1. The problem statement, all variables and given/known data A small ball of mass 'm' is released at a height 'R' above the earth's surface. The maximum depth of the ball to which it goes is R/2 inside the earth through a narrow groove before coming to rest momentarily. The groove contains an ideal spring of spring constant K and natural length R. The value of K, if R is the radius of the earth and M is the mass of the earth is, Ans: 7GMm/R^3 2. Relevant equations PE= -GMm/R KE= 1/2 m v^2 PE(of spring)= 1/2 kx^2 3. The attempt at a solution Conserving energy of the spring- mass system at point of release (at a distance 2R from the centre of the earth) and when it comes to rest at a distance R/2 from the centre of the earth, PE(of body initially)=PE(of body finally)+PE(of spring) -GMm/2R = -2GMm/R + 1/2*k*(R^2/4) k= 12GMm/R^3 I don't understand what's wrong with my answer.