When we climb up on the hill then in that case we need to do work against the force of gravity.
Here we know that our body is attracted towards the center of Earth due to the force of gravity. When we climb up the hill then we have to work against the gravity.
So here work done to climb up the hill of height "h" is given as
now we know that while moving up the work done by us is against the gravitational force so this is the energy consumed or loss of energy.
hence the natural force against which we are losing our power is gravitational force.