15. A 4.0-kg block is fastened to a spring with spring constant 100 N/m and is oscillat ing horizontally on a frictionless tabletop with an amplitude of 0.16 m. a) Draw the free-body diagram for the block when its displacement from equilibrium (distance from the center) is 0.12 m. Label all of the forces. b) Use Newton's second law to find the acceleration of the block at this point. c) What is the total mechanical energy of the oscillating block (potential plus kinetic)? Explain your answer.