·     The “Big Idea”:

-      Forces change the motion of objects in predictable ways.

·     Now we will learn:

-      How Newton’s third law relates action/reaction forces.

-      How Newton’s laws work together.



Newton’s third law

·     Every time one object exerts a force on another object the second object exerts a force that is equal in size and opposite in direction back on the first object.

***Observe what happens during our in-class skateboard demonstration.  What do you see?


Both skaters are moving away from each other.  They travel at the same speed in opposite directions.



Action and Reaction Pairs

·     The force that is exerted on an object (action) and the force that the object exerts back (reaction).


-      What is the action force acting on the skaters?

The action force is the first skater pushing off.


-      What is the reaction force acting on the skaters?

The reaction force is the second skater pushing back. 


-      Real-life examples:  Space shuttle liftoff, banging your toe on the coffee table, pushing on the table (doesn’t result in motion; still action/reaction).


Action/Reaction vs. Balanced Forces

·     Balanced forces exert force on one object.

-      Two people pulling on a backpack.


·     Action/Reaction forces act on two different things.

-    Pulling the backpack across the floor.


Newton’s Three Laws Describe and Predict Motion

·     Newton’s laws explain the motion of almost any object.

-      Kangaroo illustration, page 60

-      Explain how a squid moves forward.

The squid squirts water out from behind.  That is the action force.  The water propelling the squid forward is the reaction force. 



·     Winning your canoe race:

1.    To get the canoe moving, apply a force to overcome inertia. 

2.  A less massive canoe is easier to accelerate than a more massive one.

3.  Predict the best position for your paddle in the water.

4.  If you want to move straight ahead, push backwards on the paddle.