posted by ayla .
A 29 kg child, with a 4.5 kg backpack on his back, first stands on a sidewalk and then jumps up into the air. Find the magnitude and direction of the force on the sidewalk from the child when the child is (a) standing still and (b) in the air. Now find the magnitude and direction of the net force on Earth due to the child when the child is (c) standing still and (d) in the air.
total mass = 33.5kg
total force = 9.8 * 33.5
when standing, that's it.
in the air, no force
on earth, force is the same when standing
in the air, F = GmM/r², so it depends on how high he can jump.