Question

A 9 kg ball starts at the top of a 84 m high building. How much kinetic energy does it have three-quarters of the way to the ground?

Answer 1

It will have `5556.6 J` of kinetic energy at a point three-quarters of the way down.

Explanation:

The simplest way to approach this problem is to realize that any increase in the kinetic energy has come as a result of an equal reduction in the potential energy.

This potential energy is gravitational, and given by

`PE = mgh`

where `h` is the height of the object, `m` its mass and `g` is the gravitational field, `9.8 N"/"kg`.

At top the top of the building, this gives us

`PE = 9xx9.8 xx84=7408.8 J`

Three-quarters of the way down, its height is `21 m`, and its remaining `PE` is

`PE = 9xx9.8 xx21=1852.2 J`

Therefore, it has lost `5556.6 J` of potential energy and has gained `5556.6 J` of kinetic energy.