# What is the kinetic energy of an object with a mass of 2 kg that has been in freefall for 7 s?

Mar 11, 2018

$4705.96 J$

#### Explanation:

Here we can solve the problem in three ways.

1. Calculate the velocity after $7 s$,it will be v=9.8×7=68.6 ms^-1

So,kinetic energy will be 1/2mv^2=1/2×2×68.6^2=4705.96 J

2 . Find how much the object has come down in $7 s$ ,so it will be s=1/2×9.8×7^2=240.1 m(using $s = \frac{1}{2} g {t}^{2}$)

So,gain in kinetic energy = loss of potential energy=mgs=2×9.8×240.1=4705.96 J

3 . Here work done by gravitational force in bringing the object by distance $s$ in $7 s$ is stored in it as kinetic energy i.e F×s=mg×s=2×9.8×240.1=4705.96J