# Homework Help: Does the coefficient of friction change with the angle?

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1. Feb 18, 2015

### Astraithious

I said stay the same, the definition of co efficient is here http://simple.wikipedia.org/wiki/Coefficient_of_friction but the main part i saw not only here was

"The coefficient of friction is dimensionless, meaning it does not have any units. It is a http://simple.wikipedia.org/w/index.php?title=Scalar_quantity&action=edit&redlink=1 [Broken], meaning the direction of the force does not change its magnitude."

My teacher says it does but how can angle cause molecular change in surfaces interacting? I just figured that angle only had something to do with forces not that co efficient value.

Last edited by a moderator: May 7, 2017
2. Feb 18, 2015

### Staff: Mentor

The friction coefficient is a constant, but the friction force itself can change because it depends on other factors (what are they?).

3. Feb 18, 2015

### Stephen Tashi

4. Feb 18, 2015

### haruspex

I think that's what the OP was saying in the last sentence.
Astraithious, as you say, the answer to the question is (3).

5. Feb 18, 2015

### Staff: Mentor

Yes, I was confirming his assertion in order to relieve his doubts.

6. Feb 18, 2015

### Astraithious

hello everybody thank you for the replies, my teacher sent this back as an answer as to why mu changes but it doesn't seem right to me at all and i was hoping others could take a look

"Let's talk about it starting from the definition that you found in Wikipedia. It states: coefficient of friction
"... describes the ratio of the force of friction between two bodies and the force pressing them together.". If it is on a straight horizontal surface, both forces stay constant at all times. However, when the forces are on a incline, those forces may and do change. The normal force now depends on an angle of the incline. The formula for Force of friction also shows that relation : Ffriction="miu" x Fnormal. So, miu=Ffriction/Fnormal- Looking at this formula, changing any of those forces would change "miu". In fact, if Fnormal increases, "miu" decreases and vice verse.
The other factor that is important in calculating "miu" is a surface and because that changes a Force of Friction, it would also change the value of "miu".
I hope that helps."

I believe she is mistaking force of friction and normal force as part of the coefficient but as far as i can see at least in terms of basic physics mu is a constant.

7. Feb 18, 2015

### Nathanael

I would reply to this with something along the lines of, "μ will change only if the normal force increases without the frictional force changing proportionally, but whenever the normal force changes, the force of friction will also change in such a way that μ becomes constant."

I don't know why she thinks the force of friction is fixed. I agree with you, μ is constant. It is independent of the normal force.

With my limited knowledge, I don't find it unreasonable that μ could change with a change in normal force. But this is not what is found experimentally.

8. Feb 18, 2015

### haruspex

That is the false statement. That equation only implies a change in $\mu_k$ if the ratio of those forces changes. Indeed, the whole point of the coefficient of friction as a concept is that the ratio does not change.

9. Feb 19, 2015

### Jediknight

If it does change I've been doing alot wrong and my teacher never noticed!!, sorry for sarcasm it seems like you and your teacher are answering two different questions, both correct with respect to the question you think your answering.

cooeficient doesn't change, frictional force does, you got it

10. Feb 19, 2015

### haruspex

If post #6 is anything to judge by, you are trying too hard to give the teacher the benefit of the doubt.

11. Feb 19, 2015

### Jediknight

yea re-reading if thats word for word what the was communicated that way it is very vaugue,

yes if you change the normal force w/out proportionally changing the frictional forces exerted μ would change... but that also means your doing a different expiriment with different materials cause μ is related to material and normal force only (not even S.A. in contact)

So in summary I think your right I gave the teacher to much credit, thats no kind of answer and can only be true assuming you change a HUGE variable without stating it, which is absurd.

I imagine O.P.'s in the same boat I am, only physics teacher available isn't adequate at his job, For me I guess its kind of to be expectected (no phd in physics ever dreamed of teaching at a community college) so maybe I'm rambling now but I feel for ya. teaching yourself physic is hard

12. Feb 22, 2015

### Astraithious

thank you everybody for the help, I wrote a letter to the head of staff and he realized her mistake and I received an apology as she said if i didn't like her answer I could drop the course.

13. Feb 22, 2015

### haruspex

I feel it would be more appropriate for the teacher to drop the course - or maybe study one.