question_answer

A disc of mass m is placed on a table. A stiff spring is attached to it and is vertical. To the other end of the spring is attached a disc of negligible mass.  What minimum force should be applied to the upper disc to press the spring such that the lower disc is lifted off the table when the external force is suddenly removed?

Answer:

                The minimum applied force should be mg. When a force mg is applied vertically downwards on the upper disc, the lower disc gets pressed against the floor with a force equal to mg. The floors exerts an upward reaction equal to mg. When the external force is suddenly removed, this force of reaction lifts up the lower disc.