two forces in the pair
acts on a different object.
 Hammer pushes on stake. Stake pushes
on hammer.
 The hammer acts, the stake re-acts.
ACTION-REACTION FORCES
DO NOT CANCEL
 Action reaction forces do not act on the same object.
 Only when action reaction forces act on the same object
will they result in a net force of zero.
ACTION-REACTION PAIR EXAMPLES
RECOILING GUN
 Expanding gas pushes bullet out of gun barrel. Why does
the gun recoil?
ACTION-REACTION
 If action-reaction forces are equal
but opposite, why don't they
cancel?
SOLVING PROBLEMS
 A woman with a weight of 500
newtons is sitting on a chair.
 Describe one action-reaction
pair of forces in this situation.
SOLVING PROBLEMS
Looking for:
1.

…pair of action-reaction forces
Given
2.

Fc = 500 N
…girl’s forceW = -500 N (down)
Fw = -500 N
Relationships:
3.

Action-reaction forces are equal and opposite and act on
different objects.
Solution
4.



Draw a free body diagram
The downward force of 500 N exerted by the woman on
the chair is an action.
Therefore, the chair acting on the woman provides an
upward force of 500 N and is the reaction.
Bell Work.
Write Newton’s three
Laws of motion!
6.3 COLLISIONS


Newton’s third law tells us that any time two
objects hit each other, they exert equal and
opposite forces on each other.
The effect of the force is not always the same.
MOMENTUM
 How hard is it to stop an object.
 the mass matters
 the speed matters
 p = mv
 Yes, that’s “p ” for momentum!
6.3 MOMENTUM
 Momentum is the mass of a object times its velocity.
 The units for momentum are kilogram-meter per second
(kg·m/s).
 All moving objects have momentum.
MOMENTUM
 If the boulder and the boy have the
same momentum, will the boulder
crush the boy?
 Hint: Which would have the larger
speed?

No
6.3 MOMENTUM
The law of
conservation of
momentum states
that as long as the
interacting objects
are not influenced
by outside forces
(like friction) the
total amount of
momentum is
constant or does not
change.
6.3 MOMENTUM
The result of a
We use positive and
negative numbers to show
opposite directions.
skateboarder throwing
a 1-kg ball at a speed
of -20 m/sec is that he
and the skateboard
with a total mass of 40
kg move backward at a
speed of +0.5 m/sec (if
you ignore friction).
6.3 COLLISIONS
When a large truck
hits a small car, the
forces are equal.
The small car
experiences a much
greater change in
velocity much more
rapidly than the big
truck.
Which vehicle ends up
with more damage?
Conservation of Momentum
MOMENTUM IS CONSERVED DURING A
COLLISION
 What does this mean?
 Total momentum for the system stays the same before and after
the collision.
 Momentum lost by one object is gained by another.
HARD TO STOP, SMALL MASS
m
v
HARD TO STOP, SMALL VELOCITY
mv
mV
CONSERVATION OF MOMENTUM
Momentum Before = 0
Momentum After = 0
After firing, the opposite momentum cancel.
CONSERVATION OF MOMENTUM
Momenta are equal but opposite.
M v = m V
 4 kg v = 0.010 kg x 300 m/s
v = 0.75 m / s
CONSERVATION OF MOMENTUM
before
during
p1
p2
F2,1
F1,2
after
p 1
p 2
CONSERVATION OF MOMENTUM
CONSERVATION OF MOMENTUM
CONSERVATION OF MOMENTUM
CONSERVATION OF MOMENTUM
SOLVING PROBLEMS
If an astronaut in
space were to release
a 2-kilogram wrench at
a speed of 10 m/s, the
astronaut would move
backward at what
speed?
The astronaut’s mass
is 100 kilograms.
SOLVING PROBLEMS
1.

2.
Looking for:
… the velocity of the astronaut (backward)
Given
…velocity1 = 10 m/s; mass1= 2 kg;
 ...mass2 = 100 kg;

3.

4.

Relationships:
m1v1 = m2v2
Solution
Draw a free body diagram.