# What is the kinetic energy of an object with a mass of  15 kg that has been in freefall for  7 s?

Jan 26, 2016

Assuming the object was stationary at the beginning of the 7 seconds, its velocity will be $68.6$ $m {s}^{-} 1$ leading to a kinetic energy: ${E}_{k}$ = $\frac{1}{2} m {v}^{2} = \frac{1}{2} \cdot 15 \cdot {68.6}^{2} = 35 , 294.7 J = 35.3 k J$

#### Explanation:

Assuming that the initial velocity at $t = 0$ $s$ is $u = 0$ $m {s}^{-} 1$, the velocity, $v$, after $7$ $s$ will be:

$v = u + a t = 0 + 9.8 \cdot 7 = 68.6$ $m {s}^{-} 1$

For an object of mass $m = 15$ $k g$, the kinetic energy is then given by:

${E}_{k} = \frac{1}{2} m {v}^{2} = \frac{1}{2} \cdot 15 \cdot {68.6}^{2} = 35 , 294.7 J = 35.3 k J$