Coefficient of friction question

1. Jun 17, 2013

guay

Say if an iron disk slides down a non ferromagnetic conductor ramp and we get a certain time. And then we have an iron disk with the same mass but MAGNETIZED slide down the same ramp and get a longer time (because of of magnetic braking). The calculated acceleration of the MAGNETIZED disk will be smaller.

We then take these velocities and plug them into an energy equation (ET1 = ET2 or something like that) to find the energy lost. We use this to find 2 different energy lost amounts, and then use Wff = Ff(d)(cos180) to find the force of friction. We will get a different force of friction for each disk because they each had a different energy lost amount.

Now here is the question: if I know the normal force and stick the force of friction into the coefficient equation: Ffk = μkFn, I will get different coefficients for each disk. Why is that? I thought the coefficient depended on the material and the surface? In this case both are the same. Unless being magnetized changes the coefficient?

All help is appreciated!!

2. Jun 17, 2013

Haborix

I will just say that when you first learned about friction it was in the context of mechanics, not E&M. If I introduce an entire field of new phenomena (E&M) then I can't expect all my old intuitions to hold. Trust nature, there is no hope in trying to prove it wrong.

This probably isn't a very satisfying answer, but it's all I've got.

3. Jun 17, 2013

wukunlin

Sliding down slower shouldn't change the value of energy lost. The power value (energy over time in simple terms) would be different but in the end both disc would have lost the same amount of energy proportional to the vertical height they slid down.

4. Jun 17, 2013

guay

But the value of energy lost does indeed change, since the energy lost equation requires kinetic energy which require velocity, and the velocities for each scenario is different.

I was told by others that Normal force gets bigger. The normal force is counteracting not only gravity now, but also the magnetic force so therefore Fn is greater. The coefficient does not change.

The equation is still going to be
Ffk = μkFn

The difference however is that now
Fn = B + Fgy

Is this right? And if so, how would I find Fn (in order to find B through math)

5. Jun 17, 2013

wukunlin

nope, the magnetized disc will slide down slower with less velocity but it takes longer so overall the energy lost is the same as the other disc.

6. Jun 17, 2013

guay

So what would the values be in the equation for both scenarios? Eg1 - Ek2 = E lost

m= 0.01kg
t (NON MAGNET) = 0.80s
t (MAGNET) = 5.2s
v (NON MAGNET) = 2.5m/s
v (MAGNET) = 0.19m/s

thanks

7. Jun 17, 2013

wukunlin

the values should equate to the gravitational energy lost when the disc slid down the ramp = mgh.

8. Jun 17, 2013

guay

It does, but only for the magnetized disk. The non magnetized disk has a different (smaller) energy lost. Why is that? should they be the same?

9. Jun 17, 2013

wukunlin

Weird, they should be the same. How significant is the difference?

10. Jun 17, 2013

guay

the non magnetic is 0.018N, the magnetic is 0.048N which is the same as change in gravitational potential energy

11. Jun 17, 2013

wukunlin

what does the N stand for?

12. Jun 17, 2013

guay

oh crap i messed up, ignore the N's, they are all in Joules!!

13. Jun 17, 2013

wukunlin

hmm, hard to tell what went wrong, may I see your working?