# Calculating Coefficient of Friction for Sandpaper

1. Jan 20, 2014

### hello1910

Hello Fellow Forum,

How would you calculate the coefficient of friction for sandpaper? I'm testing mechanical resistance of a bioplastic, and I'm going to attach the bioplastic to different weights, and then rub sandpaper over the bioplastics. The one that breaks will be used to find mechanical resistance. I know F=un- and n= the weight of the object the plastic was attached to, but how do you calculate u?
Thanks
hello1910

2. Jan 20, 2014

### pikpobedy

The coefficient of friction is equal to the tangent of the angle where slippage is initiated.

3. Jan 20, 2014

### hello1910

Thanks for the quick reply! Just to clarify, the angle is between the bioplastic and the weight, right? And also, you're saying that mass has no effect on the coefficient of friction?

4. Jan 20, 2014

### pikpobedy

The friction is in direct proportion to the mass.

Friction is often considered independent of contact area. If we want to change the friction we change the materials.

Does that mean a one tonne object on a 1 square mm leg is the same friction as on 100 square meter support. Not likely for normal materials.

Is it the same once movement starts? No. Is it constant at different velocities? It depends.

You are interested in the static coefficient of friction and the kinetic coefficient of friction.
http://en.wikipedia.org/wiki/Friction

The science is tribology http://en.wikipedia.org/wiki/Tribology

Last edited: Jan 20, 2014
5. Jan 21, 2014

### sophiecentaur

The 'laws' of friction assume that there is elastic deformation of the surfaces to that the actual contact area (on a microscopic level) depends upon the normal load and not on the available area for contact between the surfaces - so that the friction force increases proportionally with the normal load - giving a 'constant' coefficient. This is an oversimplification for many situations. An abrasive paper is not likely to be that ideal - you could imagine the limiting case of barely visible grooves with vertical sides on both faces that could 'lock' together. The 'friction' force could be the same for all normal loads once the surfaces were in contact.
You would need to do some actual measurements with the abrasive that you are planning to use and find the range over which you have linearity.