# If an object with a mass of 2 kg  changes speed from 9m/s to 4 m/s, by how much does its kinetic energy change?

Feb 18, 2016

$- 65 \text{J}$

#### Explanation:

The formula for kinetic energy is given by

$\text{KE} = \frac{1}{2} m {v}^{2}$

Initially, the kinetic energy is

"KE"_"i" = 1/2 (2 "kg")(9 "m/s")^2

$= 81 \text{J}$

In the end, the kinetic energy is

"KE"_"f" = 1/2 (2 "kg")(4 "m/s")^2

$= 16 \text{J}$

The change in kinetic energy is given by

$\Delta \text{KE" = "KE"_"f} -$${\text{KE}}_{i}$

$= 16 \text{J" - 81 "J}$

$= - 65 \text{J}$