Suppose the force of kinetic friction on a sliding block of mass m is 2.5 N [backward]. What is the force of kinetic friction on the block if another block of mass 2m is placed on its upper surface?

Answer 1

Mass = M, Ff = 2.5 N.
Mass = M + 2M = 3M, Ff = 3 * 2.5 = 7.5 N. = Force of friction.