PHYSICAL SCIENCE

1. PHYSICAL SCIENCE Chapter 3 Forces 3.3 The Third Law of Motion

2. 3.3 The Third Law of Motion Objectives • State Newton’s third law of motion. • Identify action and reaction forces. • Calculate momentum. • Recognize when momentum is conserved.

3. Newton’s Third Law • Newton’s third law of motion states that when one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction. • It may be said that “for every action force, there is an equal and opposite reaction force. • Action and reaction forces occur at the same time. • Even though action and reaction forces are equal, they are not balanced because they act on different objects. • Rocket propulsion is based on Newton’s third law.

4. Newton’s Third Law Video Links • Newton's 3rd Law of Motion (1:30) - https://youtu.be/MUgFT1hRTE4 • Newton's Third Law of Motion - Science of NFL Football (4:12) - https://youtu.be/e1lzB36aHD4 • Newton's Laws Of Motion (3): Action And Reaction (5:58) - https://youtu.be/cP0Bb3WXJ_k

5. Momentum • The momentum of an object is the product of its mass and velocity. Momentum (kg•m/s) = mass (kg) • velocity (m/s) p = mv • The unit for momentum is kg•m/s. • Momentum also has a direction because velocity has a direction.

6. Momentum • Remember, acceleration is the difference between initial and final velocity. • Also, the net force on an object equals its mass times its acceleration. • Therefore, Newton’s second law may be written as… F = (mvf – mvi)/t • In the above equation, mvf is final momentum and mvi is initial momentum. • Net force exerted on an object can be calculated by dividing its change in momentum by the time over which the change occurs.

7. Momentum • The momentum of an object doesn’t change unless its mass, velocity, or both change. • Momentum can be transferred from one object to another.

8. Momentum Video Links • Richard Garriott Space Video Blog: Conservation of Momentum (1:58) - https://youtu.be/4IYDb6K5UF8