# What is the kinetic energy of an object with a mass of  5 kg that has been in freefall for  4 s?

Apr 8, 2016

About $4000 J$

#### Explanation:

In freefall, I assume acceleration due to gravity, $g$ is $10 m {s}^{-} 2$.

$g = \frac{v - u}{t}$
We need to find the final velocity $v$. We know that the initial velocity $u = 0$ and $t = 4 s$
$g \cdot t = v$
$10 m {s}^{-} 2 \cdot 4 s = 40 m {s}^{-} 1$

Kinetic energy is $\frac{1}{2} m {v}^{2} = \frac{1}{2} \cdot 5 k g \cdot {\left(40 m {s}^{-} 2\right)}^{2} = 4000 J$