A box of mass is resting on a sloping platform which is inclined at to the horizontal. The coefficient of friction between the box and the platform is . The box is released from rest and slides down the platform. Calculate:
a) the speed of the box after it has been moving for seconds.
b) the potential energy lost by the box during this time.
I've done part (a) but I cannot get my head around energy. Can I have help on part (b) please. Thanks.