The first thing we should do to answer this
question is to draw a diagram. There are four
forces acting on the block: its weight mg,
the reaction force from the ground which acts
upwards on the block, the push force applied
by the man of magnitude F and the force due
to friction, which acts on the block from
the surface. Since the block does not move,
it must be in equilibrium. Therefore, the
resultant force acting upon it must be zero.
If we just consider the horizontal forces
acting on the block we know that they must
be equal and opposite. Therefore the force
F must be equal in magnitude to the force
of friction acting on the block.
