A spring pendulum with a body of mass 
oscillates without friction on a horizontal surface - figure on the left. The oscillation frequency is 2 Hz and the amplitude is 2 cm. Then, an identical body of the same mass is now placed on the first body as shown in the figure on the right. The pendulum oscillates with the same amplitude.
What is the oscillation frequency now?
How much has the maximum kinetic energy of oscillation changed?
What must be the coefficient of friction between the two bodies so that the upper body does not slip?
