# I don't understand a symbol used in this question

## Homework Statement

I have to derive the equation:

$$I=\frac{m(g-R\alpha)R}{|\alpha_{friction}|+\alpha}$$

## The Attempt at a Solution

What is $$|\alpha_{friction}|$$ and how do i find it? And why do I use the absolute value of it? The experiment was that we had to measure the angular velocity of a rotating disk and we had to measure how much the speed went down due to friction. But in that case, it would just be the regular alpha.

EDIT: I managed to derive it. But the question of why do we use the abs value still stands.

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## Answers and Replies

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Andrew Mason
Homework Helper

## Homework Statement

I have to derive the equation:

$$I=\frac{m(g-R\alpha)R}{|\alpha_{friction}|+\alpha}$$

## The Attempt at a Solution

What is $$|\alpha_{friction}|$$ and how do i find it? And why do I use the absolute value of it? The experiment was that we had to measure the angular velocity of a rotating disk and we had to measure how much the speed went down due to friction. But in that case, it would just be the regular alpha.

EDIT: I managed to derive it. But the question of why do we use the abs value still stands.
You will have to explain the set-up better. It looks possibly like a wheel on a flat surface that is given an angular acceleration by having a weight (mg) tied to the wheel edge drop. But that is just a guess.

AM

wow, how the hell did you manage to guess that right :)

Andrew Mason
Homework Helper
Perhaps you could just give us the setup of the experiment. I am just guessing at this point. I still don't understand exactly where the friction term is coming from.

AM

You guessed it right

There is a wheel that rotates. We attached a string to it's axle and a weight was attached to the string. We dropped the mass, that unwound the string, caused the axle to start spinning, and caused the disk to accelerate.

Andrew Mason
Homework Helper
You guessed it right

There is a wheel that rotates. We attached a string to it's axle and a weight was attached to the string. We dropped the mass, that unwound the string, caused the axle to start spinning, and caused the disk to accelerate.
Ok. Well I didn't guess it quite right since I thought the mass was tied to the rim.

Is the axle mounted on something? Does the wheel roll along the surface as the weight drops or does it just spin on the axle? It appears from the equation that the moment of inertia is about an axis a distance R from the centre of mass of the wheel.

Again, can you give us the whole problem? In particular: is R the radius of the wheel? What is the radius of the axle?

AM

http://img196.imageshack.us/img196/3564/35005168.png [Broken]

R is the radius of the wheel, and the whole device is clamped to the table

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Andrew Mason