Forces acting on the pulley A : (1) A tension exerts electromagnetic force between the string and pulley A. (2) A pair of gravitational force between the Earth and pulley. Forces acting on the boy : (1) A tension exerts electromagnetic force between the string and the boy. (2) A pair of gravitational force between the Earth and the boy. Forces acting on the block C : (1) A tension exerts electromagnetic force between the string and block C.(2) A pair gravitational force between block C and the Earth.