Friction (science)

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Friction is the force that resists the relative lateral (tangential) motion of solid surfaces, fluid layers, or other materials in contact.

There are various categories of friction, for example:

[edit] Important facts about friction

Some important, and somewhat counter-intuitive, facts about sliding friction:

[edit] Static friction

The static friction acting between two surfaces in contact satisfies two important rules:

Thus, static friction opposes the tendency of two surfaces at relative rest to accelerate against each other, but cannot exceed a certain amount that depends on the nature of the surfaces, and how hard they are pressed against each other.

Static friction cannot create relative motion

The static friction between two bodies cannot cause them to slip against each other. However, static friction can cause bodies to move. For instance, if you put a tray on a table and push the table, the tray moves along with the table. This is because the static friction between the tray and the table opposes the tendency of the surfaces in contact to slip against each other, hence the tray moves along with the table.

Similarly, it is static friction that allows us to carry a stack of books simply by moving the lowest book in the stack.

Static friction is limited by normal force

Two bodies being in contact is not sufficient for static friction to act between them. It is necessary that the bodies are pressed against each other. For instance, if two blocks are lying side by side on a table, with their surfaces meeting vertically, then there is no static friction between them, so we can move one block without moving the other. On the other hand, if one block is placed on top of the other, there is a normal force between them to balance the gravitational force, and this makes static friction possible.

This is also why the stronger we grip things, the easier it is to prevent them slipping. By increasing the grip on an object, we increase the normal force between our fingers and the objects, which raises the limiting value of the force of static friction.

Static friction depends on the coefficient of static friction

The limiting coefficient of static friction, denoted μs, is a dimensionless constant dependent on both the surfaces in contact. μs usually takes values of around 0.2, but it is in principle possible for μs to be greater than 1.

Also, we typically find that the coefficient of static friction depends on how rough or smooth the surfaces are. Smooth surfaces tend to be more slippery, and hence have lower coefficients of static friction, than rough surfaces.

This also explains the intuitive idea that it is easier to grip a rough surface, than to hold to a slippery surface.

[edit] Kinetic friction

Kinetic friction acts between two surfaces in contact that are sliding against one another. Two facts about kinetic friction:

Kinetic friction opposes direction of velocity, not force

Static friction opposes the direction in which the net external applied tangential force is acting. The direction of kinetic friction is independent of the applied tangential force, and depends only on the tangential velocity.

For instance, if a block is sliding uphill on an incline, then the net external applied force (which comes from gravitational force) as well as the kinetic friction force are in the same direction.

Magnitude of kinetic friction is independent of velocity

This is somewhat counter-intuitive, and in fact does not hold for very large velocities. However, it is a reasonable approximation for low velocities. It is also highly unlike viscous force and air resistance, which have a strong dependence on velocity.

Comparison of static and kinetic friction coefficients

The coefficients of static and kinetic friction for a pair of surfaces are often very close, with the kinetic friction usually somewhat less than the static friction.

[edit] Mechanism behind friction

[edit] Physical situations related to friction

Motion on an incline

Consider a block on an inclined plane, where the inclined plane makes an angle θ with the horizontal. Suppose μs and μk are the coefficients of static and kinetic friction between the surfaces of the block and the inclined plane. An application of the laws of motion and the above outlined laws on static friction yield:

The angle tan − 1s) is sometimes termed the angle of repose for the pair of surfaces.



When a ball rolls on the floor, there is no relative motion of the surfaces of contact. Thus, the nature of friction operating is static friction, and not kinetic friction. In fact, static friction is what makes rolling possible. If there is no friction between the ball and the floor, the ball will just slide along the floor.

[edit] References

  1. Andy Ruina and Rudra Pratap (2002). Introduction to Statics and Dynamics. Oxford University Press, page 713.  Not yet released. A preprint in PDF format is available online at Introduction to Statics and Dynamics
  2. Russell C. Hibbeler (2006). Engineering Mechanics, 11th Edition. Prentice Hall. ISBN 0-13-221509-8. 
  3. Ferdinand Beer and E. Russel Johnston, Jr. (1996). Vector Mechanics for Engineers, 6th Edition. McGraw-Hill. ISBN 0-07-005367-7. 
  4. J.L. Meriam and L.G. Kraige (2002). Engineering Mechanics: Dynamics, 5th Edition. John Wiley & Sons. ISBN 0-471-40645-7. 
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