Laws of Dry Friction and Coefficients of Friction |
| Magnitude of the maximum static friction force | ||
| Magnitude of the kinetic friction force |
symbol | description |
Fm | maximum force |
Fk | kinetic friction force |
μs | coefficient of static friction |
μk | coefficient of kinetic friction |
N | normal component of the reaction surface |
(Eq1) |
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(Eq2) |
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1. | No friction (Px = 0) | F = 0 N = P + W | The forces applied to the body do not tend to move it along the surface of contact; there is not friction force. | |
2. | No motion (Px < Fm) | F = Px F < μsN N = Py + W | The applied forces tend to move the body along the surface of contact but are not large enough to set it in motion. The friction force F which has developed can be found by solving the equations of equilibrium for the body. Since there is no evidence that F has reached its maximum value, the equation Fm = μsN cannot be used to determine the friction force. | |
3. | Motion impending (Px = Fm) | Fm = Px Fm = μsN N = Py + W | The applied forces are such that the body is just about to slide, that is, motion is impending. The friction force F has reached its maximum value Fm and, together with the normal force N, balances the applied forces. Both the equations of equilibrium and the equation Fm = μsN can be used. Also note that the friction force has a sense opposite to the sense of impending motion. | |
4. | Motion (Px > Fm) | Fk < Px Fk = μkN N = Py + W | The body is sliding under the action of the applied forces, and the equations of equilibrium do not apply any more. However, F is now equal to Fk and the equation Fk = μkN may be used. The sense of Fk is opposite to the sense of motion. |
Next Lesson: Angles of Friction⇒ |