Problem 5: A man whose mass is 70 kg walks up to the third floor of a building which is 12 m above the ground in 20 s. Find his power in watt and horsepower.

Solution

Theory: When he goes upstairs, he is doing work by lifting his own weight to distance 12 m. Power is work done divided by the time taken. So, we calculate the work done and then dividing it by 20 s.

Given                     Mass of the man = 70 kg, therefore, weight of the man = mg = 70 × 9.8 = 686 N

Distance, d = height, h = 12 m

Find                       Power, P (in watts and horsepowers)

Formula             Work done, W = Fd = mgh,                          Power, P = (Work done)/time

Put the values, W = 686 × 12 = 8232 J                                       P = 8232/20 = 411.6 watts

To convert watts into horse powers, we know that

746 watts = 1 hp               OR          1 watt = (1/746) hp

Therefore,                                          1 × 411.6 watts = (1/746) × 411.6 hp

411.6 watt = 0.55 hp (Answer)