# If an object with a mass of 7 kg  changes speed from 4ms^-1 to  5 ms^-1, by how much does its kinetic energy change?

The kinetic energy changes from $56 J$ to $87.5 J$, a change of $31.5 J$.
Kinetic energy is given by ${E}_{k} = \frac{1}{2} m {v}^{2}$
If the mass, $m$, is $7 k g$, and the velocity, $v$, is $4 m {s}^{-} 1$, the ${E}_{k}$ is $56 J$. If the velocity changes to $5 m {s}^{-} 1$ the ${E}_{k}$ changes to $87.5 J$. Subtracting the former from the latter gives a change of $31.5 J$.