Vibration

[Thrashsmith]

If you had a 2m length of 20mm solid square bar and a 2m legth of 20mm hollow square bar (2.5mm wall for sake of arguement) and you held both vises in mid air so they dangled down and you hit the bottom of them, which would vibrate the most?

 

[MattH]

I was under the impression that a lot of the strength comes from the surfaces so the box would be stiffer.

So I will say the solid will vibrate more.

 

[Screwdriver]

You are putting the same amount of energy into each so they would nominally both "vibrate" by approximately the same amount. But I think what you are asking is which would sustain that vibration for the longest time or the most easily detected by human hearing.

That is "ringing" which largely depends on the number of modes of vibration and there are a lot more modes in a complex shape like a tube. Paradoxically perhaps, those additional modes of vibration willl be competing with each other to dissipate energy and being less rigid than a solid bar, even more energy will also be dissipated as heat.

The solid bar on the other hand has fewer fundamental modes of vibration which, if the size just happens to reinforce, might end up with just a few really good ones and the thing will ring like a bell for ages.

Having just made all that up, I look forwards to hearing how close that seat of the pants guesswork is to the truth

It is a constant source of amazement to me just how complicated "simple" phenomena can be. I'm a big fan of physics girl on YouTube for that very reason. Happy Physicsing!

 

[MattF]

Solid.

 

[Thrashsmith]

Sorry, to clarify: by vibrate I mean move/swing/judder. It's a movement question, not sound question.

 

[Screwdriver]

Need to see a diagram. Not quite sure how these vices end up in the air in the first place.

 

[Ashley Burton]

I don't think there would be much difference tbh. I've got similar stuff in my garage & the only stuff which seems to flex is anything under 10mm diameter.

 

[Parm]

It generally requires consideration of the mass of the object. So if they were both hit with the same amount of force/energy The tube will vibrate the most.

 

[Screwdriver]

The most what though

 

[MattH]

Just run the numbers through an online deflection calc, I think you'll be surprised.

 

[Screwdriver]

Deflection? Vibration? Move? Swing? Judder?

I think the solid bar will be more although the square tube will be greater. The solid will be slightly longer with the tube having an increased level. The solid bar will be slightly more equal with the tube differing by an identical amount.

 

[mike 109444]

Deflection? Vibration? Move? Swing? Judder?

I think the solid bar will be more although the square tube will be greater. The solid will be slightly longer with the tube having an increased level. The solid bar will be slightly more equal with the tube differing by an identical amount.

Which one of your own statements are you answering

 

[Screwdriver]

Which one of your own statements are you answering

Indeed.

 

[Thrashsmith]

Deflection? Vibration? Move? Swing? Judder?

I think the solid bar will be more although the square tube will be greater. The solid will be slightly longer with the tube having an increased level. The solid bar will be slightly more equal with the tube differing by an identical amount.

In a Viv from the Young Ones style I am now mentally hitting you with a cricket bat.

 

[Burdekin]

The hollow as it has the least mass and less stiffness.

 

[MattF]

The hollow as it has the least mass and less stiffness.

Are you sure?

 

[Parm]

Are you sure?

I’m with Burdekin

Hollow things will always resonate more.

 

[Burdekin]

Are you sure?

Less stiff hollow will defect the most so would have the highest amplitude and the extra mass of the solid bar means it will vibrate for less time. That is how I would think it would be.

 

[MattF]

What's the one necessity of a good flywheel, to maintain smooth & prolonged motion? A solid bar will see/suffer the same effect.

 

[Parm]

What's the one necessity of a good flywheel, to maintain smooth & prolonged motion? A solid bar will see/suffer the same effect.

I know what your saying but absolutely not the same thing. The fly wheels deals with momentum.