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Which weight will lift first as the rope is pulled?

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Re: Which weight will lift first as the rope is pulled?

#51

Post by Retric »

lazyant wrote: post #26
lazyant wrote: post #25

The length of the rope is constant. For every weigh with a pulley the weigh moves 1/2 length unit for every length unit it's being pulled. Weights don't matter; C will go up first, then B then A (at a relative 1/2 length ratio).

funny the right answer being downvoted without reason

You had the right answer for one set of assumptions. But, at the extreme weight is important EX: if the numbers where solar masses then gravity would suck them into each other.

If they pulley’s had sufficient friction C may move first. Or if you pull a real rope fast enough C will also move first (think KM/s speeds). Etc.

Re: Which weight will lift first as the rope is pulled?

#52

Post by loup-vaillant »

lazyant wrote: post #48
cynicalkane wrote: post #45

Earlier quoted context omitted.

The tension in the rope is greater than 30lbs, yet only one weight moves? I give up.

sorry; I made an edit with a clarification. Basically if the weights are at rest on the floor then mkn's answer is the correct one. If they are at rest on the air, then my answer is the correct one.

No: http://news.ycombinator.com/item?id=1659676

(this comment assumes that the weights are unsupported).

Re: Which weight will lift first as the rope is pulled?

#53

Post by tankenmate »

tankenmate wrote: post #11

I'm with Rider, if you view it as a "classic" physics class problem, ie weightless ropes and pulleys (and hence no angular inertia for the pulleys, and no inertia due to the ropes), and no friction, weight A would rise even without pulling the rope, weight B would remain static, and weight C would drop at the same rate that weight A would go up. If you pulled on the rope you could never make weight C go up faster tha…

If you go with the weights on the floor model, then barrkel's comment a few below is correct.

Re: Which weight will lift first as the rope is pulled?

#54

Post by Dylan16807 »

mcn wrote: post #35
RiderOfGiraffes wrote: post #2

If the system is frictionless, the rope is weightless, and the weights are not supported, then the lighter weight will rise and the heavier weight(s) will fall. Therefore if the rope is pulled very slowly, the lighter weight will rise first. If the rope really is weightless and the pulleys really are fictionless (and inertialess) then it doesn't matter how hard or fast you pull, the lighter weight will rise first. Th…

>If the rope really is weightless and the pulleys really are fictionless (and inertialess) then it doesn't matter how hard or fast you pull, the lighter weight will rise first. I'm considering a thought experiments that make me believe that this is not the whole story. Imagine a two weight system set up similar to the original diagram in which the weights are the same weight, and the gravity is very little. Yanking o…

If you take the rope from 0T to >30T instantly, then you will have all the weights lift up. As long as you keep the rope tense, they will all rise with different accelerations until they hit the top.

If you apply tension over time, even very quickly, then the lighter weight will rise first.

Re: Which weight will lift first as the rope is pulled?

#55

Post by [deleted] »

hop wrote: post #28
EventHorizon wrote: post #27

Earlier quoted context omitted.

Could it not be in a meta stable equilibrium like this if the weights were of equal mass? The numbers of the weights are unit-less. We assume they represent the mass, but for all I know the number is the serial number of the weight.

For all we know, it could be in space, they could be heavily magnetized, 100m/s wind, .5 CoF in the pulleys, the rope could stretch, and the system could be traveling near the speed of light.

[deleted]

Re: Which weight will lift first as the rope is pulled?

#56

Post by henrikschroder »

brlewis wrote: post #17
RiderOfGiraffes wrote: post #2

If the system is frictionless, the rope is weightless, and the weights are not supported, then the lighter weight will rise and the heavier weight(s) will fall. Therefore if the rope is pulled very slowly, the lighter weight will rise first. If the rope really is weightless and the pulleys really are fictionless (and inertialess) then it doesn't matter how hard or fast you pull, the lighter weight will rise first. Th…

Your conclusion is incorrect given your assumptions. You are assuming no floor, in which case all the weights will rise. Only the man will fall. I think the problem was intended to include a floor that simply isn't drawn.

Not if he weighs less than 60kg. :-)

Re: Which weight will lift first as the rope is pulled?

#57

Post by lazyant »

loup-vaillant wrote: post #52
lazyant wrote: post #48

Earlier quoted context omitted.

sorry; I made an edit with a clarification. Basically if the weights are at rest on the floor then mkn's answer is the correct one. If they are at rest on the air, then my answer is the correct one.

No: http://news.ycombinator.com/item?id=1659676 (this comment assumes that the weights are unsupported).

I supposed the system was at equilibrium without making the numbers. If it's not (as it seems) then sure, my reasoning is invalid and he's right, no problem.

Re: Which weight will lift first as the rope is pulled?

#58

Post by csmeder »

psycocoffey wrote: post #15
cynicalkane wrote: post #3

Well, the diagram is impossible unless the weights are all resting on the floor, so let's assume they are. This system will find a steady state only at a local minimum of potential energy, so the lightest weight will be lifted before the heavier two get off the ground.

Assumptions: pulleys: idealized, massless, frictionless rope: idealized, massless, doesn't stretch weights: resting on ground. As tension is applied to the rope, Weight A will be lifted first, until it is lifted to the ceiling. Then Weight B, and finally Weight A. It helps to visualize Weight A as being massless. In that case, there would just be extra slack in the rope, and B&C would not move until the slack was tak…

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Re: Which weight will lift first as the rope is pulled?

#59

Post by [deleted] »

mcn wrote: post #35
RiderOfGiraffes wrote: post #2

If the system is frictionless, the rope is weightless, and the weights are not supported, then the lighter weight will rise and the heavier weight(s) will fall. Therefore if the rope is pulled very slowly, the lighter weight will rise first. If the rope really is weightless and the pulleys really are fictionless (and inertialess) then it doesn't matter how hard or fast you pull, the lighter weight will rise first. Th…

>If the rope really is weightless and the pulleys really are fictionless (and inertialess) then it doesn't matter how hard or fast you pull, the lighter weight will rise first. I'm considering a thought experiments that make me believe that this is not the whole story. Imagine a two weight system set up similar to the original diagram in which the weights are the same weight, and the gravity is very little. Yanking o…

[deleted]

Re: Which weight will lift first as the rope is pulled?

#60

Post by lookACamel »

lazyant wrote: post #41
mkn wrote: post #34

I loved these pulley problems in statics. Assume the weights are resting on a surface, the pulleys and rope are massless, the pulleys are frictionless, and the system is maintained in quasi-equilibrium as the rope is pulled (steady state & small accelerations). In this case, the tension T in the rope is constant everywhere. Now, take a horizontal section through the ropes. ("Cut" them and replace the missing portions…

"When ... T = 10, weight A begins to rise". No; the Tension the guy is supporting at rest is bigger than 10 (there are 2 other weights) so at T = 10 he would be moving backwards. He needs to apply a bit more than the tension at rest. (edit: this is for when all 3 weights are in the air, I see now some people see them in a floor that is not drawn)

There must be a floor ... or maybe the man is floating!
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