# What is the kinetic energy of an object with a mass of 3 kg that has been in freefall for 2 s?

May 26, 2018

E_k =600 J

#### Explanation:

from the first movument law
s= s° + v°t + 1/2 a t^2
but s°=0 and v° = 0 so
$s = \frac{1}{2} a {t}^{2} = \frac{1}{2} 10 \frac{m}{s} ^ 2 {\left(2 s\right)}^{2} = 20 m$
the kinetic energy is like the potential energy lost
${E}_{w} = m g h = 3 k g \times 10 \frac{m}{s} ^ 2 \times 20 m = 600 J$
${E}_{k} = 600 J$
otherwise you can have also
$v = a t = 10 \frac{m}{s} ^ 2 \times 2 s = 20 \frac{m}{s}$
${E}_{k} = \frac{1}{2} m \times {v}^{2} = \frac{1}{2} \times 3 k g \times {\left(20 \frac{m}{s}\right)}^{2} = 600 J$