WHAT IS FRICTION ?

FRICTION is an especially interesting example of a force. It is the resistance to motion which takes place when one body is moved upon another. Friction is generally defined as "that force which acts between two bodies at their surface of contact, so as to resist their sliding on each other".

Suppose that a block of metal, weighing 40 pounds, is resting on a flat, horizontal table top. If, using an accurate tension scale, you exert a small horizontal force on the block, the block will not move. Now suppose you increase the horizontal force until the block moves, and you notice that the value of the force is 8 pounds.

You now have enough data to calculate an important friction parameter known as the coefficient of friction (μ), which defines the nature of the resistance to motion these two bodies exert on each other. The value of the coefficient of friction (m) is the horizontal force needed to move the block ( 8 lbs.) divided by the vertical force pressing the block and the table together (40 lbs.) μ = 8 / 40 = 0.20 )

There are several interesting properties of friction between dry, unlubricated surfaces, summarized as follows:

1. At low velocities, the friction is independent of the velocity of rubbing. As the velocity increases, the friction decreases. In other words, the force required to overcome friction and start a body into motion is greater than the force required to sustain the resulting motion. That fact is reflected in the existence of two different coefficients of friction for each material pair: the static coefficient and the dynamic coefficient.
2. For low contact pressures (normal {perpendicular} force per unit area), friction is directly proportional to the normal force between the two surfaces. As the contact pressure increases, the friction does not rise proportionately, and when the pressure becomes very high, friction increases rapidly until seizing takes place.
3. For a constant normal force, the friction, in both its total amount and its coefficient, is independent of the surface area in contact (as long as the pressure is not high enough to enter the seizing region).

Now suppose you apply a thin film of oil on the table under the block. The oil reduces the coefficient of friction to somewhere in the neighborhood of 0.025, so the block can now be moved with a horizontal force of about 1 pound (0.025 * 40 = 1).

The properties of friction between well-lubricated surfaces are considerably different from those above for dry surfaces.

1. The frictional resistance is almost independent of the contact pressure if the surfaces are flooded with oil.
2. For low contact pressures, the friction varies directly with velocity. For high contact pressures, the friction is very high at low velocities, dropping to a minimum at about 2 feet-per-second, then increasing as the square root of velocity.
3. For well-lubricated surfaces, the friction decreases dramatically with increasing temperature, from the influence of (a) rapidly-decreasing oil viscosity and (b) for a journal bearing, increasing diametral clearance.
4. If the bearing surfaces are flooded with oil, the friction is almost independent of the nature of the materials of the contact surfaces. As the lubrication diminishes, the coefficient of friction becomes more dependent on the materials.