The Open Door Web Site
HOME PAGE BIOLOGY CHEMISTRY PHYSICS ELECTRONICS HISTORY HISTORY of SCI & TECH MATH STUDIES LEARN FRENCH STUDY GUIDE PHOTO GALLERY
ATOMIC and NUCLEAR ELECTRICITY and MAGNETISM MEASUREMENTS MECHANICS OPTICS PRACTICAL WORK QUESTIONS RELATIVITY THERMAL PHYSICS WAVES
MECHANICS
Google
Custom Search
Friction
Body in Equilibrium on a Horizontal Surface  
(Ok, maybe I should say that the forces acting on the body are in equilibrium but let's not be too pedantic.)   
  The surface needs to exert a force equal but opposite to the force due to gravity.   
   
This force is called the normal reaction.  
It can be considered to exist as a "reaction" to the force of gravity pulling the body onto the surface and it acts at 90 to the surface.   
   
Because it acts at 90 to the surface, this force has no component acting parallel to the surface.   
   
 
   
Body in Equilibrium on an Inclined Surface   
     
   
   
If equilibrium is to be maintained, the surface must continue to exert a force which is equal but opposite to the force of gravity but now this force has a component which acts parallel to the surface  
   
   
   
   
   
   
     
It is often useful to resolve the reaction force into two components, as shown here, one acting at 90 the surface and one acting parallel to the surface.   
   
The component acting at 90 to the surface is still called the normal reaction  
   
The component acting parallel to the surface is called the force of friction between the body and the surface.   
   
   
   
Similarly, we can resolve the force of gravity into two perpendicular components.   
     
To maintain equilibrium:   
   
1. the magnitude of the force of friction must be equal to the magnitude of the component of the force of gravity acting down (parallel to) the slope   
   
2. the magnitude of the normal reaction must be equal to the magnitude of the component of the force of gravity acting at 90 to the surface.   
   
   
Therefore, if the angle between the surface and the horizontal is θ, the force of friction will have a magnitude given by   
   
and the magnitude of the normal reaction is   
   
   
The force of friction acting between two (reasonably regular, non "sticky") surfaces  
1. opposes their relative motion  
2. depends on the strength of the force pushing the two surfaces together (indicated by the magnitude of the normal reaction)  
3. does not depend* on the relative speed of the two surfaces  
4. does not depend* on the area of the surfaces in contact.  
   
* There are, of course, an almost infinite number of things on which the force of friction does not depend! These two are mentioned because many people find them surprising...  
   
The Coefficients of Friction  
Experiments show that the magnitude of the force of friction acting between two surfaces is directly proportional to the magnitude of the normal reaction force.  
Therefore  
 
The constant of proportionality is called the coefficient of friction for the two surfaces, symbol μ.  
 
   
The value of μ depends on the nature of the two surfaces (what materials they are made of and how "smooth" they are).  
Be careful with this word, smooth. In physics, when we describe surfaces as smooth, we mean that there is absolutely no force of friction.  
   
Therefore, a surface which is described as smooth can only exert a force at 90 to itself.  
   
Experiments (or just everyday observations) also show that, for any given pair of surfaces, we need two different coefficients of friction, μs the static (or limiting) coefficient, and μd the coefficient of dynamic friction.  
It is usually more difficult to start two surfaces slipping past each other that to keep them moving once they have been started (even ignoring the effects of the inertia of the bodies).  
   
Conclusion: for two given surfaces  
 
   
Click here for worked example.  
SITE MAP
WHAT'S NEW
ABOUT
PRIVACY
COPYRIGHT
SPONSORSHIP
DONATIONS
ADVERTISING
 

© The Open Door Team
2016
Any questions or
problems regarding
this site should be
addressed to
the webmaster

David Hoult 2017

Hosted By
Web Hosting by HostCentric

 
SiteLock
 
 
Mechanics Index Page