Register to reply 
Coefficient of Friction and Normal Force 
Share this thread: 
#1
Jul508, 03:43 PM

P: 69

I know that the force of friction is the (coefficient of friction) x (normal force). My question is, why isn't area involved? That is, why wouldn't a larger surface have more friction than a smaller surface, if the normal force is the same?
PS: Sorry, just in case this is in the wrong section 


#2
Jul508, 04:36 PM

Sci Advisor
HW Helper
P: 1,275

There is no reason why the coefficient of friction can't implicitly depend on area.



#3
Jul508, 05:49 PM

Sci Advisor
HW Helper
Thanks
P: 26,148

Because the larger the surface, the more its weight is spread out. The friction per area depends on the pressure between the surfaces …*the harder they're pressed together, the more friction you'd expect. Pressure is force divided by area. Halve the area, and the pressure is halved, so the friction per area is halved, so the total friction is still the same. 


#4
Jul508, 05:59 PM

Sci Advisor
HW Helper
P: 1,275

Coefficient of Friction and Normal Force



#5
Sep1908, 12:12 PM

P: 5

could u put forward ur point properly 


#6
Sep1908, 03:02 PM

Sci Advisor
P: 2,284

CS 


#7
Sep1908, 03:09 PM

Sci Advisor
P: 2,284

F does not depend on the area. CS 


#8
Sep1908, 03:12 PM

Sci Advisor
P: 2,284

CS 


#9
Sep1908, 06:48 PM

HW Helper
P: 7,131

In the real world it depends on the surfaces involved. In the case of tires, maximum static friction force does not increase lineary with normal force, the ratio is called load sensitivity:
http://en.wikipedia.org/wiki/Tire_load_sensitivity For tires, a larger area reduces the force per unit area, increasing friction (there is a point of diminishing returns due to unsprung weight). Also in the second half of the second video on this web page, 4 objects of the same density are placed on a smooth board, and the smallest object slides last, although I'm not sure if this is friction force or something related to air between the objects and board. http://www.gyroscopes.org/1974lecture.asp 


#10
Sep1908, 09:48 PM

HW Helper
P: 2,277

In simple words, within its range of applicability Coulomb's theory of dry friction has given good results. For cases where Coulomb's theory is not applicable such as lubricated surfaces, other theories must be employed. 


#11
Sep2008, 01:33 PM

P: 5

Now read this..
When the surface area is reduced , the pressure on the contact points between the two surfaces increases. As a result some contact points undergo deformity in shape and hence more number of contact points are obtained (imagine the contact points to be like mountains if the height of peak is reduced then there would be more number of hills with the same height than were previously). When there is greater surface area , pressure on the contact points are less. So they do not undergo deformity, but, due to greater surface area number of contact points are more already. So in both the cases the total number of contact points remain same and hence friction remains same in both the cases. 


Register to reply 
Related Discussions  
Normal Force, Static and Kinectic Friction.  Introductory Physics Homework  1  
Normal Force, Friction Force, Accel problem  Introductory Physics Homework  2  
HELP! problem involving force of friction and normal force  Introductory Physics Homework  7  
Centripital Force and Coefficient of Friction  Introductory Physics Homework  2  
Friction and the Normal Force  Introductory Physics Homework  2 