PH105
Coefficient of Friction
The coefficient of static friction is defined as the ratio of the maximum frictional force to
keep an object from sliding to the normal force acting on the object. The coefficient of
kinetic friction is the ratio of the frictional force to keep an object moving at constant
(nonzero) velocity to the normal force.
s 
f s ,max
f
, k  k
n
n
The normal force may or may not be the weight of the object, depending on what other
forces might be acting on the object and whether it is on a level surface or an incline. For
simplicity, we usually assume that the coefficient of friction is independent of the value
of the normal force. For example, if you double the normal force you also double the
friction so that  remains the same. We also assume that the coefficient of kinetic
friction is independent of speed. These are only approximations, however.
There are several ways in which you can measure the coefficient of static or kinetic
friction between an object and a surface. For example 1) Measure the force required to start an object sliding on a level surface or to keep it
moving at constant speed.
2) Measure the deceleration of an object sliding to rest on a level surface.
3) Place an object on an inclined plane and measure the maximum angle before slipping
occurs, or measure the angle so that it slides at constant speed.
Your group will be asked to use one of the above methods to measure the coefficient of
static and/or kinetic friction between a block of wood and a metal track. If possible, you
should also test whether the approximations regarding the coefficient of friction being
independent of normal force and speed are correct. If you have the time and opportunity,
you might measure the coefficient of friction for other surfaces or objects.