The power is the work done W divided by the time t: [tex]P= \frac{W}{t} [/tex] In our problem, the work done corresponds to the potential energy converted into kinetic energy of motion of the wagon, so W=700 J, while the time is t=20 s, so the power is: [tex]P= \frac{700 J}{20 s} =35 W[/tex]
