Question

A 1-kilogram rock is dropped from a cliff 90 meters high. After falling 20 meters, the kinetic energy of the rock is approximately

Answer 1

Firstly

using s=ut+1/2at²

20=0×t+1/2×10×t²

or, 20=5×t²

or, t=2secs

using v=u+at

v=0+10×2

therefore velocity at that instant would be 20m/s

so KE=1/2mv²

=1/2×1×20²

=200J