Question

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

Answer 1

`4705.96J`

Explanation:

Here we can solve the problem in three ways.

1. Calculate the velocity after `7s`,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 `7s` ,so it will be `s=1/2×9.8×7^2=240.1 m`(using `s=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 `7s` is stored in it as kinetic energy i.e `F×s=mg×s=2×9.8×240.1=4705.96J`