Show that when hitting the ground it has a kinetic energy of 30 J.
Answers (1)
Potential energy is mass times g times height. You've been given the weight (mass times g) in Newtons so if you multiply that by the height, you know how much potential energy it had before it fell. 20 times 1.5 = 30 (Joules).
When it falls, potential energy is converted to kinetic energy. Just before it hits the floor, all the 30J of potential energy are now kinetic energy (and are about to converted into other types of energy when it slams into the floor and stops moving).