# Homework Help: Coefficient of static friction experiment

1. Sep 15, 2014

### physicshelpppp

1. Problem

I am currently doing a home experiment about the coefficient of static friction. I've set up a ramp and am testing at which angle a tissue box begins to movie (with five different masses in it)... Although for every mass I use the angle seems to remain the same. The angle at which an object will start moving is independent of mass right? Mass is important when there is no friction?

2. Sep 15, 2014

### CWatters

Have you covered the maths at school yet? If you have then try proving it that way.

3. Sep 15, 2014

### Domenico94

The acceleration is given by a factor of $$g*(sin(Θ) - μcos(Θ) )$$
yes..it doesn't thepend from mass

4. Sep 15, 2014

### physicshelpppp

I have been looking at questions, when I do a worked question...

and the force down (weight force) on the object would be mgsinpheta and the force perpendicular is mgcospheta
which has same magnitude as normal force

so frictional force is = μsN (where is normal force is equal to perpendicular force)

therefore Ffr = μsmgcospheta
the object will start to move when the force down the slope is equal to the maximum friction force

therefore

mgsinpheta = μsmgcospheta

(therefore we can cancel the mg out on both sides)

and then we are left with μs = sinpheta/cospheta therefore μs = tanpheta

therefore it is independent of mass?

5. Sep 15, 2014

### CWatters

Correct. Nice to see theory and experiment agree.

6. Sep 17, 2014

### Domenico94

it isn't....remember the galileo experiment with gravity :)

7. Sep 17, 2014

### Staff: Mentor

The galileo experiment shows that the rate of fall of an object is likewise independent of mass. But that is not what is involved in the present static friction problem under investigation.

Chet