Warm-up Mon. 3/19/07

5 min to perfect your lab in silence…

No Talking.

You don’t talk, you get time to work.

You talk, You get a fat zero on the lab.

Newton’s Third Law

Action-Reaction

Chapter 4

Interactions

• What is an interaction?

– Exchange of actions between two objects– Requires a FORCE from both objects

Force as Interaction

• Let’s look at the binder-clips and rubber bands:

1. Try to open your binder-clip, using only one hand on one side of the clip

2. Press it on the desk… is the desk pushing?

3. Now, working with a partner open the binder clip with only one hand on one side (each)

Rubber Band

1. With a partner, stretch the rubber band.

2. Try to stretch the rubber band using just one hand.

3. Hook it on your desk…

4. …is the desk pulling?

Non-living objects

can push and pull!

Newton’s Third Law

When object A exerts a force on a object B, then object B object exerts an EQUAL and OPPOSITE force on object A.

For every action, there is ALWAYS an equal and opposite reaction

Example Time

• The book has a weight of 10N

• What force is the book exerting on the table?

• What force is the table exerting on the book? Table on

Book: 10N

Book on Table: 10N

Action-Reaction

Action = Book’s 10N Force down on the table

Reaction = Table’s 10N Force up on the book

Magnitude = Same

Direction = Opposite

• The man is pushing on the wall with a force of 25N to the right.

• Action Force: ?

• Reaction Force: ?

Action-Reaction

• We call these Action-Reaction forces…

FORCE PAIRS– Forces always occur in pairs– Equal magnitude and opposite direction!

Let’s look at a balloon…

• If I blow it up, then let it go…

– What is the interaction?• Force exchange between balloon and escaping air

– What is the action?• Balloon pushing DOWN on air with some Force

– What is the reaction?• Air pushing UP on balloon with an EQUAL and

OPPOSITE Force!!!

ACTION: Balloon pushes on air with a Force

REACTION: Air pushes back on balloon with an EQUAL and OPPOSITE Force

Think About It…• What is the force pair that allows a bird

to fly?– What is the interaction– What is the Action Force?– What is the Reaction Force?

Interaction: Between Wing and Air

Action: Wing pushes DOWN on air with some Force

Reaction: Air pushes UP on wing with an EQUAL Force

• What is the interaction in this picture?

• What is the Action?

• What is the Reaction?

Foot exerts a force on the ball

Ball exerts an EQUAL and OPPOSITE force on the foot

Let’s look at some car crashes…

• Think of the INTERACTIONS in these accidents!!!!

• Just a sec….

Car vs. Wall

A car hits a wall with a force of 500N…• What is the interaction

– Exchange of forces between car & wall

• What is the Force Pair?– Action

• Car exerts a force on the wall

– Reaction

• Wall exerts an equal and opposite force on the car

Draw this in your notebook!

Somebody show me what you got on the board…

Warm-Down…

• Newton’s Third Law– For every Action there is an EQUAL and

OPPOSITE R_____________

• Forces happen in P_______– An interaction includes an A_____ F_______

and a R_______ F____

eaction

airsction orce

orceeaction

“Dark and Stormy Night” Activity…