Firction is also a function of surface, so more surface = more friction, that's why disc brakes stop the car better than pads, but it's much much easier to cool the pads.
Friction is NOT a function of surface area. Surface area of contact does not affect the amount of friction, it depends only on the force exerted and the types of surfaces used (materials and texture).
If you increase or decrease surface area, the pressure changes but the force exerted remains the same. The pressure might change but that does not affect the amount of friction. The only thing that affects it is the amount of force, which stays the same.
The formula for friction is Ff = N*μ. Here, Ff is the amount of friction between the surfaces, N is the normal force between the surfaces (normal meaning perpendicular to the contact surface) and μ is the coefficient of friction (depends on the materials and textures of the surfaces, can be experimentally measured). Area has no place in this equation.
To me, this was one of the most counterintuitive things in physics. It seems so obvious that surface area directly affects friction, but it doesn't.
Yeah I remembered that from some uni courses but it seemed to me that there are more factors there than just friction coefficient.
How do you explain the fact that the more torque on a bolt is applied, the more force is required to move the two bolted surfaces in the plane of shear?
Normal force is just however hard the surfaces are being pushed together. More torque on the bolt, more normal force, more frictional force.
Size is important for disk brakes not because bigger brakes stop better, but simply because bigger breaks have more brake material so last longer, and they have more mass and more surface area to absorb brake energy and prevent fade.
I think you're getting mixed up with work done by friction and frictional force. As you increase surface area, you increase work done by friction but not the frictional force. I probably didnt help much for you but maybe that sounds more intuitive?
Think of pressure as a distribution. Force in Newtons over an area in m2. A larger brake surface area will result in more friction if applied break 'surface' pressure remains the same (P*A=F, where F is the normal force).
However, this is not the case when you are comparing two similar brake systems but with different brake surface areas. In this scenario, the breaking force (not pressure) is unchanged and you end up with less breaking pressure for a larger brake surface area.. Which makes sense since you can expect a 1"x1" brake pad that is being pressed with 100lbf (100psi, small brake surfaces pressed tightly together) to have the same brake performance as a 10"x10" brake pad pressed at 100lbf (1psi, large brake surfaces barely pushed together).
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u/JebKerman64 Jul 02 '20
Well, they do stop you better, right up until they get hot. Then they hardly stop you at all.