|MadSci Network: Physics|
Some forms of friction do increase with velocity, but certainly not all. Your statement is generally correct for viscous friction, friction involving the flow of fluids. For example, fish or submarines in the water, and balloons or subsonic airplanes in the air all may have their friction increase with velocity. The fish, submarines, balloons, and airplanes all experience drag from viscosity. Both air and water have viscosity, so motion through air or water produces a velocity 'field' that dissipates energy. That is a form of frictional energy loss. In water, waves may form that also dissipate energy. A boat experiences friction from both viscosity (skin friction) and wave generation. A lightweight boat may hydroplane at some speed. For the narrow speed range during which this planing happens, the total friction of the boat drops with speed. After planing take place, further speed increases will add to the friction. For this example, friction increases with speed, but in a piecewise manner. Pneumatic tires on the road appear to have their friction increase with velocity. This is called rolling slip. However, this slip is from deflection of the tire casing and tire tread. Most of the tire tread has static friction with the road. Velocity between the tire tread and the road occurs primarily at the trailing (back) edge of the contact. Here the tread of the tire slips as it unloads. This edge slippage is what makes the tire squealing sound when starting, stopping, or cornering rapidly. Hydraulic shock absorbers force oil through an orifice as they move. These generate a damping force by fluid flow that does increase with speed. Most dry surface friction does not increase with speed. If you have used a rope tow when skiing, you should have noticed the jerking action as your gloves stopped slipping on the tow rope. This jerk results from a large increase of friction as motion stops. Here, the friction increases as the speed decreases. Friction often may stay nearly constant with velocity, as with car brake linings against a disc brake rotor. However, because dry friction results from surface contact, it may change from nearly any surface contaminant. A sub-microscopic film can change friction. I hope this help you understand better. Your statement about friction increasing with velocity was correct, but only some of the time. For more information about friction and velocity, you might wish to check the following web sites: davidson mit gsu
Try the links in the MadSci Library for more information on Physics.