Question

In a scene from the movie, Superman was shown running after a train. The train moves at 54 m/s. How much work must he do in order to stop the train? Mass of train = 425 000 kg.

Answer 1

$$\Delta KE=W,$$ $$KE_f-KE_i=W,$$ $$0-\dfrac{1}{2}mv^2=W,$$ $$W=-\dfrac{1}{2}\cdot425000\ kg\cdot(54\ \dfrac{m}{s})^2=-6.19\cdot10^8\ J.$$

The Superman must do $6.19\cdot10^8\ J$ of work in order to stop the train. The sign minus means that Superman pushes the train in the direction opposite to the motion of the train.