friction question

[student85]

The formula for the calculation of friction, F=uN, where u is the friction coeficient, doesn't include anything related with its area of contact.
That is, to what I know, a rectangle for example, weighing 3 kg that is real long and has little height, has a big area of contact with the surface it is on, compared to a block for example, of the same weight . I think that the rectangle is harder to move because of its much bigger area of contact.
I know that because it has a big area of contact then its weight is distributed and so it has less Newtons per cubic m.
However I still think that the rectangle should have a greater friction than the cube..am I right?

[whozum]

You are wrong, but at least you are thinking.

The larger area will be compensated for by a smaller density. Ultimately in kinetics we adopt the model of a point particle of mass 3kg, and measure the normal force on this. However, the normal force has no component on area simply because the area doesnt matter.

Think of something like a car. A car has a surface area of contact of maybe .5m^2 (tires), but if you got rid of the entire car and just had the 4 tires, it would be much easier to move. The surface area plays no role in this calculation.

[Andrew Mason]

The formula for the calculation of friction, F=uN, where u is the friction coeficient, doesn't include anything related with its area of contact.
That is, to what I know, a rectangle for example, weighing 3 kg that is real long and has little height, has a big area of contact with the surface it is on, compared to a block for example, of the same weight . I think that the rectangle is harder to move because of its much bigger area of contact.
I know that because it has a big area of contact then its weight is distributed and so it has less Newtons per cubic m.
However I still think that the rectangle should have a greater friction than the cube..am I right?Experimentally it has been shown that the kinetic friction force and maximum static friction force are proportional to the normal pressure x area of contact. The pressure determines how strongly the two surfaces 'mesh'. The area determines the extent of contact of the two surfaces. But pressure x area = total Normal force.

So you are right in the sense that a larger surface with the same pressure will have the greater friction force. But in order to have the same pressure, normal force must increase in proportion to area. So in the end, it is easier to think of friction as simply proportional to normal force.

AM

[student85]

Thanks guys