Problem: A man of mass $m$ jumps vertically into the air from a sitting position in which his center of mass (CoM) is at a height $h_1$. When his feet are just about to leave the ground his CoM is at height $h_2$. When he is at the highest position in the jump, his CoM. is at height $h_3$. What is the average force exerted by the ground on him?
My solution: The average force must be equal to the amount of work done divided by distance i.e. $m \cdot g \cdot (h_3 - h_1) / (h_3 - h_1) = m \cdot g$
But the actual solution involves all three heights, which I don't understand; if he is sitting on a chair, the force exerted by the chair must also be included in any force exerted by the ground
shouldn't it?
Actual answer:
$m \cdot g \cdot (h_3 - h_1) / (h_2 - h_1)$