MadSci Network: Physics

Re: Why does friction increase with velocity?

Date: Thu Aug 17 19:37:07 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 966259781.Ph

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 

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:

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