Experimental facts about friction

1.

f_s$\le$$\mu_{s}^{}$N where $\mu_{s}^{}$ is the coefficient of static friction and N is the magnitude of the normal force. Equality holds when the object is on the point of slipping: f_s(max) = $\mu_{s}^{}$N .

2.

f_k = $\mu_{k}^{}$N where $\mu_{k}^{}$ is the coefficient of kinetic friction and is approximately constant for any given pair of materials.

3.

Values of $\mu_{s}^{}$ and $\mu_{k}^{}$ depend on the nature of the surfaces that are in contact. Usually $\mu_{k}^{}$ < $\mu_{s}^{}$ . Examples: rubber on concrete $\mu_{s}^{}$ = 1.0, $\mu_{k}^{}$ = 0.8 ; waxed wood on wet snow $\mu_{s}^{}$ = 0.14, $\mu_{k}^{}$ = 0.10.

4.

The direction of the force of friction is opposite to the direction the object wants to move.

5.

$\mu_{k}^{}$ and $\mu_{s}^{}$ are nearly independent of the area of contact between the two surfaces.

6.

$\mu_{k}^{}$ is nearly independent of the velocity of the object under consideration.