If the potential energy of the system is zero, the force on the object is zero. when air resistance is taken into account, a ball thrown directly upward spends more time on the way down than on the way up. you drop a ball to the ground from rest, and, from the same height, throw a second identical ball down to the ground. their change in gravitational potential energies are different. the potential energy of a system can be negative. if the force on an object is zero, the potential energy of the system is zero.