Chapter 6Newton’s Third Law of

Motion- Action and ReactionConceptual Physics

Hewitt, 1999

6.1 Forces and Interaction Interaction- mutual action of two

objects• Required for a force to occur

You can’t touch without being touched

6.2 Newton’s Third Law “Whenever one object exerts a force on

a second object, the second exerts an equal force in the opposite direction on the first.”• FA on B = -FB on A

Action force- causes the force Reaction force- reacts to the force in an

equal but opposite way to the action force

6.3 Identifying Action and Reaction As objects react to each other, action-

reaction objects doesn’t really matter.• Action: Object A exerts a force on Object B• Reaction: Object B exerts a force on Object A

6.4 Action and Reaction on Different Masses

Mass and acceleration are inversely proportional Force and acceleration are directly proportional

Example: Mallet & Post• Mallet: large mass, small acceleration• Post: small mass, large acceleration• Both have an equal but opposite force on them

Example: Bullet being fired• http://www.youtube.com/watch?v=h3PUXbo3tCo&feature=rel

ated• http://www.youtube.com/watch?v=f9tCpLNBwxQ&NR=1• http://www.youtube.com/watch?v=xSkxQxPThi4&feature=rela

ted

6.5 Do Action and Reaction Forces Cancel?

If they cancelled, nothing would ever move!

We need to consider what is in the “system”• System- the area around the force(s) that we

are concerned with• If we only care about a football accelerating,

we call the football the system• If we care about how the football and the foot

interact, we can include them both in the system

6.6 Horse-Cart Problem What is pulling what?

• FGW- force of the ground on the wagon (friction)• FWG- force of the wagon on the ground

• FHW- force of the horse on the wagon• FWH- force of the wagon on the horse (inertia)

• FHG- force of the horse on the ground (applied force)• FGH- force of the ground on the horse (friction)

Assessments Review Questions 1-18

Think & Explain 1-15