Chapter 11 - Heat - Learning Path Questions and Exercises - Exercises - Page 412: 5

(a) $60$ (b) $5min$

We know that $1 cal=4.184J$ $\implies 2800 cal=2800\times 4.184=11715.2J$ Now the work done in lifting $20Kg$ mass is given as $W=mgh$ $\implies W=20\times 9.81\times 1.0=196.2J$ (a) We can find the required number of lifts as $No. of\space lifts=\frac{11715.2}{196.2}\approx 60$ (b) The required time can be determined as $60\times 5.0=300s=5min$