Question

A 0.275 kg apple falls from a tree to the ground, 3.6 m below. Ignoring air resistance, determine the apple’s kinetic energy, KE, when the apple’s height above the ground is 2.8 m.

Answer 1

The law of conservation of energy says

$$\rm KE_i+PE_i=KE_f+PE_f$$

Hence

$${\rm KE_f=KE_i+PE_i-PE_f=0+mgh_i-mgh_f}\\ =0.275\times 9.8\times (3.6-2.8)=2.156\:\rm J$$