This is a simulation of a mass oscillating on the end of a spring, which then has another mass dropped onto it. You can choose to have the mass drop when the oscillating mass is at its equilibrium position or at its maximum displacement from the equilibrium. You can also use the slider to adjust the mass of the object being dropped. Watch how the amplitude and period of the oscillation change when the mass is dropped. Also look for the effect on the total mechanical energy of the mass-spring system. Think about the differences between the situation where the mass is dropped at the equilibrium position vs. when it is dropped at the maximum displacement from the equilibrium.