Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 8 - Think and Rank: 54


B, C, A.

Work Step by Step

A counterclockwise torque from the hand is needed to counteract the clockwise torque produced by gravity. A large gravitational torque on a rod will require a large torque from the hand to hold it steady. Torque is force multiplied by lever arm. All three masses are the same, so the gravitational force on them is the same. The lever arm is the shortest distance between the applied force (i.e., the line along which the force acts) and the rotational axis. This is discussed on page 139, and examples are shown in Figures 8.19-8.20. Rod B will experience the largest gravitational torque because the lever arm is the longest; it is roughly the horizontal distance from the hand to the vertical line passing through the hanging mass. Rod C has the next longest lever arm and requires slightly less torque from the hand to hold it steady. Rod A has the shortest lever arm for the gravitational force, and requires the least torque from the hand to hold it steady.
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