# Finding Moment of Interia of a 'Loop' when given Density & Cross sectional area

1. Jul 28, 2011

### anr91

1. The problem statement, all variables and given/known data

I eventually have to solve for maximum angular acceleration of the loop in a magnetic field, and I have gotten everything with the exception of the moment of inertia, so I won't include the emf and B known variables.
Known: a copper wire with a density of $\rho$ = 8960 kg/m3 is formed into a circular loop of radius 0.50 m. Cross sectional area of the wire is 1.00 x 10-5m2.

2. Relevant equations
I=MR2
(and eventually) $\tau$ = $\alpha$I

3. The attempt at a solution
I know since mass isn't given, I need to integrate something so I can use the density. However, it's been a really long time since I've integrated, so I'm not very familar with it. I've been unable to find an equation to find the volume of the 'loop,' so I know integration is the only way.

2. Jul 28, 2011

### Mike Pemulis

Re: Finding Moment of Interia of a 'Loop' when given Density & Cross sectional area..

No integration required. A loop is just a cylinder bent into a circle. Imagine bending the loop back into a "normal" cylinder, and find the volume of that object. (If you don't remember the formula for the volume if a cylinder, it is easy to find).

3. Jul 28, 2011

### WatermelonPig

Re: Finding Moment of Interia of a 'Loop' when given Density & Cross sectional area..

You can approximate the loop as a circle (line) because the cross section is much smaller than the radius.

4. Jul 29, 2011

### Mike Pemulis

Re: Finding Moment of Interia of a 'Loop' when given Density & Cross sectional area..

Yes, I forgot to add that part. But first, OP needs to find the mass, which requires finding the volume.

Once that is done you would forget about the finite width and simply use I = MR2