Question

What is the kinetic energy of an object with a mass of `6` `kg` that has been in freefall for `8` `s`?

Answer 1

There are a number of ways to do this, but I have outlined one below.

The kinetic energy is `E_k=18.4` `kJ`

Explanation:

We know how to find the velocity of an object that starts at rest and has been accelerating at a known rate for a known time. We also know how to calculate the kinetic energy of an object if we know its mass and velocity.

`v=u+at`, but `u=0`, so `v=at`.

The acceleration, of course, is just the acceleration due to gravity. I'll use the value `9.8` `ms^-2`.

`E_k=1/2mv^2` and we can substitute in our expression for the velocity (and the value for the mass).

`E_k=1/2(6)(9.8xx8)^2=18,440` `J=18.4` `kJ`

Not that technically, given we were given the data to only one significant figure, technically the answer is `20` `kJ`.