Question

What is the kinetic energy of an object with a mass of `2 kg` that has been in freefall for `4 s`?

Answer 1

The kinetic energy will be 1537 J.

Explanation:

After `4s` of freefall, the speed of a body will be

`v="g"t=9.8(4)=39.2m/s`

Its kinetic energy will be

`KE = 1/2mv^2=1/2(2)(39.2^2) = 1537 J`

You can also find this by conservation of energy methods

During the 4s it will fall a distance of

`y=1/2"g"t^2 = 1/2 (9.8)(4^2) = 78.4m`

The decrease in potential energy of the body will be
`DelatU=mgy=(2)(9.8)(78.4) = 1537 J`

This decrease in potential energy will equal its increase in kinetic energy.