Newtonʼs 2nd Law of
Motion
• This law says that the
more force applied to an
object, the more it will be
accelerated, and thus the
more it will change its
motion.
Newtonʼs 2nd Law:
RELATIONSHIP BETWEEN FORCE,
MASS, AND ACCELERATION
• Example #1. The harder you
throw a baseball, the more
distance it will travel.(other
variables not considered here)
Force and Acceleration
Force and Acceleration
• Whatʼs different about
throwing a ball
horizontally as hard as
you can and tossing it
gently?
• When you throw hard,
you exert a much
greater force on the
ball.
• Acceleration = Motion that
is not constant .
• The hard-thrown ball has
a greater change in
velocity, and the change
occurs over a shorter
period of time.
Getty Images
Getty Images
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Force and Acceleration
Mass and Acceleration
• Recall that acceleration is the change in
velocity divided by the time it takes for the
change to occur.
• If you throw a softball
and a baseball as hard as
you can, why donʼt they
have the same speed?
• So, a hard-thrown ball has a greater
acceleration than a gently thrown ball.
• The difference is due to
their masses.
Getty Images/Lars A. Niki
Mass and Acceleration
• If it takes the same amount of time to throw
both balls, the softball would have less
acceleration.
• The acceleration of an object depends on its
mass as well as the force exerted on it.
Newtonʼs Second Law
• Newtonʼs second law of motion states that
the acceleration of an object is in the same
direction as the net force on the object, and
that the acceleration can be calculated from
the following equation:
• Force, mass, and acceleration are
related.
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Calculating Net Force with the
Second Law
Calculating Net Force with the
Second Law
• To solve for the net force, multiply both
sides of the equation by the mass:
• Newtonʼs second law also can be used to
calculate the net force if mass and
acceleration are known.
• To do this, the equation for Newtonʼs
second law must be solved for the net force,
F.
Letʼs see what you got!
• What is the
acceleration of
“The Bat” if it
has a mass of
1,200kg and is
propelled by a
force of 24,000
N?
• The mass, m, on the left side cancels,
giving the equation:
Just one more!
• What is the net
force Aaron
Rodgers applies
to a football
when the mass
of the ball is
0.4kg and is
accelerating at
20 m/s2
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Newtonʼs 3rd Law of
Motion
Newtonʼs 3rd Law
ACTION/ REACTION
Newtonʼs Third Law
• But, the action force doesnʼt cause the
reaction force. They occur at the same
time.
• When you jump on a trampoline, for
example, you exert a downward force on
the trampoline.
• Simultaneously, the trampoline exerts an
equal force upward, sending you high into
the air.
• This law states
that for every
action, there is
a opposite and
equal reaction.
• These are called
action/reaction
forces.
• Example. #1
• Heading a soccer ball
• Answer:
• Action= ball moves
upward
• Reaction= head and neck
compress.
Action and Reaction Forces
Donʼt Cancel
• According to the third law of motion, action
and reaction forces act on different objects.
• Thus, even though the forces are equal, they
are not balanced because they act on
different objects.
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Newtonʼs 3rd Law of
Motion
• 2) Describe how
Newtonʼs 2nd
Law works on
the release of a
balloon filled
with air?
• What are the
action/reaction
forces?
Section Check
Section Check
Question 1
Inertia is __________.
A. the tendency of an object to resist any
change in its motion
B. the tendency of an object to have a positive
acceleration
C. The tendency of an object to have a net
force of zero.
D. The tendency of an object to change in
speed or direction.
Newtonʼs 3rd
Law of Motion
• Example#3:
On ice skates, you push
against the boards, you
move backwards. What do
the boards do?
Answer:
The boards go backwards
What are the action/reaction
forces?
Section Check
Answer
Inertia is the tendency of an object to resist
any change in its motion. An unbalanced force
must act upon the object in order for its motion
to change.
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Section Check
Section Check
Question 2
Newtonʼs second law of motion states that
_________ of an object is in the same
direction as the net force on the object.
A.
B.
C.
D.
Section Check
Answer
The answer is A. Acceleration can be
calculated by dividing the net force in newtons
by the mass in kilograms.
acceleration
momentum
speed
velocity
Section Check
Section Check
Question 3
According to Newtonʼs third law of motion,
what happens when one object exerts a force
on a second object?
Answer
According to Newtonʼs law, the second object
exerts a force on the first that is equal in
strength and opposite in direction.
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