How Does Air Force Affect Kinetic Energy in a Fancart?

[magma_saber]

Homework Statement

A fancart of mass 0.8 kg initially has a velocity of < 0.7, 0, 0 > m/s. Then the fan is turned on, and the air exerts a constant force of < -0.4, 0, 0 > N on the cart for 1.5 seconds.

What is the change in momentum of the fancart over this 1.5 second interval?

What is the change in kinetic energy of the fancart over this 1.5 second interval?

Homework Equations

K = 1/2mv²
K = p²/2m

The Attempt at a Solution

I got the change in momentum. It's <-.6,0,0>
I can't find the change in kinetic energy.
I tried K = 1/2mv² and got 0.196
I also tried p²/2m and got 0.225

 

[Delphi51]

You must find the KE before and after. Take the difference to get the change.