# B A stick in space

1. Dec 23, 2017

### Erunanethiel

Which one has the greater angular acceleration given an impulse on one end?

2. Dec 23, 2017

### jbriggs444

Which one has the greater moment of inertia?

3. Dec 23, 2017

### Erunanethiel

As far as I know, moment of inertia is the distance between the point the force is applied and the point where the object rotates around. In that case, it wouldn't be any different between the two.

4. Dec 23, 2017

### jbriggs444

The [perpendicular] distance between the point where a force is applied and the axis of rotation is the "moment arm". If you multiply the magnitude of the force by the length of the moment arm, the result is "torque".

The moment of inertia is a measure of how an object will accelerate under an applied torque. The greater the moment of inertia, the more torque (or the more time) it will take go achieve a particular rotation rate. It can be computed by considering an object as a bunch of tiny pieces, taking the mass of each piece multiplied by the square of its distance from the axis of rotation and adding up all of those results.

An object with a large mass or with a mass that is very spread out will have a high moment of inertia. It will take a lot of torque to change its rotation speed.

You would be well served to try to educate yourself on rotational mechanics. https://physics.info/rotational-dynamics/

5. Dec 23, 2017

### Erunanethiel

Yes and that is not different between the heavy ball with two sticks and the two heavy balls joined by a stick is it?

The link is not working, it gives and error

6. Dec 23, 2017

### jbriggs444

Excuse me? What is not different? And the link works fine for me.

7. Dec 23, 2017

### Erunanethiel

The perpendicular distance between the point the force is applied and the point the objects rotates around (center of mass) given the centers of mass of the objects are in the same place

8. Dec 23, 2017

### jbriggs444

That is the "moment arm" and is indeed the same. But what about the "moment of inertia"?

9. Dec 23, 2017

### Erunanethiel

Okay I get it know thank you!

By the way, do we agree the answer to the original question is "the middle"?

10. Dec 23, 2017

### jbriggs444

The original question is not well enough posed to have a definite answer. But yes, for a reasonable understanding of the intended question, the answer is "the middle".

11. Dec 23, 2017

### Erunanethiel

PeroK put it way better than I did:

When you give an impulse to one end of the rod, the COM moves forward and the object rotates around the COM. The initial velocity of the other end if the rod is the sum of the forward velocity of the COM and a backwards velocity from the rotation.

Question: what mass distribution will result in the other end of the rod initially moving backwards from its starting position with the greatest speed?

12. Dec 23, 2017

### jbriggs444

Answer: undefined. There is no optimum. The distribution with all of the mass exactly in the center results in an undefined speed and, accordingly, is ineligible.

13. Dec 23, 2017

### Erunanethiel

Do you mean that the location of center of mass does not matter for this, the only thing that does is how "centralized" the mass is to CoM, irrespective of it's location. In short: Make every where that is not CoM make as light as possible.

Correct?

14. Dec 23, 2017

### jbriggs444

The location of the center of mass relative to the ends matters. But so does the degree to which the mass is centralized. Both are relevant. One can even write an equation.

Try it. Apply an impulse "p" at right angles to one end of a rod of length "l" and mass "m" with center of mass offset "r" from the end where the impulse will be applied. If the rod has a moment of inertia "I", what speed will the other end have as a result?

Edit: Take it a step at a time. For instance, what rotation rate will result from the applied impulse?

15. Dec 23, 2017

### Erunanethiel

Well, not that the other bits have no mass, they are as I said made as light as possible

16. Dec 23, 2017

### Erunanethiel

If the location of the center of mass relative to the ends matters, would the optimum position be in the middle, or closer to the side which the force is applied?

17. Dec 23, 2017

### jbriggs444

Sorry, you are going to have to put some effort of your own into this problem. Until you do, I'm out.