Question

A rock tumbles down from a cliff it is moving at 20 m/s when it hits. What the height of the cliff?

The mass of the rock is 8 kg.

Answer 1

The height of the cliff is `=20.41m`

Explanation:

The kinetic energy of the rock when it hits the ground is

`KE=1/2mv^2`

The potential energy of the rock at the top of the cliff is

`PE=mgh`

We have conservation of energy

`KE=PE`

`mgh=1/2mv^2`

The speed is `v=20ms^-1`

The acceleration due to gravity is `g=9.8ms^-1`

The height of the cliff is

`h=v^2/(2g)=20^2/(2*9.8)=20.41m`