I would love to see a video demonstration of the answer (my intuition was apparently wrong which would make a video that much more entertaining).
Which weight will lift first as the rope is pulled?
21–30 of 63 posts
Re: Which weight will lift first as the rope is pulled?
#22Well, 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…
* Then Weight B, and finally Weight A.*
I believe you meant C, correct?
Re: Which weight will lift first as the rope is pulled?
#23Earlier quoted context omitted.
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.
In physics problems things are assumed idealized unless they aren't. Also, velocity doesn't matter here, and no math is required to solve the problem.
Re: Which weight will lift first as the rope is pulled?
#24None. Instead he'll pull the ceiling down as they failed to screw it into the studs. ;-)
There's an elephant in the way?
Re: Which weight will lift first as the rope is pulled?
#25The 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).
Re: Which weight will lift first as the rope is pulled?
#26The 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
Re: Which weight will lift first as the rope is pulled?
#27This 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.
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.
Re: Which weight will lift first as the rope is pulled?
#28This 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.
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.
Re: Which weight will lift first as the rope is pulled?
#29Well, 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.
huh.. i didn't think of them resting on the ground and thought it was a trick question.
Re: Which weight will lift first as the rope is pulled?
#30The 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 go against the consensus without a solid explanation. For instance, why don't the weights matter? Would they matter if they were respectively 1mg, 1kg, and 1megaton?