Newtons 3rd Law of Motion

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Newtons 3rd Law of Motion
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What is Newtons 3rd Law of Motion?

• For every action, there is an equal and opposite
  reaction.
• Example
• Ice skaters pushing off each other sending them
  to opposite sides of the rink

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Where does Newtons 3rd Law occur?

• Everywhere!
• Examples
• Walking you push on the ground and the ground
  pushes back on you
• Hitting a hammer hit the nail and the nail
  pushes back on the hammer

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Can action and reaction forces cancel out? Why?

• No, they cant cancel each other out because they
  are in opposite directions they cant be added
  together because they involve different objects.
• Example 2 balls hitting each other then
  bouncing back

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What is momentum?

• Its a measure of the quantity of motion.
• Its the product of the objects mass and its
  velocity.

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How do we calculate momentum?

• Momentum
• Momentum mass x
• Momentum mass x distance
• time

mass
velocity
x
distance time
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What are the units of momentum?

• Remember momentum mass x distance
• time

Momentum
kg
m
x
s
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What can change momentum?

• A change in
• A change in
• A change in
• A change in

velocity
speed
direction
mass
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• A blue car is moving at 40 km/h and a red car is
  moving at 50 km/h. The blue car has a mass of
  2000 kg and the red car has a mass of 1200 kg.
  Which car has the greater momentum?

Momentumb
2000 kg x 40 km/h
80,000 (kgkm)/h
Momentumr
1200 kg x 50 km/h
60,000 (kgkm)/h
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• A train is traveling down the track at 30 km/h.
  It has a momentum of 300,000 kg-km/h. What is
  the mass of the train?

Momentum mass x velocity
300,000 kg-km/h mass x 30 km/h
300,000 kg-km/h mass 30 km/h
10,000 kg mass
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• A golf ball travels at 16 m/s and a baseball
  travels 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 the greater momentum?

Momentumg
0.045 kg x 16 m/s
0.72 (kgm)/s
Momentumb
0.14 kg x 7 m/s
0.98 (kgm)/s
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• Which has more momentum, a 250 kg dolphin
  swimming at 6 m/s or a 450 kg manatee swimming at
  2 m/s?

Momentumd
250 kg x 6 m/s
1500 (kgm)/s
Momentumm
450 kg x 2 m/s
900 (kgm)/s
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Conservation of Momentum

• The Law of Conservation of Momentum states that
  the total momentum of the objects doesnt change
  when they interact, unless outside forces act on
  the objects.

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Scenario 1

• What happens to the momentum of two moving
  objects when they hit?
• The total momentum stays the same momentum is
  conserved.
• The velocities of the objects may change, but the
  total momentum is the same.

They hit
10 m/s
10 m/s
5 m/s
5 m/s
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Scenario 2

• What happens to the momentum of one moving object
  when it hits and bounces off a stationary object?
• The total momentum stays the same momentum is
  conserved.
• The source of the momentum changes.

stopped
stopped
They hit
10 m/s
10 m/s
0 m/s
0 m/s
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Scenario 3

• What happens to the momentum of one moving object
  when it connects to a stationary object?
• The total momentum stays the same momentum is
  conserved.
• The mass being moved increased when joined
  together, but velocity decreases.

stopped
They connect
10 m/s
5 m/s
5 m/s
0 m/s