A man of mass m is standing on a plank of equal mass m resting on a smooth horizontal surface. The man starts moving on the plank with speed u relative to the plank. The speed of the man relative to the ground is

Explanation

Let the velocity of the board relative to the surface = v rightward.
Net external force on the board + man system in the horizontal direction = 0
`m(u+v)+Mv=0`
`v=-("mu")/(m+M)`
`rArr v_("man")=u-("mu")/(m+M)=("mu")/(m+M)`
Speed of man relative to the surface `=(Mu)/(m+M)` rightwards.