PHYSICAL SCIENCE

Chapter 3 Forces

3.3 The Third Law of Motion

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.

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.

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

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.

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.

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.

Momentum Video Links

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