An object of mass 5 kg is placed on a rough horizontal surface. If the coefficient of kinetic friction between the object and the surface is 0.4, what is the frictional force opposing the motion of the object? [g = 10 m/s²]

The normal reaction N = mg = 5 × 10 = 50 N. The frictional force F = μN = 0.4 × 50 = 20 N. Option A results from dividing mass by coefficient of friction, while option D is the normal force alone without applying the coefficient.