Someone has drilled a hole straight through the center of this planet to the other side and is about to drop a small object of mass m into the hole. We can show that the object will experience simple harmonic motion in the hole by showing that the gravitational force on the object will obey Hooke’s law, Fgrav = -kx, where k is the force constant and x denotes the displacement from equilibrium position, which is the planet’s center. Enter an expression for k, in terms of R, M, m, and the gravitational constant, G.