The average work done by a human heart while it beats once is $0.5 \, J$. Calculate the power used by the heart if it beats $72$ times in a minute.

(0.6 W) The work done per beat $= 0.5 \, J$.
Total work done for $72$ beats is given by:
$W = 72 \times 0.5 \, J = 36 \, J$.
The time taken for $72$ beats is $1 \, \text{minute} = 60 \, s$.
Power is defined as the rate of doing work:
$P = \frac{W}{t} = \frac{36 \, J}{60 \, s} = 0.6 \, W$.
Thus,the power used by the heart is $0.6 \, W$.