The surface of objects is usually rough. This is the reason why when two objects are in contact and one of them is in relative motion (or begins to move) with respect to the other, in addition to the normal force, there is also a friction force (or friction) that acts between them.
Although this is not always the case, the friction force often opposes the motion of the body on which it acts and the magnitude of its maximum value is proportional to the normal:
Where μ is the coefficient of friction between the two surfaces in contact.
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The coefficient of friction is a dimensionless scalar and its value depends on the pairs of materials in contact. There are two types of friction coefficient: static and kinetic.
- Static Friction μs: is that which, when multiplied by the normal, gives the force to overcome so that one of the bodies begins to move with respect to the other.
- Kinetic Friction μk: is that which, when multiplied by the normal, gives the force to overcome so that the two bodies continue to move with a uniform relative motion once the motion has begun.
The value of the two coefficients of friction is determined experimentally.
The coefficient of static friction is greater than the coefficient of kinetic friction.
Here you can find Newton’s laws problems.
