Gravitational Potential Energy
The work done by an object by a force is
W = F x d
F = m x a
W = m x a x d
If the object is in free fall, then a = g and the distance is its height, d = h.
Thus the work done by gravity on the object (GPE) is
W = = GPE = m g h
When a skydiver jumps out of a plane we consider the forces in 3 stages:
Immediately after jumping: There is zero drag, the only force acting is the weight and the acceleration is a = g = 9.81m/s^2
Before Terminal velocity is reached: The drag force increases and so the net force decreases this means that a < g
At Terminal velocity: drag = weight and the acceleration is zero. Falling at constant speed.