Really, this thread is at this point just re-hashing a pretty comprehensive consensus that had been reached after about the first page: by physics problem *convention * “light” implies “ignore the mass of this”.
If you don’t know/follow such convention/s then light taken literally means something else.
My point exactly. Although I used 9.8 m / sec[sup]2[/sup] in my example, I could have just used G.
It’s not an extra assumption, it’s just undetermined. There is not enough information provided to confirm D as the right answer. We don’t know the mass of the string per cm, we don’t know the friction in the pulley, we don’t know how far the weights are moved, we don’t know the mass of the weights. Once you introduce the string’s mass to the analysis, the outcome is undetermined. If the string is assumed to be massless, the outcome is fully determined.
Ultimately, the whole point of massless strings, frictionless pulleys and no air resistance is to focus the analysis on certain parts of the problem, not to ask what would happen if we setup an experiment with imaginary materials. You can’t add up all the forces that act on this setup at once, you have to break it up into piece parts, net out the forces for each simple part, and combine them at the end.
First you net out the force of gravity on the two masses, then you add in the gravitational force on the string, then add in the friction of the pulley, then add in the air resistance, or buoyancy, or any other factor that comes into play. This question is about getting that first analysis correct, so you (should) tell the student to ignore the other stuff, for now, because they don’t yet know how to deal with it.
I think the problem is in using “light” instead of “massless” to set the bounds of the analysis. It’s not clear enough unless it’s strictly defined somewhere that “light” means “ignore”.
Better learn what the conventions mean, then. I’ve seen it argued in apparent good faith that operator precedence (BIDMAS) only applies “if you follow it”.
But as long as we’re still arguing about this, you might consider what effect the imbalance caused by a few milligrams of string has on a system with a net mass measured in kilograms. A milliNewton of force isn’t going to result in much movement, and then only if the pulleys are really low in friction.
From Physics (3rd Ed.) by Halliday and Resnick (which AFAIK was the standard freshman physics text book at the time (30 years ago :()), example 6:
The next example has the case in the OP
Figure 5-8a shows the blocks at two different heights. They didn’t specify “massless” for the string in this example, but it’s clearly assumed in obtaining a = 0.
I can’t find anything near there about the string mass merely being small instead of zero. Nor in the Wikipedia page on the Atwood machine, or in another physics text book with an example discussing the Atwood machine. So I disagree with your statement that I’m demonstrating “a lack of understanding of the way physics problems are set up.” I seem to have it exactly right.
Although any question on a test is by definition hypothetical, I notice this question seems to be worded very deliberately to describe real world conditions.
“What will happen when Ms Lee lets go of the two masses?”
I think if anybody here were presented the actual apparatus as described, and asked what would happen under those circumstances, the answer would be “B”
In the real world, the friction of the pulleys would most likely be enough to resist movement of the weights if the only imbalance is a tiny length of “light” string.
Tangential question: even if we don’t assume the pulley is massless, that doesn’t change the question either. Granted, a massive pulley would not accelerate as quickly as a massless one, but even a massive (but frictionless) pulley would begin to turn, albeit perhaps very slowly, the instant some net force was applied to it. Yes?
Correct.
OK, then, if we really must consider the string as literally massless, then the problem still does not have the correct answer among its answer choices, as the correct answer is then “the string flies away at the speed of light”.
You should write all the physics text book authors to complain about this glaring problem they all have.
Why would that happen?
And poor Ms. Lee, if it did happen. No matter which direction the string went, if it instantly moved at the speed of light, wouldn’t it displace the air around it? I mean, “massless,” is not “volume-less,” is it? And aerodynamics doesn’t apply to a volume moving that fast – the air would not have time to flow around the string. The leading side of the string would fuse with the air molecules on its way out and irradiate Ms. Lee with gamma rays.
No, because it’s securely anchored to something that does have mass. :smack:
I this it’s written that way because it’s geared toward kids who haven’t studied physics. As such, the wording is unfortunate, because it isn’t clear to someone without a deeper level of understanding.
This reminds me of a science teacher I had in junior high school. Once tests had been graded and recorded, they would be handed back to the students and the teacher would spend the hour discussing each question one by one. If a student felt that an answer that had been marked as incorrect was really correct, the student was given an opportunity to argue the point. I can remember a couple times the teacher conceded the point and adjusted the score of everyone who had chosen the answer that was now judged to be correct. I think I learned as much during the test review as in the lectures that preceded the test. This science teacher would have allowed either B or D as correct answers.