# B Moment of inertia dynamics

1. Apr 13, 2017

### Pinon1977

Good evening, good people of PF. I have a fairly simple question. Please see the attached drawing. My question is this: does the moment of inertia change from position one to position two (as the weight is rotating around a center line). In essence, does it maintain the same moment of inertia at position one as it does it position two. What about the kinetic energy? According to the formulas I see online, all point to yes.

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2. Apr 13, 2017

### Mastermind01

Moment of inertia is the distribution of mass about axis point we're considering. If the axis or mass does not change the moment of inertia wouldn't change. Similarly rotational kinetic energy is moment of inertia and angular velocity dependent only, unless these change, and in your example they don't, the rotational kinetic energy doesn't change.

3. Apr 13, 2017

### Pinon1977

Thank you for the response, and I agree! The reason I ask is my SolidWorks 3d modeling software is showing the moment of inertia changing as the weight is moved around the axis. With the highest MOI values being at 12 and 6 o'clock position and and lowest being at 9 and 3 o'clock. Why would that be, I wonder? Solid works is usually spot on in simulation.

4. Apr 13, 2017

### sophiecentaur

Perhaps you could check on your positioning of the axis of rotation with respect to your coordinate origin? That could make a difference.

5. Apr 13, 2017

### Nidum

The system shown has a net eccentric mass .

Think about the torque needed to maintain simple static balance with arm in different positions .

6. Apr 13, 2017

### Dr.D

A moment of inertia value always requires a reference point. Thus we speak of "The moment of inertia about point A" or "the moment of inertia about the x-x axis." If the MOI is shifting, I would assume that the axis is shifting as well.

The one that should definitely not change is the MMOI with respect to the CM. See what value you are given for this.

7. Apr 13, 2017

### sophiecentaur

That doesn't necessarily matter. You can have a moment of inertia about anywhere in space. The problem has to be something to do with the data entry into the software. The MI is independent of angle of rotation. If it were not, the object would not spin at a uniform rate because angular momentum cannot change - and that's not going to happen.

8. Apr 14, 2017

### Vinodgopal

Hi if a man accidentally falls from a 65 feet rooftop but clings on a rope 10 feet from ground which for 3 seconds holds good but cuts not able to handle his weight, will his inertia or kinetic energy and potential energy come down enough to save his life?

9. Apr 14, 2017

### Dr.D

What does this have to do with the original question? May I suggest that you start a new thread?

10. Apr 15, 2017

### _PJ_

How much is the variatiin, or the relative scales of mass, distance etc.?

I ask since there may be a floating point inaccuracy (very common in angular /trigonometric calculations) that may be involved?

11. Apr 15, 2017

### sophiecentaur

Yes. What is the error that's been found? Quite frankly, this shows how the non analytic / numerical / simulation approach can be a minefield. The 'algebra' of this is very trivial. If a numerical calculation produces some error then that is what you can expect. It's surprising if such a simple system can produce seriously wrong results. We can believe the theory so, if the results are really bad, there is something wrong with what's being put into the simulation. Reality is not about to crumble.

12. Apr 15, 2017

### Nidum

Have you told Solid works what axis to use ?

13. Apr 15, 2017

### sophiecentaur

Try working it out by hand - it's very easy to do.

14. Apr 17, 2017

### Pinon1977

Yes, when I go to "mass properties" in SolidWorks, it presents all the information relative to the model. My model's orgin, my axis orgin, and UCS orgin are all lined up at a cingular point in space. They all share a common 0, 0, 0 position in space. When I move the weight from position 1 to position 2, the MOI value in SW changes in all categories. I have everything lined up and oriented just as it's supposed to be. Now if I take the weight and balance the system out, SW will output a uniform MOI in the Mass Properties dialog box regardless if it's in position 1 or 2. So how can that be?

15. Apr 17, 2017

### Pinon1977

I am not 100% certain what you are referring to as a floating point? I will do my best to try and provide more details if you can elaborate a little further. Thanks.

16. Apr 17, 2017

### Pinon1977

I think you may be on to something with that statement, sir. Reason being is that's exactly how it's behaving. It's almost as if it providing torque values relative to the weight's position in space.

17. Apr 17, 2017

### Pinon1977

So what does "net eccentric mass" entail? How is it calculated or considered in such a situation?

18. Apr 17, 2017

### Pinon1977

Yes I checked that as well. Everything is positioned just as it should be (and as I always do). It's really strange.

19. Apr 17, 2017

### sophiecentaur

Have you actually calculated this by hand, to see which one is 'wrong'? The calculation is very easy.

20. Apr 17, 2017

### Pinon1977

I=mr² or I= 3.37 kg•m² for the 5lb mass at 48" and .37 kg•m² for the Mass at 16". The total MOI should be 3.74 kg•m². Correct? Well Solid Works produces a MOI that varies depending on the location of the Mass as it moves from position 1 to position 2. The greatest MOI values are witnessed at position 1 and steadily decreasing as it moves to position 2. Now, if I take the same weight and make it balanced left to right , Solid Works calculate a consistent MOI value regardless of position. What do you think about that one comment where the guy mentions a "net eccentric mass"?