Why is the magnitude of its acceleration less on the descent

[katana]

This is sort of a concept problem.
It says: A block is given a push so that it slides up a ramp. AFter the block reaches its highest opint, it slides back down. Why is the magnitude of its acceleration less on the descent than on the ascent?

I know it as to do with friction but I don't see how the friction forces are different on the ascent and the descent.

 

[Doc Al]

Here's a hint: Friction always acts to oppose slipping between surfaces. So... when the block slides up the ramp, which way is it slipping with respect to the ramp surface? So which way must friction act? Do the same analysis for the block sliding down the ramp. Then compare total force on the block in both cases.

 

[QuantumCrash]

You could also try to analyse it in terms of Conservation of Energy if you want. When you initially give the box a push, the box will use up its kinetic energy to gain GPE and do work against friction. Afterwards, the GPE will be used to do work against friction again and give the box kinetic energy to reach the bottom.

Another way of tackling this is to resolve the forces acting on the box, which you will find:
Going up: mg sin @ + friction (both downwards)
Going down: mg sin @ (downwards) - friction (upwards)