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Rotational Mechanics

Need some help
rotation.jpg

@comoving_frame

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Original post by TeeEm
Need some help
rotation.jpg

@comoving_frame


Am I being stupid?

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Reply 2
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Original post by atsruser
Am I being stupid?

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have I mistyped?
Reply 3
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Original post by atsruser
Am I being stupid?

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there is energy lost as the string gets taut and the pulley is set in motion
Original post by TeeEm
there is energy lost as the string gets taut and the pulley is set in motion


Yes, you're right. You can't use conservation of energy here - it's analogous to a completely inelastic collision of two particles, since the "separate" mass and pulley turn into a single system after the string becomes taut. I think I see how to get started though(*), but I'd still need to think about the subsequent motion. Tricky problem.

(*)

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Original post by TeeEm
Need some help
rotation.jpg

@comoving_frame


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Original post by atsruser
Yes, you're right. You can't use conservation of energy here - it's analogous to a completely inelastic collision of two particles, since the "separate" mass and pulley turn into a single system after the string becomes taut. I think I see how to get started though(*), but I'd still need to think about the subsequent motion. Tricky problem.

(*)

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correct plus the speed of the particle post jerking
(edited 8 years ago)
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Original post by atsruser

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I got different fraction but I could easily have made a mistake.
(I get free proof reading here)
what about work-energy principle?
Assuming that the magnitude of the opposing couple is constant (thus its torque), how could we determine its work?
Is the initial rotational kinetic energy zero? What about the final total kinetic energy? What's the total work done on the system?
(edited 8 years ago)
Original post by depymak
what about work-energy principle?


See the comments above already. It's not a perfectly elastic process.
Original post by TeeEm
I got different fraction but I could easily have made a mistake.
(I get free proof reading here)


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Original post by atsruser

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we have to wait for the Mechanics master-kid .
(Hopefully I catch him !!!, as he looks unbeatable at the moment)

I get 4/3 mg (I could be wrong)
my proposal is as follows:
KFINAL-KINITIAL=WNET
WC=2aCΔθ
Δθ=2h/a
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Original post by depymak
my proposal is as follows:
KFINAL-KINITIAL=WNET
WC=2aCΔθ
Δθ=2h/a


my proposal is to do a full solution, as you see it, take a photo and post it
Original post by TeeEm
my proposal is to do a full solution, as you see it, take a photo and post it

not sure though but ..in a while
(edited 8 years ago)
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Original post by depymak
not sure though but ..in a while

no worries
let me know your thoughts.
prop.jpg
Reply 17
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Original post by depymak
let me know your thoughts.
prop.jpg


overly simplified

there are three parts
Before getting taut
The instant that it gets taut
the motion after
Original post by TeeEm
we have to wait for the Mechanics master-kid .
(Hopefully I catch him !!!, as he looks unbeatable at the moment)

I get 4/3 mg (I could be wrong)

Shouldn't the dimensions be Nm and not N?
Reply 19
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Original post by A Slice of Pi
Shouldn't the dimensions be Nm and not N?


put up a solution and will check it ...
I might have missed an a when I typed
(edited 8 years ago)

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