Simple Nature - Light and Matter

Accelerating a cart example 35If you push on a cart and accelerate it, there are two forces actingon the cart: your hand’s force, and the static frictional force of theground pushing on the wheels in the opposite direction.Applying the work theorem to your force tells us how to calculatethe work you do.Applying the work theorem to the floor’s force tells us that thefloor does no work on the cart. There is no motion at the pointof contact, because the atoms in the floor are not moving. (Theatoms in the surface of the wheel are also momentarily at restwhen they touch the floor.) This makes sense, because the floordoes not have any source of energy.The kinetic energy theorem refers to the total force, and becausethe floor’s backward force cancels part of your force, the totalforce is less than your force. This tells us that only part of yourwork goes into the kinetic energy associated with the forward motionof the cart’s center of mass. The rest goes into rotation of thewheels.Discussion QuestionsA Criticize the following incorrect statement: “A force doesn’t do anywork unless it’s causing the object to move.”B To stop your car, you must first have time to react, and then it takessome time for the car to slow down. Both of these times contribute to thedistance you will travel before you can stop. The figure shows how theaverage stopping distance increases with speed. Because the stoppingdistance increases more and more rapidly as you go faster, the rule ofone car length per 10 m.p.h. of speed is not conservative enough at highspeeds. In terms of work and kinetic energy, what is the reason for themore rapid increase at high speeds?166 Chapter 3 Conservation of Momentum