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 than weight A until it hit the top and stopped moving. So the pulling bit is a red herring, as weight A would always hit the top before the other two, either pulling or holding it still. Assuming the weights are in kilograms you would have to let the rope go at a rate of 3.13 m/s (11.27 kph, 7 mph) to prevent weight A from rising.
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?
#12Its not a static system; the question is unfair.
Why does that make it unfair?
The diagram has omissions. Either:
1. There is a floor that the weights (and the man) are standing on.
2. Weight C is dropping whether he pulls the rope or not.
I think that the diagram is meant to show all the weights resting on a floor, and all the confusion is due to misunderstanding of that. In the other case, the diagram neglects to mention something to the effect of "supports have just been removed".
Re: Which weight will lift first as the rope is pulled?
#13Its not a static system; the question is unfair.
Why does that make it unfair?
Is the person currently pulling, are the weights on the ground, are the weigths currently moving to get into a stable position? Friction, weight of cord, acceleration, etc. etc. Any combination of the previous?
Depending on your level with maths/physics you'll probably give different answers and make different assumptions.
Re: Which weight will lift first as the rope is pulled?
#14This tricks people because it doesn't start out in static equilibrium. For it to look like this, the weights would have to be resting on a table to relieve the tension in the rope.
If the man does nothing, the heaviest weight will fall and the lightest will rise. If it's frictionless and he starts pulling, the same thing will happen, only the lengths will lessen.
Re: Which weight will lift first as the rope is pulled?
#15Well, 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.
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 taken in.
Re: Which weight will lift first as the rope is pulled?
#16[deleted]
Re: Which weight will lift first as the rope is pulled?
#17If 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.
Re: Which weight will lift first as the rope is pulled?
#18This reminds me of the airplane on the conveyor belt, in that the only confusion comes from the question being insufficiently specified.
- Friction of the pulleys
- Mass of the pulleys
- Moment of inertia of the pulleys
- Mass of the rope
- Unit of mass of the weights
- Is there a surface that the weights are resting on?
- What's the local gravity like?
- Others
Re: Which weight will lift first as the rope is pulled?
#19[deleted]
Re: Which weight will lift first as the rope is pulled?
#20If 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.
Do you really want me to write the full six page analysis? No, I didn't think so. Unfortunately, concise replies such as the one I gave and which require the reader to think a little often result in replies such as yours where it is difficult to discern whether you are being deliberately clever, deliberately trollish, or genuinely confused.
Let me expand.
I only assumed the weights were unsupported, not the man. I did that to assist the reader in understanding the analysis. I did not assume the man was unsupported - I had hoped my initial description of what happens implied that. Possibly it didn't.
I would expect that the problem is intended to include the floor - that's not my point. Having made the analysis for the unsupported weights, the evolution of the situation when there is a floor becomes obvious.
So I hope you were joking, and not a troll.