Chapter 10 – Forces
Chapter 10 – Forces

Section 3 – Newton’s 1st and 2nd Laws
Standards
 2.e – Students know that when the forces on
an object are unbalanced, the object will
change its velocity (that is, it will speed up,
slow down or change direction)
 2.f – Students know the greater the mass of
an object, the more force is needed to
achieve the same rate of change in motion
Newton’s 1st Law of Motion
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
Inertia (in ur shuh) – the tendency of an
object to resist change in its motion
Newton’s First Law of Motion – an object
at rest will remain at rest, and an object
that is moving at constant velocity will
continue moving at constant velocity,
unless acted upon by an unbalanced force
(called the law of inertia)
Newton’s 2nd Law
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

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Newton’s 2nd Law – the net force on an
object is equal to the product of its
acceleration and its mass
Force = Mass x Acceleration
Acceleration = Force / Mass
Mass = Force / Acceleration
F
m a
Units

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Mass – kilograms (kg)
Acceleration – meters per second per
second (m/s2)
Newton – the force required to accelerate
one kilogram of mass at 1 meter per
second per second (kg.m/s2 which is a
Newton)
Force Problem Example

A 52 kg water skier is being pulled by a
speedboat. The force causes her to
accelerate at 2 m/s2. Calculate the net
force that causes this acceleration.

Force = Mass x Acceleration

Force = 52 kg x 2 m/s2

Force = 104 kg.m/s2

Force = 104 N
Force Problem Example

What is the net force on a 1,000-kg object
accelerating at 3 m/s2?

Force = Mass x Acceleration

Force = 1000 kg x 3 m/s2

Force = 3000 kg.m/s2

Force = 3000 N
Force Problem Example

What net force is needed to accelerate a
25-kg cart at 14 m/s2?

Force = Mass x Acceleration

Force = 25 kg x 14 m/s2

Force = 350 kg.m/s2

Force = 350 N
Chapter 10 – Forces

Section 4 – Newton’s 3rd Law
Standard
 2.e – Students know that when the forces on
an object are unbalanced, the object will
change its velocity (that is, it will speed up,
slow down or change direction)
Newton’s 3rd Law of Motion


Newton’s 3rd Law of Motion – states that
if one object exerts a force on another
object, then the second object exerts a
force of equal strength in the opposite
direction on the first object
Newton called one force an
action, and the other he
called a reaction
Action-Reaction Pairs
Momentum

Momentum of an object is the product of
its mass and velocity (p = m * v)
Momentum is the ‘quantity of motion’

The unit for momentum is kg * m/s

Conservation of Momentum
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When two objects collide in the absence
of friction, momentum is not lost
Law of Conservation of Momentum states
that the total momentum of the objects
that interact does not change
The total momentum of any group of
objects remains the same unless outside
forces act on the objects (friction is an
example of an outside force)
Momentum Problem Example

Which has more momentum: a 3.0-kg
sledgehammer swung at 1.5 m/s, or a 4.0kg sledgehammer swung at 0.9 m/s?

Momentum = Mass x Velocity

Momentum = 3.0 kg x 1.5 m/s

Momentum = 4.5 kg.m/s

Momentum = Mass x Velocity

Momentum = 4.0 kg x 0.9 m/s

Momentum = 3.6 kg.m/s

Momentum was greatest for 3.0 kg hammer
Momentum Problem Example

A golf ball travels at 16 m/s, while a baseball
moves at 7 m/s. The mass of the golf ball is
0.045 kg and the mass of the baseball is
0.14 kg. Which has greater momentum?

Momentum = Mass x Velocity

Momentum = 0.045 kg x 16 m/s

Momentum = 0.72 kg.m/s

Momentum = Mass x Velocity

Momentum = 0.14 kg x 7 m/s

Momentum = 0.98 kg.m/s

Momentum was greatest for baseball
Momentum Problem Example

What is the momentum of a bird with a
mass of 0.018 kg flying at 15 m/s?

Momentum = Mass x Velocity
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Momentum = 0.018 kg x 15 m/s
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Momentum = 0.27 kg.m/s
Chapter-10 Forces
Conservation of Momentum
In the absence of friction, momentum is conserved when two
train cars collide.
BRAIN BREAK!
With your partner, discuss each of
Newton’s 3 Laws and write a simple
example of each