What is conserved in collisions: linear momentum or angular momentum?

[jbriggs444]

It is no riddle, the ball will move in a north-east direction and the angle will be determined by a parallelogram and the M/m ratio , and the its speed and KE will increase accordingly. What is the problem?

The cue ball struck the target ball at right angles to its path. How can work have been done if the force applied and the direction of movement were at right angles. That is the linear analogue of the conundrum that you present with gyroscopes.

Now suppose your ball hits perpendicularly the edge of a spinning wheel on gimbals like in video #18, it starts to rotate or not, does it gain speed and KE in that direction or not, does the total KE of the gyro increase or not, has work been done or not?

I'm not responding to this until you can answer the riddle.

 

[bobie]

First, you need to try to work it yourself..

post#12

Please refer to DaleSpam's post 21.

My patience has been exhausted. .

Try to answer this riddle before we come back to jerks and gyroscopes.

I'm not responding to this until you can answer the riddle.

I think your attitude toward me is rather patronizing, jbriggs, to use an understatement.
Surely it is not constructive.
This unwieldy thread might have ended at post #11 if you had given me a direct, exaustive reply (we are not in the HW forum).
And I think it is not fair that you ask for my (stupid, I know) opinion/explanation first and then you refuse to give yours. And you seem to get patience by fits and starts.

Before you further hurt my feelings, I beg you to read these threads and tell me if I can deal with linear momentum and if you would have been able to give such elegant solutions:

https://www.physicsforums.com/showthread.php?p=4586458#post4586458
https://www.physicsforums.com/showthread.php?t=715553#post4562489

Thanks for your attention

 

[Dale]

I know, you're telling me to wind this up!
Please, just tell me when to stop.
For future posters' sake I'd hate do cause the closure.
Thanks

It would be good to wind this up. In my experience, the longer a thread is the less useful it becomes.

But mostly I am telling you exactly what I said: "study simple cases". You have a strong tendency which I have noticed over multiple threads to ask about a half-dozen random and extremely complicated cases before you have even mastered the simplest case. That simply is not an effective way to learn. If you find a half-dozen cases, then look at them for the simplest one, and ask questions about that one case (never introducing the others) until you fully understand it. And if people tell you that there is an even simpler case you should study first, then do that. Approach learning step-by-step, instead of trying to run before you can even crawl.

 

[Dale]

I have repeatedly and clearly explained what I mean :
if an object is rotating in one plane it has k KE, if you make it rotate in 2 different planes it has undeniably KE > k, somebody must have given it some KE and therefore must have spent some energy. Is that vague to you? Is this wrong or arguable in any case?

It is not vague, but it is wrong. Use the 4th formula I posted in 72. Calculate the initial KE before the precession. Calculate the final KE after the precession. Compare them. Don't just assume KE>k, actually calculate it.

After you have done that you have learned to crawl, so then try to stand up: Calculate the angular momentum during the precession using the 2nd formula (this is the hardest step and I will be glad to help). Remember that the torque is, at all times, perpendicular to the axis of rotation.

Then, once you are comfortable with that, try to walk: Calculate the work done during the precession by using the 3rd formula on the results from the last part. Compare that to the results you got in the first part.

 

[jbriggs444]

I think your attitude toward me is rather patronizing, jbriggs, to use an understatement.

It is not intentional. My approach to problem solving is to simplify, simplify, simplify. To distill problems down to the simplest level possible. When conversing with someone and failing to have a meeting of the minds, my tendency is to do the same thing -- to simplify down to a level where we can find some common ground. In doing so, the appearance may be that I am underestimating the intelligence and background knowledge of my correspondent. For that, I apologize.

I have not tried to characterize your attitude and will not do so now.

And I think it is not fair that you ask for my (stupid, I know) opinion/explanation first and then you refuse to give yours.

When I've given what seem to me to be patient explanations, you have seemed not to acknowledge the points being made.

And you seem to get patience by fits and starts.

A fair assessment, certainly.

I beg you to address the riddle. A pool ball is subject to an impulsive collision at right angles to its path.

It is clear that its kinetic energy has increased. Since the impulse was at right angles, it seems clear that no work can have been done on the ball. We are assured by the work energy theorem that if no work has been done then kinetic energy cannot increase. And yet the kinetic energy has increased.

How can one reconcile these things?

That is the simple problem that contains (my guess at) the kernel of the misunderstanding that we are currently faced with.

 

[Dale]

My approach to problem solving is to simplify, simplify, simplify.

An approach that I endorse also.

 

[A.T.]

This example is not appropriate, because the body is already moving, and you are not increasing its speed or KE (same applies to orbits, etc.).

This objection is not appropriate, because the spinning gyro is also already moving, and a torque perpendicular to its angular velocity doesn't increase its KE. Just like a force perpendicular to linear velocity doesn't increase KE.

if an object is rotating in one plane it has k KE, if you make it rotate in 2 different planes it has undeniably KE > k, somebody must have given it some KE and therefore must have spent some energy. Is that vague to you? Is this wrong or arguable in any case?

Yes it is wrong. In particular the "undeniably" & "must" part.

 

[Dale]

Since this thread is getting a little tense, and has already gone over 100 posts, the other mentors and I have decided that it is past time to close it.

Bobie, please look at the great advice that you have received from many people and try to actually work out some of the details on the very simplest possible scenarios. Don't assume that you know the answer until you have actually worked out the math.