How Do You Calculate Moment of Inertia for Different Axes?

[mcnealymt]

Homework Statement

a)Find the moment of inertia about an axis that passes through mass A and is perpendicular to the page. In kg m^2

b)Find the moment of inertia about an axis that passes through masses B and C.In kg m^2

http://session.masteringphysics.com/problemAsset/1070524/4/12.EX16.jpg

Homework Equations

I=mr^2

The Attempt at a Solution

a) (Mass of a * r + Mass of b * r)/ (Total Mass)

.005 They wanted the answer in two sigfigs so I put .05*10^-1 and it was wrong.

b) Used Pythagorean Theorem

sqrt(.10^2- .062)= .08

 

[Doc Al]

I=mr^2

Good. Just find I for each mass (around the given axis) and add them up to get the total I.

a) (Mass of a * r + Mass of b * r)/ (Total Mass)

You forgot to square the distances. What distance did you use? And why did you divide by the total mass?

 

[mcnealymt]

Good. Just find I for each mass (around the given axis) and add them up to get the total I.


You forgot to square the distances. What distance did you use? And why did you divide by the total mass?

That makes sense, for some reason I was trying to find the center of mass which is completely irrelevant.

The distances I used were for a were: (.1kg)(.1m)^2+ (.1kg)(.1m)^2


The distance used for b was .08m (found using Pythagorean)
(.08m)^2(.2kg)

 

[mcnealymt]

Thank you Doc Al, I appreciate it. Have a nice day.