**Problem 2:** A 70 kg man runs up a long flight of
stairs in 4.0 seconds. The vertical height of the stair is 4.5 m. Calculate his
power.

**Solution**

**Theory: **When the man goes up stairs, he is doing work
by lifting his own weight to a vertical height. This work is equal to the gain
in the potential energy of his body. Power is the rate of doing work (P = W/t).
Hence, the gain in potential energy will be calculated which is equal to the
work done and then divided by the time he takes will give us the power of the
man.

**Given** Mass
of the man = 70 kg OR weight of the man = 70 × 9.8 = 686 N

Vertical height = 4.5 m, Time taken = 4.0 s

**Required** Power
of the man

Now gain in the P.E = mgh = 686 × 4.5 = 3087 J = the work done the man do go upstairs.

Power = work done/time = P = 3087/4.0 = 771.75 = 772 = 7.7 ×
10^{2} watts

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