Question

1. a 40 kg boy runs up a flight of stairs 4 m high in 4 s. his power output is (a) 160 w (b) 392 w (c) 40 w (d) 1568 w (e) none of the above

Explanation:

Step1: Calculate the force

The force exerted by the boy is equal to his weight. Using the formula $F = mg$, where $m = 40\ kg$ and $g= 9.8\ m/s^{2}$. So $F=40\times9.8 = 392\ N$.

Step2: Calculate the work - done

The work - done $W$ in moving through a height $h = 4\ m$ against gravity is given by $W=Fh$. Substituting the values of $F$ and $h$, we get $W = 392\times4=1568\ J$.

Step3: Calculate the power

Power $P$ is defined as the rate of doing work, $P=\frac{W}{t}$. Given $t = 4\ s$ and $W = 1568\ J$, so $P=\frac{1568}{4}=392\ W$.

Answer:

(b) 392 W