Question

Two surfaces are rubbed together , first with a small force and then with a greater force. In which case will the friction between the surface be higher ? Why ?

Answer 1

The friction between the surfaces will be higher in the second case. Because when two bodies are rubbed together with a greater force, the friction also increases remaining equal and opposite to the applied force. With smaller applied force, there is less friction. As we already know that, friction is directly proportional to the force applied.

Therefore, the rubbing will be effective in the second case(with greater force applied).

Answer 2

It depends on two things, the force being applied tangentially to the surface and the force acting normal to the surface. If a great force is being applied tangentially to the surface, then yes, the force which is greater will give rise to higher friction.

Friction is also proportional to the weight of the object being rubbed (force normal to the surface in question). This proportionality can be eliminated by the equation Ffr = μ*W ;
Where Ffr = Frictional force
            μ = Co-efficient of friction
            W = Weight of the object(if surface is horizontal)/Force normal to the surface