Coefficient of friction

We can say that friction forces are inevitable in our daily lives. If we couldn’t beat them, they would make all objects that were moving, in all directions, stop.

But on the other hand, if there were no friction, cars couldn’t move and we couldn’t even hold a pencil to write. We can say that without friction, a nail would be useless.

Thus, we can say that friction is contact forces between two surfaces. To verify the existence of these forces, we can do the following test: place a book on the table and push it. We can see that the book moves for a short time, that is, it travels through a small space. It does not continue to move because there is friction between the table surface and the book.

In the figure below we have a book at rest on a table. Acting on it is only the gravitational force which is balanced with the normal force. If we exert a force F on it, trying to move it to the left, in response there is a friction force f to the right, which balances the force we apply. We call this force the static friction force (f s ). The book remains motionless.

But as we increase the intensity of the force applied on the book, so that it starts to move, suffering an acceleration to the left, the friction force that opposes the movement in this new situation is called the kinetic friction force. (fc ) .

Thus, we can say that the magnitude of the force of kinetic friction, which acts on bodies that are moving, is less than the maximum magnitude of the force of static friction, which acts on objects at rest.

We can then determine both the magnitude of the static friction force and the magnitude of the kinetic friction force. Friction forces depend on the normal force and also on the coefficient of friction. In this way, we have:


µ s is the coefficient of static friction
µ c is the coefficient of kinetic friction

We cannot forget that the coefficients µ s and µ c are dimensionless, that is, they do not have a unit of measure.

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