Starting from rest, a 5 kg block slides 2.4 m down a rough 30 degree incline in 2 seconds. Find a) the work done by the force of gravity b) the change in mechanical energy due to friction I'm stuck on where to start. For work, i'm using the equation w=fdcosx. I don't know what to use for f. Is the force just 9.8 or is it 9.8m? Or should i find the perpindicular force of the block? Thanks!