# Understanding μk

1. Jan 13, 2014

### T_razz

I'm currently a first year physics student and missed some nets and concepts and need help before exams. It would be a HUGE help if someone could explain it to me and how to use it to find things like Fn and a→ when mass and μk is given. Please i really could use this info and thanks.

2. Jan 13, 2014

### Simon Bridge

Welcome to PF;
By $\mu_k$ do you mean the coefficient of kinetic friction?
It's used the same way as any coefficient of friction.

You use friction as an additional force in the free-body diagram.
People tend to trip up on the direction (it changes if the velocity changes direction) and on the transition between kinetic and static cases.

See:
http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html

Hyperphysics appears to repeat an "obsolete idea" about friction re surface roughness.
See: http://amasci.com/miscon/miscon4.html#fric
... does not affect the calculation though.

Last edited: Jan 13, 2014
3. Jan 13, 2014

### T_razz

yeah i was talking about it as as a coefficient of friction kinetic friction and thanks for the help!

4. Jan 13, 2014

### Staff: Mentor

I'm impressed you were able to decode that, Simon! I didn't have a clue what the OP was asking...

5. Jan 13, 2014

### Simon Bridge

@T_razz: no worries, do some examples and let me know if you get stuck.

@berkeman: experience :D
I need to edit my original reply slightly though: the coefficient is not the force. I'm sure nobody got confused...

6. Jan 14, 2014

### sophiecentaur

Psychic?

7. Jan 14, 2014

### Simon Bridge

I dunno - lets see... ummmm... I'm picking up an impression of a square or rectangle of light in front of you... also a general blockyness like an ordered cubism regularly under your hands.
There's another in the background - it's fuzzy - associated with the letter D or C.
Does this make any sense to you?