The force that resists motion of a body is called friction . It produces heat . For example----in winters when we rub our hands against each other ( motion ) , it produces friction which produces heat . It is due to force of friction that when we apply brakes in cars , the tyres are able to stop .

As it is a kind of force , it's SI unit is that of force i e ---- Newtons .

the resistance that one surface or object encounter when moving over another .

si unit of friction is newton

Friction

PHYSICS

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Friction

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Matter

Wear

Viscosity

Dry damping

Coefficient of friction

Stick-slip friction

Static friction

Kinetic friction

Internal friction

Rolling friction

Friction,forcethat resists the sliding or rolling of onesolidobject over another. Frictional forces, such as the traction needed to walk without slipping, may bebeneficial, but they also present a great measure of opposition tomotion. About 20 percent of the enginepowerofautomobilesis consumed in overcoming frictional forces in the moving parts.



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The major cause of friction betweenmetalsappears to be the forces of attraction, known asadhesion, between the contact regions of the surfaces, which are always microscopically irregular. Friction arises from shearing these “welded” junctions and from the action of the irregularities of the harder surface plowing across the softer surface.

Two simple experimental facts characterize the friction of sliding solids. First, the amount of friction is nearly independent of the area of contact. If a brick is pulled along a table, the frictional force is the same whether the brick is lying flat or standing on end. Second, friction is proportional to the load or weight that presses the surfaces together. If a pile of three bricks is pulled along a table, the friction is three times greater than if one brick is pulled. Thus, the ratio of frictionFto loadLis constant. This constant ratio is called thecoefficient of frictionand is usually symbolized by the Greek letter mu (μ). Mathematically,μ=F/L.Because both friction and load are measured in units of force (such aspoundsornewtons), the coefficient of friction is dimensionless. The value of the coefficient of friction for a case of one or more bricks sliding on a clean wooden table is about 0.5, which implies that a force equal to half the weight of the bricks is required just to overcome friction in keeping the bricks moving along at a constant speed. The frictional force itself is directed oppositely to the motion of the object. Because the friction thus far described arises between surfaces in relative motion, it is calledkinetic friction.

Static friction, in contrast, acts between surfaces at rest with respect to each other. The value of static friction varies between zero and the smallest force needed to start motion. This smallest force required to start motion, or to overcome static friction, is always greater than the force required to continue the motion, or to overcome kinetic friction.

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Rolling friction occurs when a wheel, ball, or cylinder rolls freely over a surface, as in ball and roller bearings. The main source of friction in rolling appears to bedissipationofenergyinvolved indeformationof the objects. If a hard ball is rolling on a level surface, the ball is somewhat flattened and the level surface somewhat indented in the regions in contact. The elastic deformation or compression produced at the leading section of the area in contact is a hindrance to motion that is not fully compensated as the substances spring back to normal shape at the trailing section. The internal losses in the two substances are similar to those that keep a ball from bouncing back to the level from which it is dropped. Coefficients of sliding friction are generally 100 to 1,000 times greater than coefficients of rolling friction for corresponding materials. This advantage was realized historically with the transition fromsledgetowheel. (Seemechanics.)

This article was most recently revised and updated byErik Gregersen, Senior Editor.

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