Demos: 1M-04 Work-Energy

A small wooden block is placed on a holder attached to a pendulum arm. The arm is pulled back and released. The holder strikes the base at the bottom most point, causing the block to slide along the horizontal base until it comes to rest. We show that the potential energy of the block equals the work done against friction.

The potential energy of the block before it is launched is mgh. To determine the force of kinetic friction we perform a separate experiment. A string is attached to the block and passed over a smooth pulley. Small weights are added until a uniform motion of the block is obtained. At this point the hanging weight equals the kinetic friction force, f = Mg. Then



Computing each of these separately, we see that they agree within the limits of experimental error.

Directions: Place the small block on the holder and pull the holder back until the arm is parallel with the 45° “flag.” Release the holder. The holder hits the base causing the block to slide along the surface. Measure the distance d. You might want to do this two or three times to get an average. In determining f, you should place a small weight on the end of the string and give the block a gentle push. If it stops, you have too little weight. If it picks up speed, you have too much. Try it a few times until you are satisfied that the block is moving at constant speed.

Suggestions for Presentation: In discussing conservation of energy, you might point out that the block begins with potential energy, then gains kinetic energy, and finally has neither potential nor kinetic energy. Where did the energy go?

Applications: Situations such as a sled sliding down a hill and coming to rest are good examples.

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