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

Jun 25, 2018

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

The kinetic energy is ${E}_{k} = 18.4$ $k J$

#### 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.

$v = u + a t$, but $u = 0$, so $v = a t$.

The acceleration, of course, is just the acceleration due to gravity. I'll use the value $9.8$ $m {s}^{-} 2$.

${E}_{k} = \frac{1}{2} m {v}^{2}$ and we can substitute in our expression for the velocity (and the value for the mass).

${E}_{k} = \frac{1}{2} \left(6\right) {\left(9.8 \times 8\right)}^{2} = 18 , 440$ $J = 18.4$ $k J$

Not that technically, given we were given the data to only one significant figure, technically the answer is $20$ $k J$.