Physics (10th Edition)

Published by Wiley
ISBN 10: 1118486897
ISBN 13: 978-1-11848-689-4

Chapter 6 - Work and Energy - Problems - Page 170: 76


a) $W_T=4.12\times10^5J$ b) $W_{mg}=-4.12\times10^5J$

Work Step by Step

The elevator moves at a constant speed, meaning the net vertical forces equal 0, or the tension in the cable and gravitational force equal each other. $$T=mg=1200\times9.8=11760N$$ a) $T$ pulls the elevator up along its displacement, so $\cos\theta=\cos0=1$. Therefore, $$W_T=Ts=11760\times35=4.12\times10^5J$$ b) $mg$ pulls the elevator down opposite from its displacement, so $\cos\theta=\cos180=-1$. Therefore, $$W_{mg}=-mgs=-4.12\times10^5J$$
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