How long will it take a 7.08 hp motor to lift a 250 kg beam directly upward at constant velocity from the ground to a height of 45.0 m? Assume frictional forces are negligible. (1 hp = 746 W)

Question

Quizplus · Accepted Answer

The power output of the motor is:$$P = 7.08 	ext{ hp} \left( \frac{746 	ext{ W}}{1 	ext{ hp}} ight) = 5280 	ext{ W}$$The work done in lifting the beam is:$$W = Fd = mgh = (250 	ext{ kg})(9.8 	ext{ m/s}^2)(45.0 	ext{ m}) = 1.10 	imes 10^5 	ext{ J}$$The time it takes to do the work is:$$t = \frac{W}{P} = \frac{1.10 	imes 10^5 	ext{ J}}{5280 	ext{ W}} = 20.9 	ext{ s}$$

How Long Will It Take a 7