A boy holds a 20N rock while standing still on a trampoline. Suppose he hands the rock to someone else. Explain how he will move, in terms of the forces acting on him

The boy will bounce/move upwards.

Explanation:

The boy holding the 20N rock standing still on a trampoline has two important forces acting in this case that keeps the whole system at equilibrium.

The total weight, (the weight of the boy, Wb + weight of rock, Wr = W) exerted on the trampoline and the normal reaction, N the trampoline exerts back on the boy. (Newton's third law)

At equilibrium when he's standing still on the trampoline with the rock in hand, The forces balance each other, that is, the normal reaction, N is equal to the total weight, W.

N = W = Wb + Wr

If the boy suddenly gives the rock away to someone else not on the trampoline, at that instant, the total weight suddenly becomes only the weight of the boy, that is, W = Wb

And the normal reaction, N suddenly becomes larger than the total weight exerted upon the trampoline.

That is, N > W

This imbalance causes motion, as the imbalance is turned into elastic force by the trampoline to make the boy bounce in the upwards direction.

Hope this helps!!!