# Moment of inertia in plane translation

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1. Mar 18, 2015

### arvindsharma

I have a doubt in my mind regarding moment of inertia.in my text book a question is somewhat like this The center of a disc of mass 'M' and radius 'R' is at a distance 'd' (here 'd' >'R') from a certain point 'O'.what is the moment of inertia of the disc about an axis perpendicular to the plane of the disc and passing through point 'O'?If,
(1)Disc is stationary
(2)Disc is having translatory motion such that perpendicular distance between center of disc and point 'O' remains same?
(3) disc is rotating about an axis perpendicular to its plane and passing through its center of mass?
(4)disc have both type of motions as in part 2 and part 3?

As far as i know moment of inertia is defined for rotation only so i could not answer part 1 and part 2(can i apply parallel axis theorem for first two parts?)for part 3 i applied parallel axis theorem and got right answer.unfortunately answer for part 4 was not even given in the book.can someone help me how to solve part 1,2 and 4th.
i will be thankful.

2. Mar 18, 2015

### Simon Bridge

Moment of inertia plays the same role for rotational motion that mass plays in linear motion.
An object can have a moment of inertia without rotation just like it can have mass without translating.

The questions ask about the moment of inertia wrt a particular axis.
What are the things that have to change for the moment of inertia about this axis to change?