# I Rotational movements

1. May 2, 2018

### physea

Hello!

I am a bit confused with the multiple terms like centre of gravity (ok I know that), moment of inertia, radius of gyration, etc.

1) Can you compile a complete list of such terms?
2) What are their differences and when do I use each?

Thanks!

2. May 2, 2018

### sophiecentaur

That's a bit of an open question. First, what is your understanding of the meaning of those terms?

3. May 2, 2018

### Dr.D

Might be well for the OP to list terms about which he/she has questions. The list of terms is just about endless, and far to broad to answer in general.

4. May 2, 2018

### A.T.

https://en.wikipedia.org/wiki/Glossary_of_physics

5. May 3, 2018

### physea

OK, then we can start from the ones I already listed in my first post.

6. May 4, 2018

Anyone?

7. May 4, 2018

### sophiecentaur

Have you found a definition of Centre of Mass? Do you have a problem with it.
PF has a specific aim (read the blurb) regarding students and 'learners'. The aim is to help people without spoon feeding them with knowledge that they can easily find elsewhere. We need to be aware that you (the student) have actually gone to some trouble with this. You will be amazed at the difference in response that you will get when you give first and ask afterwards.

8. May 4, 2018

### physea

I don't understand the difference between moment of inertia, centroid, and radius of gyration as well as when do we consider each.

9. May 4, 2018

### A.T.

What do they have in common?

10. May 4, 2018

### ZapperZ

Staff Emeritus
Unfortunately, even after several prodding, you don't seem to want to show any indication that you have made an effort to understand these things. It appears that you simply want to sit back and be spoonfed.

You simply can't say "I don't understand so-and-so." You need to explain and elaborate WHY and WHERE you stopped understanding the concept. Describe what you already know. In this case, what is the concept of "moment of inertia", "centriod", and "radius of gyration" that you already know? Without knowing what you know, how are we going to be able to build something on top of that? We don't know where the foundation is!

Here's another things that you need to be aware of. Sometime, in the act of trying to write it down and explain as clearly as possible your problem to another person, THAT in itself can trigger something that might help you understand what it is. Many of us who have to teach this material, even with all our academic degrees, often find that we discover something we either didn't realize or wasn't aware of in the process of making our lecture notes or presentations. In trying to convey the material in a clear, systematic, and understandable manner, it is not unusual that we discover some information that we didn't know before.

So stop simply claiming that you don't understand something. You MUST understand SOMETHING, otherwise you won't be in such a class that requires you to deal with these concepts. At the very least, you should be able to find these definitions either in your text/lecture notes (preferable), or online. Then describe up to what point you understood the concept, and where you stop understanding it.

A patient can't simply walk into a doctor's office and claim that he/she "does not feel well", and expect to get an accurate diagnose. Use that analogy here!

Zz.

Last edited: May 4, 2018
11. May 4, 2018

### sophiecentaur

@physea Alternatively find yourself a paid tutor. He who pays the piper calls the tune.
Your attitude is really very tiresome for people in PF who are only too ready to help someone who shows a willingness to make an effort.

12. May 4, 2018

### physea

I am honestly not sure what moment of inertia and radius of gyration are.
I think moment of inertia is an expression of the 3D dispersion of the mass of a 3D structure. But not sure where this should be used.
Also, radius of gyration as well.

13. May 4, 2018

### ZapperZ

Staff Emeritus
It appears that you chose not to heed any of the things I said in my post. So I'm out.

Zz.

14. May 4, 2018

### physea

If some people don't want to explain, that's fine by me, pointing me to a good reference would be fine.

15. Jun 1, 2018

### dean barry

you could make up a problem and involve some mechanics to solve it, imagine you have a trolley with 4 freely mounted wheels, you are going to accelerate the trolley with a constant force, the wheels don't slip under acceleration, what is the acceleration rate of the trolley ? ignore air drag and rolling resistance.

16. Jun 1, 2018

### Staff: Mentor

Thread closed due to lack of effort from the OP. Thanks for trying, everyone.