Newton's Third Law with bathroom scales

Purpose:
To let students experience third-law forces

Equipment:
2 analog bathroom scales, 2 linear carts

Equipment Location:
Mara Left

Procedure:
Introduce the bathroom scale as a measure of force. Ask for two volunteers, one small female and one large male. Give each a bathroom scale and instruct them on how to hold the scale. Describe the experiment: the two students will place the scales back to back then (on cue) push on the scales, the opposite of a tug of war. The objective, though, is for them to reach equilibrium, so that neither of them are moving, not for one person to move the other. Ask the students in the class to make a prediction. What will the scales read? Perform the experiment. Compare to the predictions. Now, consider a=F/m. They are applying the same force, but they have different masses, why didn't one or both of them "move?" The floor is a bit of a problem, so let's change the experiment. Place each student on a linear cart, thus taking the floor out of consideration. Ask the students in the class to make a prediction. What will happen now when the two students push on each other? Perform the experiment. Compare to the predictions.