Give an example to show that friction depends on the force with which the two surfaces are pressed together.
Suppose, a person is pushing a heavy box, then he is applying say force F1.
Now if the same person pushes two boxes of same mass as the previous case. Then, in this case he will have to apply more force to overcome the larger friction as in the earlier case. Thus is because friction depends on the mass of the object. Heavier the mass, more the force is exerted by the object on the surface and thus more friction is produced.