1. In lifting a heavy book above your head, are you doing work on the book? Once
ID: 2022822 • Letter: 1
Question
1. In lifting a heavy book above your head, are you doing work on the book? Once you have lifted it and hold it steady in the air, do you continue to do work on it?2. A billiards player strikes the cue ball with a cue stick, thereby putting the cue ball in motion. Part A: Assuming the cue ball does not come in contact with any of the other billiards balls, what forces will eventually cause it to come to a stop? Part B: The same billiards player “breaks” by striking the cue ball with the cue stick and sending it into a stationary formation of billiard balls, thereby putting the balls in motion. What does the law of conservation of momentum tell us happens to the total momentum of the cue ball and billiard balls?
Explanation / Answer
1) Yes, you do work on the book when you lift it into the air. Work = force*distance, and you apply a force to raise it a certain distance, so work is done on the book. For the second part, when you hold it steady, you do no work on the book: there is force holding it there, but there it isn't acting over a distance, and thus no work is done. 2) Eventually the frictional forces of the table, the frictional force of the ball, air resistance and the force of gravity will cause the ball to come to a stop. The law of conservation of momentum tells us that momentum will be conserved: Momentum = mass*velocity, so that momentum will be passed onto all of the balls (in different amounts depending on placement and configuration of the balls), but the net momentum of all those balls will equal the momentum of the cue ball. Hope that helps, let me know if you have any questions, I'm happy to answer. Good luck!
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