For example, the strength of static friction is not constant, but instead often depends on the length of time two surfaces are in contact.
Dynamic friction is almost always dependent on the relative sliding velocity.
It is often impossible to achieve a constant velocity, because of instability leading to stick-slip motion. After describing some of the recent progress concerning friction between surfaces, I focus on the interesting case of sheared granular matter, which is of some relevance to earthquakes. Granular materials are different from both fluids and solids. In recent experiments we have determined the nature of stick-slip motion in sheared granular matter, the time variation of the frictional force when slip occurs, and the slow creep that happens when there is no apparent motion. The results are compared and contrasted to that of friction in solids and in confined liquid films.