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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…