# Tricky physics question

Discussion in 'Physics' started by redacejr, May 30, 2008.

1. ### redacejr Thread Starter Active Member

Apr 22, 2008
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If a 3400rpm 3v motor is going at full speed with its terminals slowly heating up due to friction, at what MACH is an unbalanced weight at the end travelling??

2. ### Papabravo Expert

Feb 24, 2006
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1,850
Need the diameter of the shaft or the radial distance of the weight from the center of rotation. Are you suggesting that it might be going supersonic?

3. ### Caveman Active Member

Apr 15, 2008
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Sounds like a trick question. Technically, you need the fluid that the motor is travelling through as well to determine it's mach number.

4. ### triggernum5 Active Member

May 4, 2008
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So 3V saturates the field at 3400rpm.. Thats info about the electrical properties of the coil, which can give you the current, which can give you the power, which can be calculated against the angular velocity to give you the moment of inertia.. But I get stuck there since different shapes of different lengths can yield the same moment of inertia.. Or are we to assume the typical bar's mr^2?

5. ### beenthere Retired Moderator

Apr 20, 2004
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The terminals "slowly heating due to friction" sounds like the shaft might be stationary while the motor revolves around it. It's a clue of some sort, though.

How does one define "unbalanced weight at the end", though? End of what?

6. ### triggernum5 Active Member

May 4, 2008
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I overlooked the point that they were heating from friction, assumed resistance.. Would magnetostriction count as friction? If the question implies mechanical friction than I can't see how any bounds/parameters can be derived since its unquantified, and heat is generated by any motion out of vacuum..

7. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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3400*60*pi*D/Vs where Vs is the speed of sound in length per second in the surrounding media, and D is the D is the diameter in same units length of the path traveled by the weight.

8. ### triggernum5 Active Member

May 4, 2008
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In that context its more of a homework question than electronics forum puzzle..

9. ### redacejr Thread Starter Active Member

Apr 22, 2008
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HINT :

the speed is below MACH1
the shaft diamiter is not needed
it is very tricky
the motor is HORIZONTAL!!!!

10. ### redacejr Thread Starter Active Member

Apr 22, 2008
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unbalanced as a p[iece of weight that is attached to the shaft NOT AT ITS C.O.G

11. ### recca02 Senior Member

Apr 2, 2007
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Not that I claim that I'll be able to solve this one, but

Is it not required to know the type of motor? AC/DC?
What does it mean by "terminals" here? The supply terminals?(brushes?)

Perhaps a small rough schematic will also help.

12. ### triggernum5 Active Member

May 4, 2008
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I'll be the first to admit that your hints were not sufficient (as of yet atleast)..
The only way I can see the horizontal fact being relevant is if gravity also involved in the calculation as a component of SHM equations..

13. ### thingmaker3 Retired Moderator

May 16, 2005
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Okay. I get it now. Zero is the answer.

14. ### redacejr Thread Starter Active Member

Apr 22, 2008
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OMG the closest guy was very far away from the answer.
immagine a horizontal 3v dc motor with an unbalaced weight.... this would make the motor do extra effort. THERE IS NO SPECIFIC MACH!!!!! the motor is slowing down until 0 rpm.... now youre saying... damn that was so easy... the numbers are all useless

15. ### redacejr Thread Starter Active Member

Apr 22, 2008
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and at the moment the motor is switched on the weight is at 0.34 mach

16. ### thingmaker3 Retired Moderator

May 16, 2005
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The above makes no sense.

17. ### triggernum5 Active Member

May 4, 2008
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Alot of it doesn't make sense.. What if the armature was a mile long, at 3400rpm, that would be waaay more than mach 1..
Your question asked us to give an answer in terms of velocity relative to sound, and you stressed that there was infact an answer.. As far as I'm concerned, there is no answer according to your more recent explanation.. Besides, explain to me how anything with greater than 0 length (required for an unbalanced load) could be stationary (Mach 0) at '3400rpm'..
What a bummer, I was expecting something cool..

18. ### redacejr Thread Starter Active Member

Apr 22, 2008
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look, its not i who wrote this question and the guy who gave me the answer is a friend. i just thought it was an interesting question to post and it ended up in armagaddeon =.=

the motor is not travveling at one specific speed (mach) but rather constantly changing its speed due to frictional heat. (well thats wt my firend said)

there is no other explenation for it... i wrote all the details and if someone figures it out in another way please tell me XD

and also i agree that the horizontal motor detail should be a clue but i cant figure it out

19. ### redacejr Thread Starter Active Member

Apr 22, 2008
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and the guy posting above... whe i said there is an answer i meant theat there should be one as every question has an answer .... got it and IM NOT SAYING IT IS TRAVELLING AT 0 MACH!!!!.... ITS JUST SLOWING DOWN

Last edited: Jun 2, 2008