# What is the kinetic energy of an object with a mass of 5 kg that has been in freefall for 7 s?

Apr 24, 2018

see below

#### Explanation:

$K E = \frac{1}{2} m {v}^{2}$

where the letters take their usual meaning

we need to find the velocity after $7 s$
$t = 7 s$
$u = 0 m {s}^{- 1}$

v=?

$g = 9.8 m {s}^{- 2}$

$v = u + a t$

$v = 0 + 9.8 \times 7 = 68.6 m {s}^{- 1}$

$\therefore K E = \frac{1}{2} \times 5 \times {68.6}^{2}$

$K E = 11764.9 = 11800 J$(3sf)