1. Newton’s Third Law of Motion
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Newton’s Third Law of Motion
Key Concepts
What is Newton’s Third Law of Motion?
How can you determine the momentum of an object?
What is the Law of Conservation of Momentum?
Key Terms:
Momentum

2. Newton’s Third Law of Motion
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 “For every action there is an equal but opposite reaction”

3. Action – Reaction Pairs
Examples from book
Gymnast pushes down on vaulting horse, the horse pushes him up and over
Kayak paddle pushes the water back, the water pushes the kayak forward
A dog pushes down on the ground, the ground pushes up on the dog causing it to leap into the air
Other Examples…….

4. oppositely directed force on the first one. If the example above
Detecting Motion
You can’t always detect motion when paired forces are in action.
If one of the pairs is much more massive, you will only see the less massive object move
Example – When you push down on the Earth, you won’t see the Earth move, only you jumping in the air

5. Do Action – Reaction Forces Cancel?
If 2 equal act in opposite directions they are balanced and cancel each other out. There is no movement
Action – Reaction forces do not cancel out because they are acting on 2 different objects

6. Momentum
A characteristic of a moving object that is related to the mass and velocity of the object
Can be determined by the following formula”
Momentum = mass X Velocity
Unit is kg *m/s
Like velocity and acceleration, momentum has a direction. Its direction is the same as its velocity

7. Sample Momentum Problem
Which has more momentum: a 3.0 kg sledgehammer swung at 1.5m/s, or a 4.0 kg sledgehammer swung at .9m/s?
Mass 1 = 3.0kg
Velocity 1 = 1.5 m/s
Momentum = mass X velocity = 3.0kg * 1.5m/2 = 4.5kg*m/s
Mass 2 = 4.0kg
Velocity 2 = .9m/s
Momentum = mass X velocity = 4.0kg * .9m/s = 3.6kg*m/s

8. Math Practice 1
A golf ball travels at 16m/s, while a baseball moves at 7 m/s. The mass of the golf ball is .045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum?

9. Math Practice 2
What is the momentum of a bird with a mass of kg flying at 15 m/s?

10. Momentum The more momentum an object has the harder it is to stop
The more mass an object has, the more momentum. You can stop a baseball traveling 20m/s, but not a truck
The greater the velocity an object has, the harder it is to stop. An arrow shot from a bow has a large momentum even though it has a small mass

11. Conservation of Momentum
Conservation – same amount before and after an event
Total amount of momentum objects have when they collide is conserved
Law of Conservation of Momentum – states that in the absence of outside forces, the total momentum of objects that interact does not change
Friction is an example of an outside force

12. Collision with Moving Objects

13. Sources http://www.sciencetoymaker.org/balloon/images/newton.gif