Question

Work, energy, and energy conservation

1. How much work is done on a 2 kg object when it changes its velocity from 20 m/s to 40 m/s?

2. A pile of books of mass 50 kg is placed on top of a cabinet 3 m high. What is the gravitational

potential energy of the pile of books?

3. A box is pulled by a force of 200 N making an angle of 30 degrees with the horizontal. How

much work is done on the box if it is displaced by 20 m?

Answer 1

1. $W=\Delta KE=mv^2/2-mv_0^2/2=2\cdot40^2/2-2\cdot20^2/2=1200\ (J)$

2. $PE=mgh=50\cdot9.8\cdot3=1470\ (J)$

3. $W=Fs\cos\alpha=200\cdot20\cdot\cos30°=3464\ (J)$