It is used to check the motion of the wagon.
How does a lock check the motion of the wagon?
The lock is pressed against the wheels, so that enough friction is produced to check the motion of the wagon.
Why does it require more force to draw a rusty plough through the ground, than to draw one that is bright and smooth?
Because the rust increases the friction between the plough and the ground, and this makes more force necessary in order to move it.
Why cannot we hold a live eel in our hand, as easily as we can a live fish?
Because the skin of the eel is so smooth, that there is much less friction between it and our hand, than there is between the fish and our hand. Dry sand or dry ashes sprinkled on the eel will increase the friction so that it can be held.
