# Can someone please explain to me what moment of inertia means?

Can someone please explain to me what moment of inertia means??

I googled it and I found that moment of inertia is an objects resistance to angular momentum??? What does that mean exactly? Can someone please briefly explain to me (in English) what it means or give me an example? Because I really need to understand the concept in order to answer a question like this:

If global warming continues over the next one hundred years, it is likely that some polar ice will melt and the water will be distributed closer to the equator.
(a) How would that change the moment of inertia of the Earth?
increase
decrease
no change

(b) Would the duration of the day (one revolution) increase or decrease?
increase
decrease
no change

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lewando
Homework Helper
Gold Member

Just like how an object with mass resists change to its linear motion (Newton's 1st Law), an object with mass also resists change to its rotational motion.

Just like how linear momentum is a function of mass and velocity, angular (rotational) momentum is a function of mass and angular (rotational) velocity. The unique thing about rotational motion (that is related to your homework questions) is that mass distribution is important.

An example of this importance is when a figure skater rotates with arms outstretched and then pulls the arms close to the body (same mass, different mass distribution). In this example (and also for your homework), angular momentum is conserved, though the angular velocity has increased.

This is where moment of inertia comes in. It combines mass and mass distribution into a single usable value. For example the moment of intertia for a solid sphere is (2/5)mr2 whereas for a solid disk, it is (1/2)mr2 (rotating like a wheel, not like a penny spinning on edge--that is different because the mass distribution about the axis of rotation is different).

The equations are all out there--I just hope this gives you a little more insight into "I" (moment of inertia).