1. Newton’s Third Law of Motion and Momentum
Section 3
Newton’s Third Law of
Motion and Momentum

2. Key Concepts What is Newton’s third law of motion?
What is needed for an object to have a large momentum?
How is momentum conserved?
When this bumper car
collides with another car, two forces
are exerted. Each car in the collision
exerts a force on the other.

3. Newton’s Third Law
A force cannot exist alone. Forces always exist in pairs.
According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
Action and Reaction Forces: These two forces are equal in size and opposite in direction.

4. Newton’s Third Law Action-Reaction Forces and Motion
Video 1
Video 2
Action-Reaction Forces and Motion
The action force causes the water to move in the direction of the action force.
However, the water also exerts its equal and opposite reaction force on the swimmer.
The reaction force acts on the swimmer and pushes her forward through the water.

5. Newton’s Third Law Action-Reaction Forces Do Not Cancel
The reason is that the action and reaction forces do not act on the same object.
The action force acts on the water, and the reaction force acts on the swimmer

6. Newton’s 3rd Law of Motion
For example, consider the interaction between a baseball bat and a baseball.
The baseball forces the bat to the right (an action);
the bat forces the ball to the left (the reaction).

7. Momentum
the product of an object’s mass and its velocity. An object with large momentum is hard to stop.
An object has a large momentum if the product of its mass and velocity is large.
The momentum for any object at rest is zero.

8. Momentum Momentum Formula Momentum = Mass x Velocity
units of kilogram-meters per second or kg•m/s

9. Momentum
Which has more momentum, a kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second?

10. Conservation of Momentum
collisions obey the law of conservation of momentum.
law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change.
In a closed system, the loss of momentum of one object equals the gain in momentum of another object— momentum is conserved.

11. Momentum

12. Momentum

13. Reviewing Concepts
1. Using Newton’s third law, explain what is meant by action and reaction pairs of forces.
2. State in your own words the formula for momentum.
3. What is a necessary condition for the conservation of momentum?
4. If an eagle and a bumblebee are traveling at 8 km/hr, which has more momentum? Explain.