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Solutions05 - Physics 140 Discussion 05 Solutions 4.20...

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Physics 140 - Discussion 05 - Solutions 4.20. I DENTIFY : Weight and mass are related by . w mg = The mass is constant but g and w depend on location. S ET U P : On earth, 2 9 80 m/s . g = . E XECUTE : (a) , w m g = which is constant, so E A E A . w w g g = E 17 5 N, w = . 2 E 9 80 m/s , g = . and A 3 24 N. w = . 2 2 A A E E 3 24 N (9 80 m/s ) 1 81 m/s . 17 5 N w g g w . = = . = . . (b) E 2 E 17 5 N 1 79 kg. 9 80 m/s w m g . = = = . . E VALUATE : The weight at a location and the acceleration due to gravity at that location are directly proportional. __________________________________________________________________________________ 4.29. I DENTIFY : Since the observer in the train sees the ball hang motionless, the ball must have the same acceleration as the train car. By Newton’s second law, there must be a net force on the ball in the same direction as its acceleration. S ET U P : The forces on the ball are gravity, which is w , downward, and the tension T r in the string, which is directed along the string. E XECUTE : (a) The acceleration of the train is zero, so the acceleration of the ball is zero.
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