Newton's Third Law

	                                     Name:  ____________________________

A common (but confusing) statement of Newton's Third Law is For every action there is an equal and opposite reaction. In this activity you will measure forces with force sensors and try to understand the real meaning of Newton's Third Law.

Predictions

We will set up two objects, both based on force sensors, sitting on the table in front of you. On the left, a big combination of force sensor plus two mass bars: call this the "truck". On the right, a force sensor alone, much less massive: call it the "car". How big are the forces exerted by the "truck" on the "car", and vice versa?

First situation: hold the truck with your left hand and the car with your right hand. Slide them together gently, and press them together as you hold them together. Don't let them slide around on the table as you press them together. Which object exerts a bigger force on the other?

Second situation: leave the truck sitting by itself on the table. Hold the car with your right hand, and slowly slide it so that its bumper presses against the bumper of the truck. Then push harder on the car, so that it causes the truck to start to slide backwards on the table. Keep pushing so that the car pushes the truck backwards at an accelerating rate. Now which object exerts a bigger force on the other?

Discuss your answers with the others in your group and record your consensus answers.


   Situation               Magnitude of force                Measurements
                             of car on truck is
                               < or = or > 
                             force of truck on car
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 Vehicles push each     |                               |             
   other in stationary  |                               |             
   position, no motion  |                               |
                        |                               |
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 Car pushes truck       |                               |             
   backwards at an      |                               |             
   accelerating rate    |                               |
                        |                               |
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