Now find the gravitational potential energy U(z) of the object when it is at an arbitrary height z. Take zero potential to be at position z=0. Keep in mind that the potential energy is a scalar, not a vector. Express U(z) in terms of m, z, and g. c) In what direction does the object accelerate when released with initial velocity upward? d) Find F(z), the electric force on the charged particle at height z.