A worker is lifting a heavy block of the mass M with the help of the lever. The far end of the lever is put on the solid floor, and the other end is slowly lifted by the worker. The length of the lever is L. The heavy block is lying on the lever at the distance x from the far end. What force does the worker apply? Prove that the total work by all forces in this process is zero. Prove that the work by the worker is converted into the potential energy of the heavy block.