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Friction
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Friction is a contact force that opposes the relative motion of two bodies. The basic facts concerning friction between dry, unlubricated surfaces are fairly simple and were established long ago.
| The lows of friction | (1) The force of friction is proportional to the load. (2) The force of friction is independent of the area of contact. By "load" we mean the force pressing the two surfaces together. (3) The force of friction is independent of the speed. |
The fact that the force of friction is proportional to the load seems reasonable. Its independence of the area of contact is surprising. To understand this we must distinguish between the apparent area of contact and the actual area of contact. Even highly polished surfaces are not smooth at the microscopic level. The two surfaces meet only at the hilltops (called asperities), which means that the
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| FIGURE 3.11 |
actual area of contact may be less than one ten-thousandth of the apparent area of contact. Therefore, friction does increase with the real area of contact, but it is independent of the apparent area of contact.
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