Definition of Friction

When the surface of a body moves or tends to move over that of another, each body experiences a frictional force. The frictional forces act along the common surface, and each is in such a direction as to oppose the relative motion of the surfaces.

Types of Friction

1) Static Friction: The frictional force that opposes the force which tends to make an object want to move (a push or a pull for example) from its resting position is called static friction. When an external force is applied to bodies that are in contact, even though they might not be in motion, the static frictional force is opposing the motion of the object.

2) Kinetic Friction: The frictional force which exists between two adjacent surfaces which are in motion relative to each other is usually slightly less in quantitative value than static frictional force between the surfaces and is called kinetic or dynamic friction.

Rolling friction

Sliding Friction

Causes of Friction

On a microscopic level, even a highly polished surface has bumps and hollows. So as two objects slides over each other, their contact surface is not smooth, but on the contrary, very rough and jagged. Therefore, the surfaces kind of grind and drag against each other. This is where friction comes from.

But friction is not always a nuisance. In fact, without it, we wouldn’t be able to walk, sit in a chair, climb stairs, or use a mouse to surf the web. Everything would just keep slipping and falling all over the place.

Properties of Friction

Contrary to common belief, the frictional force between two hard surfaces does not have to do with the total surface area that is in contact. For example a book that is being slid across a table on its front side will have roughly the same frictional force acting on it as when the book is slid on its spine, assuming both sides have the same smoothness. The force of friction is dependent upon two things, a value called the coefficient of friction and the weight of the object in question.
1) Coefficients of Friction: There are two coefficients of friction, thecoefficient of kinetic friction and coefficient of static friction. Both of these are experimental values that are constant for any two designated substances (see examples in the chart below). Both coefficients have no units of measurement.

Surfaces Coefficient of Static Friction Coefficient of Kinetic Friction
Wood on Wood 0.4 0.2
Rubber on Dry Concrete 1.0 0.8
Ice on Ice 0.1 0.03

2) Normal Force: The normal force


The efficiency of the system that does work is related with the amount of energy lost/gained to friction. In some system, as the efficiency increases, the amount of energy loses due to friction decreases. However, there are some other cases that reduce friction will decrease the efficiency. In other words. it is also true that the system will be more efficient as the friction increases. For example, it is much easier for people to walk with more friction. In that case, more friction means more efficiency. Therefore, people use different materials or design the system in a specific fashion to make it work more efficiently. Theoretically, rolling can reduce more friction than sliding. Things with wheels run much faster than they slide on the same material. On the other hand, special materials, such as the materials with superior lubricating property, produces less friction. If a system were frictionless, it would be 100% efficient. Nevertheless, there is nothing frictionless in the world, nothing is 100% efficient so that people find different ways to make the system gain as much as energy.