Centripetal forces | finding coeff of friction with only velocity

almosthavoc

1. Homework Statement
I don't remember exactly what the question was because it was on a quiz today. It said something along the lines of "a ladybug is on a turntable and begins to slip at .5 m/s. Find the coefficient of friction.

2. Homework Equations
Fc = (mv^2)/r
a = v^2/r
Ff = uFn

I think that's all I should need really for this?? Please let me know if I should be using anything else

3. The Attempt at a Solution
There hardly was an attempt unfortunately. I first set up the sum of the forces in the x direction. From that I had Fc - Ff = 0. I was able to cancel out mass but still had an unknown radius. Then I reread the problem, and the context made it sound like the lady bug wasn't under constant velocity (the lady bug begins to slip at .5 m/s - (to me at least) inferring that the ladybug was getting up to that speed). Overall I'm very confused. The physics department at my school is horrible, and all I really have for resources are yahoo answers and online forums.

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jbriggs444

Homework Helper
We have to reconstruct a more complete problem before we can help you solve it.

Suppose that the lady bug starts at the center of the turntable and crawls toward the rim. She crawls slowly. The turntable is spinning at 33 1/3 revolutions per minute. When the bug gets far enough from the center that her tangential velocity is 0.5 m/s, she begins to slip.

How far can you get solving that version?

Edit: Corrected to match rotation rate of a 60's era LP

Last edited:

almosthavoc

We have to reconstruct a more complete problem before we can help you solve it.

Suppose that the lady bug starts at the center of the turntable and crawls toward the rim. She crawls slowly. The turntable is spinning at 33 1/3 revolutions per second. When the bug gets far enough from the center that her tangential velocity is 0.5 m/s, she begins to slip.

How far can you get solving that version?
where did you get the 33 1/3 from?

jbriggs444

Homework Helper
where did you get the 33 1/3 from?
Phonograph records (LP's) turn at a rate of 33 1/3 rotations per minute. I've edited my post to indicate minutes instead of seconds.

almosthavoc

Phonograph records (LP's) turn at a rate of 33 1/3 rotations per minute. I've edited my post to indicate minutes instead of seconds.
Wouldn't I be able to solve for r now since you gave me rpm? T = 1.8 and plugging it into v=(2pir)/T would give a radius of .14 meters.
Now I can set umg = (mv^2) / r. M cancels out. After plugging in I get u = .18
Did I do this correct? Even if I did, it still doesn't help me because the rpm wasn't given on the quiz

jbriggs444

Homework Helper
Wouldn't I be able to solve for r now since you gave me period? T = 1.8 a and plugging it into v=(2pir)/T would give a radius of .14 meters.
Now I can set umg = (mv^2) / r. M cancels out. After plugging in I get u = .18
Did I do this correct?
Let's see if I can follow. T=1.8a. That looks like a typo. The time for one rotation is 1.8 seconds. So far so good.
0.14 meters radius is right (within rounding error anyway).
0.18 for $\mu$ matches what I get as well.

It would be a good exercise to work the problem again with pure algebra rather than plugging in numbers. See if you can solve for $\mu$ in terms of the rotation rate ($\omega$), the tangential velocity at which the bug begins to slip (v) and the acceleration of gravity (g).
Even if I did, it still doesn't help me because the rpm wasn't given on the quiz
Since we do not know what was on the quiz, there is not much we can do to help.

If you do the algebra suggested above, it should become clear that the answer depends on the given value for $\omega$.

almosthavoc

Let's see if I can follow. T=1.8a. That looks like a typo. The time for one rotation is 1.8 seconds. So far so good.
0.14 meters radius is right (within rounding error anyway).
0.18 for $\mu$ matches what I get as well.

It would be a good exercise to work the problem again with pure algebra rather than plugging in numbers. See if you can solve for $\mu$ in terms of the rotation rate ($\omega$), the tangential velocity at which the bug begins to slip (v) and the acceleration of gravity (g).

Since we do not know what was on the quiz, there is not much we can do to help.

If you do the algebra suggested above, it should become clear that the answer depends on the given value for $\omega$.
The quiz literally only gave velocity. I did not leave out any givens.

almosthavoc

The quiz literally only gave velocity. I did not leave out any givens. The core of the question was slipping at .5 m/s velocity and find the coeff of friction. Everything else was useless info like draw a free body diagram

jbriggs444

Homework Helper
The quiz literally only gave velocity. I did not leave out any givens. The core of the question was slipping at .5 m/s velocity and find the coeff of friction. Everything else was useless info like draw a free body diagram
Again, you are asking for help with a question that you cannot provide.

almosthavoc

Again, you are asking for help with a question that you cannot provide.
I’m seriously giving the whole question. My wording definitely isn’t correct, but I’ve provided everything. Maybe the quiz was messed up? I highly doubt it, but it could be possible

jbriggs444

Homework Helper
I’m seriously giving the whole question. My wording definitely isn’t correct, but I’ve provided everything. Maybe the quiz was messed up? I highly doubt it, but it could be possible
How can we know? All you are able to give us is your recollection of the question. You have admitted that the recollection is not perfectly accurate.

almosthavoc

How can we know? All you are able to give us is your recollection of the question. You have admitted that the recollection is not perfectly accurate.
The only thing that’s not accurate is the exact wording. I remember all the key points. The question first starts off by saying a ladybug on a turntable begins to slip at .5 m/s. The next sentace said 1 point is awarded for drawing a free body diagram. The next sentence said 4 points are rewarded for finding the coeff of friction

jbriggs444

Homework Helper
The only thing that’s not accurate is the exact wording. I remember all the key points. The question first starts off by saying a ladybug on a turntable begins to slip at .5 m/s. The next sentace said 1 point is awarded for drawing a free body diagram. The next sentence said 4 points are rewarded for finding the coeff of friction
Then perhaps the test writer assumed that it is common knowledge that turntables rotate at 33 1/3 rpm. [Unless, of course, it's a 45, a 78 or a 16 2/3. https://blog.electrohome.com/vinyl-record-speeds-33-45-78-mean/]

almosthavoc

Then perhaps the test writer assumed that it is common knowledge that turntables rotate at 33 1/3 rpm. [Unless, of course, it's a 45, a 78 or a 16 2/3. https://blog.electrohome.com/vinyl-record-speeds-33-45-78-mean/]
Well then that would just further prove my point how horrible the physics department here is. Anyways, thank you for helping out as much as you could. I appreciate it

haruspex

Homework Helper
Gold Member
2018 Award
Well then that would just further prove my point how horrible the physics department here is. Anyways, thank you for helping out as much as you could. I appreciate it
You can apply dimensional analysis here.
You are given a velocity, you know a relevant acceleration, g, and you are asked for a dimensionless quantity. There is no way to combine a velocity and an acceleration to obtain such. You need another piece of information, whether it is a rotation rate or a radius, or whatever.

almosthavoc

You can apply dimensional analysis here.
You are given a velocity, you know a relevant acceleration, g, and you are asked for a dimensionless quantity. There is no way to combine a velocity and an acceleration to obtain such. You need another piece of information, whether it is a rotation rate or a radius, or whatever.
You can apply dimensional analysis here.
You are given a velocity, you know a relevant acceleration, g, and you are asked for a dimensionless quantity. There is no way to combine a velocity and an acceleration to obtain such. You need another piece of information, whether it is a rotation rate or a radius, or whatever.
I emailed my professor, and there was an error after all

"Centripetal forces | finding coeff of friction with only velocity"

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