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LAWS OF
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
Chapter 6.1 Learning Goals
Describe how forces cause changes
in motion.
Demonstrate and describe Newton’s
first law.
Explain the meaning of net force.
6.1 Force changes motion
 A force is a push or pull, or any
action that is able to change motion.
6.1 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.
6.1 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
According to these vectors, in
velocity UNLESS
there is a net force.” what direction is the net force?
6.1 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.
6.1 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?
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
Chapter 6.2 Learning Goals
Define Newton’s second law by
relating force, mass, and acceleration.
Apply Newton’s second law
quantitatively.
Describe the relationship between net
force and acceleration.
Investigation 6A
Newton’s First and Second Laws
Key Question:
 What is the relationship between force
and motion?
6.2 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.
6.2 The newton
 The S.I. unit of
force (newton) is
defined by the
second law.
 A newton is the
amount of force
needed to
accelerate a 1 kg
object by 1m/s.
6.2 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.
6.2 Newton’s second law
 Unbalanced forces cause changes in
speed, direction, or both.
6.2 Acceleration and mass
The greater the mass, the smaller the
acceleration for a given force.
This means acceleration is inversely
proportional to mass.
6.2 Acceleration, force and mass
The acceleration caused by a force is
proportional to force and inversely
proportional to mass.

6.2 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
Solving Problems
 Your bicycle has a mass of 9.1 kilograms.
You accelerate at a rate of 1.79 m/s2.
Calculate the net force that is accelerating
the bicycle.
 Calculate the acceleration of a car if the
force on the car is 450 Newtons and the
mass is 1300 kilograms.
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
Chapter 6.3 Learning Goals
Describe action-reaction force pairs.
Explain what happens when objects
collide in terms of Newton’s third law.
Apply the law of conservation of
momentum when describing the
motion of colliding objects.
Investigation 6B
Newton’s Third Law
Key Question:
What happens when equal and opposite forces
are exerted on a pair of Energy Cars?
6.3 Newton’s Third Law
 Newton’s Third
Law (actionreaction) applies
when a force is
placed on any
object, such as a
basketball.
6.3 The Third Law: Action/Reaction
 Newton’s Third Law
states that every action
force creates a reaction
force that is equal in
strength and opposite
in direction.
 There can never be a
single force, alone,
without its actionreaction partner.
6.3 The Third Law: Action/Reaction
 It doesn’t matter
which force you call
the action and which
the reaction.
 The forces do not
cancel because we
One force acts on the
ball, and the other force
can only cancel
acts on the hand.
forces acting on the
same object.
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 _______________.
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.
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).
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?
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. Looking for:
 … the velocity of the astronaut (backward)
2. Given
 …velocity1 = 10 m/s; mass1= 2 kg;
 ...mass2 = 100 kg;
3. Relationships:
 m1v1 = m2v2
4. Solution
 (10 m/s)(2 kg) = (100kg) v2
 20(kg·m/s) = (100kg) v2
 0.2 m/s
Solving Problems
A rocket engine expels 875 kg of exhaust
gasses at a speed of 3,200 m/s. As a result,
the space ship increases its speed by 930 m/s.
What is the mass of the spaceship?
A cannon (m = 550 kg) fires a cannon ball (m =
4.5 kg) at a speed of 420 m/s. What is the
recoil speed of the cannon?