Objectives

• Define force as part of an interaction. (6.1)• State Newton’s third law of motion. (6.2)• Given an action force, identify the reaction force. (6.3)• Explain why the acceleration caused by an action force

and by a reaction force do not have to be equal. (6.4)• Explain why an action force is not cancelled by the

reaction force. (6.5)• Describe the horse-cart problem. (6.6)• Explain why you can’t touch without being touched.

(6.7)

7.1 Forces and Interactions

• A force is always a part of a mutual action that involves another force.– Forces always occur in pairs.

7.2 Newton’s Third Law

• Describes the relationship between two forces.

• Newton’s third law – whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the other object.– Once force is called the

action force, the other, reaction force.

Questions

1. What happens when a force exerts a force on another object?

2. Does a stick of dynamite contain force?3. A car accelerates along the road. Strictly

speaking, what is the force that moves the car?

7.3 Identifying Action and Reaction

• A simple way of determining action-reaction forces:– First Identify the interacting objects.• Action: Object A exerts force on Object B• Reaction: Object B exerts force on Object A

Question

• We know the Earth pulls on the moon. Does the moon also pull on the Earth? If so, which pull is stronger?

7.4 Action and Reaction on Different Masses

• The Earth and a falling boulder. We know the boulder falls to the Earth. But does the Earth also fall to the boulder?– You must draw on Newton’s second

law to help answer this question.– Given a force exerted on a small

mass produces a greater acceleration than the same force exerted on a larger mass.

• Lift– Helicopter• Rotating blades push down on air (action) air pushed

on blades (reaction).

– Planes• Wings receive a greater force from air underneath the

wing (action), wing provides a force back on the air (reaction).

7.5 Defining a System

• Action and reaction forces do not cancel each other when either of the forces is external to the system being considered.

7.6 The Horse-Cart Problem

• If the horse in the horse-cart system pushes the ground with a greater force than it pulls on the cart, then there is an net force on the horse, and the horse-cart system accelerates.– It’s the force outside the

horse-cart system that pushes the system.

7.7 Action Equals Reaction

• For every interaction between things, there is always a pair of oppositely directed forces that are equal in strength.