Question #6bde4
1 Answer
And yes...
The real world is never quite so tidy as our mathematical models. If you are taking an introductory physics course, the answer is no. However, if you are designing brake pads or tire treads, you better actually measure it.
Static friction usually holds stronger than kinetic friction. At some velocity between zero and moving slowly there is a transition from one to the other. Careful adjustment of the velocity should show that this transition has some features.
A more complicated transition between to two states happens in a stick-and-slip situation. This happens when brakes squeal. The two surfaces alternate between kinetic and static friction. If this takes place at an audio frequency you will hear a sound. This is the normal way that a violin bow works. The bow periodically grabs and releases the string as it slides across. In the stick-and-slip mode we can model the average friction as decreasing with velocity.
See this excellent article from Wikipedia for lots more detail:
Wikipedia: Friction
