A box of mass $\displaystyle 10kg$ is resting on a sloping platform which is inclined at $\displaystyle 20^\circ$ to the horizontal. The coefficient of friction between the box and the platform is $\displaystyle 0.2$. The box is released from rest and slides down the platform. Calculate:

a) the speed of the box after it has been moving for $\displaystyle 3$ seconds.

b) the potential energy lost by the box during this time.

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I've done part (a) but I cannot get my head around energy. Can I have help on part (b) please. Thanks.