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# Rotational Mechanics watch

1. Need some help

comoving_frame
2. (Original post by TeeEm)
Need some help

comoving_frame
Am I being stupid?

Spoiler:
Show
The mass has lost of PE when it comes to rest, which can only be converted into the work against the couple. This turns through so
3. (Original post by atsruser)
Am I being stupid?
Spoiler:
Show
The mass has lost of PE when it comes to rest, which can only be converted into the work against the couple. This turns through so
have I mistyped?
4. (Original post by atsruser)
Am I being stupid?
Spoiler:
Show
The mass has lost of PE when it comes to rest, which can only be converted into the work against the couple. This turns through so
there is energy lost as the string gets taut and the pulley is set in motion
5. (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.

(*)
Spoiler:
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conservation of AM to get initial of pulley
6. (Original post by TeeEm)
Need some help

comoving_frame
Spoiler:
Show
?
7. (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.

(*)
Spoiler:
Show
conservation of AM to get initial of pulley
correct plus the speed of the particle post jerking
8. (Original post by atsruser)
Spoiler:
Show
?
I got different fraction but I could easily have made a mistake.
(I get free proof reading here)
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?
10. (Original post by depymak)
11. (Original post by TeeEm)
I got different fraction but I could easily have made a mistake.
(I get free proof reading here)
Spoiler:
Show
I get the velocity of the mass after the jerk to be

and the equation of motion for the pulley after the jerk to be:

with initial angular velocity

The deceleration of the mass is via SUVAT and we have

I'll leave you to check this and compare to your version.
12. (Original post by atsruser)
Spoiler:
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I get the velocity of the mass after the jerk to be

and the equation of motion for the pulley after the jerk to be:

with initial angular velocity

The deceleration of the mass is via SUVAT and we have

I'll leave you to check this and compare to your version.
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)
13. my proposal is as follows:
KFINAL-KINITIAL=WNET
WC=2aCΔθ
Δθ=2h/a
14. (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
15. (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
16. (Original post by depymak)
not sure though but ..in a while
no worries
17. let me know your thoughts.
18. (Original post by depymak)
overly simplified

there are three parts
Before getting taut
The instant that it gets taut
the motion after
19. (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?
20. (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

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