Which of the following is equivalent to one hour?
a) 36000 milliseconds
b) 3600 milliseconds
c) 3600000 milliseconds
d) 360000 milliseconds

Explanation

The kinetic energy of the suitcase was zero when it is was at the platform and it aain became zero when it was put on the head. The change in kinetic energy is zero and hence the total work done on the suitcase is zero. Two forces act on the suitcase, one due to gravity and the other due to the porter. Thus, the work done by the porter is negative of the work done by gravity. As the suitcase is lifted up,the work done by gravity is `W=-mgh`
`=-(20 kg)(9.8m/s^2)(2m)=-392J`. The work done by the porter is 392 J =390 J.