A 7 lb collar is constrained to travel along a frictionless vertical ring of radius R = 1...

A 7 lb collar is constrained to travel along a frictionless vertical ring of radius R = 1 ft. The spring attached to the collar has a spring constant k = 20 lb/ft. Treating the collar as a particle, neglecting air resistance, and knowing that, while at rest at A, the collar is displaced gently to the left, determine the spring’s unstretched length if the collar is to reach point B with a speed of 15 ft/s.

Figure P4.87