Calculate the magnitude of the force the jumper must exert on the ground to produce an upward acceleration 4.00 times the acceleration due to gravity.

According to Newton's Second Law of Motion, the net force is equal to the product of the system's mass and acceleration. There are two forces acting on a jumper once he jumps: gravitational force and upward force. The solution is as follows:

Net Force = mass * acceleration
Upward Force - Gravitational Force = mass*4*gravity
F - mg = 4mg
F = 4mg + mg
F = 5mg

Thus, the force must be five times greater than the gravitational force.