Coefficient of friction

Also called mu

A dimensionless number, written mu, that sets how much friction a given pair of surfaces can produce for a given normal force. It is a property of the two surfaces together rather than of one object, and it has no units because it is a ratio of two forces.

Friction gets reported as a ratio. Divide a friction force by the normal force pressing two surfaces together and the newtons cancel, leaving a bare number, μ\mu. That is the whole reason a coefficient of friction has no units.

Every pair of surfaces has two of them. μs\mu_s sets the largest friction force available before sliding starts; μk\mu_k sets the force once sliding is underway. The AP Physics 1 CED states that the coefficient of kinetic friction depends on the material properties of the surfaces that are in contact, and that the static coefficient is typically greater than the kinetic coefficient for a given pair.

The equation sheet carries friction as an inequality, ∣F⃗f∣≤∣μF⃗N∣|\vec{F}_f| \leq |\mu \vec{F}_N|, so a single printed line covers both cases.

What μ\mu is not:

  • It is not a property of one object. Swapping in a heavier block on the same table changes the normal force, not the coefficient.
  • It does not depend on how much surface area is in contact.
  • It has no canonical value to memorize. Real pairs are measured, and everyday values run from roughly 0.03 for lubricated metal to above 1 for rubber on dry road, so an exam question states the value it wants you to use.

The measuring procedure is in how to find the coefficient of friction, and the values with their spread are in the coefficient of friction table.

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