What is causing it to oscillate? This isn’t a pendulum we’ve got here.
So you would say that “light” implies negligible, but “moves easily” does not imply frictionless?
Do I take it that in your view an eleven year old should assume (without being told) that string in problems is massless and inextensible and that pulleys are frictionless? I’m not saying you are wrong. I’m genuinely looking for your answer. To put it another way, if the examinee gives an answer that does not rely on these assumptions, is he wrong?
Let’s assume an ordinary amount of friction in the pulley, and string of typical weight and extensibility, then. All the more reason why it won’t move - any slight imbalance caused by the difference in the weight of the string (tiny in comparison to the weights) is swamped by the friction in the system.
This.
This is a test for 11-year-olds?
In the real world, this could never work. Everyone knows you have to put the string over the pulley first, and then tie the masses together.
There are only two possible answers, B and D. The weights either stay where they are, or mass B (the one at the lower position) goes lower and the Mass A goes higher.
If the string is massless, and we ignore variations in gravity due to height, the forces are identical on either side, so there is no motion.
If the string has mass, and/or we include gravity variation, the downward force on the side with Mass B is greater, so that side goes down.
If we assume friction in the pulley, all that does is increase the threshold the differences in force need to overcome before the Mass B can go further down.
For the test, I’d answer B.
One other reason I’d choose B is that answer B indicates a direct understanding of Newton’s First Law. The forces exactly cancel out, so the objects at rest will stay at rest. Test questions tend to be written around specific concepts, rather than setting up random scenarios for the student to figure out the possible outcomes.
Answer D, while reasonable, does not clearly define a single scientific concept, it’s more likely that someone can answer this way just from observing a rope thrown over a tree limb, the long side comes down while the short side goes up.
Heh, I was coming round to Malacandra’s pragmatic POV and then I read your post and remain as ambivalent as ever.
So what is the ‘official’ answer?
Yes of course the motor is needed to overcome inertia and set the system in motion. What I meant was, if somebody called an empty elevator down from the 50th floor, causing the counterweight to zoom up to the top of the building, the counter weight would be almost useless if answer D were correct. Even more so once people start piling into the elevator. The counterweight weighs the same whether it’s on the ground floor or the 50th floor.
And I am not going to consider silly things like a few micrograms of string. The question is clearly phrased (“a light piece of string”) so that we can ignore the mass of the string. And such a tiny imbalance would, I think, be nowhere near enough to overcome the inertia of the system and get the weights moving.
I still say B.
I have a degree in Physics, and I approve Cheesesteak’s message. Best answer so far.
I think it is a great question and worded as best it can be for an 11 YO.
The answer is clearly B. Bozuit’s cite explains this.
You can’t expect an 11YO to know what “massless” or “frictionless” means. Yeah, most do, but this is a science test, not a vocabulary test. If they called it a “massless string” and “frictionless pulley”, not only would you have kids asking what these words mean (or penalize them if they can’t ask questions), but they would be accused of having a meaningless question on the test (there is no such thing as a “massless string”). No matter how they phrase the question, they will get complaints.
By using the wording they have, they get their point across without using these words. They are trying to see if the kids understand the concept that the weights would not move, regardless of their starting position, if they are balanced. They don’t want to get drawn into a meaningless discussion on where they got this “massless string” or the “frictionless pulley” or where someone might be able to obtain such items. They do word the question is such a way that if you were to construct the device yourself, you have the information needed to do it and you would get the correct answer.
B
excavating (for a mind)
Dude, you think eleven year olds who are smart enough to answer this question couldn’t understand the words “ignore the mass of the string”?
On third thought, there’s another reason it can’t be D. If this is a “real world” problem, you can’t guarantee that D will happen without explicitly defining the weight (and length) of the string and the friction of the pulley, for your AP Physics student to calculate the variety of forces needed to determine the outcome.
Since D is uncertain without additional information that is pointedly not provided, I don’t think that can be the test answer. Answer B is certain, once you make a single, typical, assumption about the scenario, that the string’s weight is to be ignored.
According to the test writer? Gotta be B.
If the weights move where is the unbalanced force coming from?
I hated these sort of questions because an argument could be made for D. That weight weighs more since it is closer to the Earth’s center of gravity (inverse square law to address cheesesteak’s point of using a single scientific concept) yet on a multiple choice you never get to explain WHY you chose a particular answer.
If it helps, when I was at school, especially in earlier physics lessons, we would have probably assumed frictionless pulleys and massless strings. The important thing would have been how the two masses affected one another. We would ignore air resistance in the same way, and g was always 9.81. In my limited experience of physics, you’re almost always ignoring some factor or using approximations somewhere.
??
B!
Bet money on it!
I once had a roommate who was a physics major, and I took physics in high school, so clearly my credentials are beyond reproach.
B.
F=ma
When they were first hung at the same height, they didn’t move, and the force on m[sub]1[/sub] was:
F[sub]1[/sub]=m[sub]1[/sub](9.8 m/sec[sup]2[/sup]).
The force on m[sub]2[/sub] was: F[sub]2[/sub]=m[sub]2[/sub](9.8 m/sec[sup]2[/sup]).
And since they didn’t move, m[sub]1[/sub](9.8 m/sec[sup]2[/sup]) = m[sub]2[/sub](9.8 m/sec[sup]2[/sup]).
Now we move the masses. Unless we count the extra weight of the “light” string or the extremely tiny difference in gravity on one side, both of which have got to be extremely close to zero, the same equation is used:
F[sub]1-new[/sub] = m[sub]1[/sub](9.8 m/sec[sup]2[/sup])
F[sub]2-new[/sub] = m[sub]2[/sub](9.8 m/sec[sup]2[/sup])
And we know that m[sub]1[/sub](9.8 m/sec[sup]2[/sup]) = m[sub]2[/sub](9.8 m/sec[sup]2[/sup]). So we know that F[sub]1-new[/sub] = F[sub]2-new[/sub].
So the answer is B.
Am I close?
I have a degree in Mathematics (including sixty credits in mechanics) and I approve **Cheesesteak’**s message.
Close. You’ve calculated the forces on the masses if there were no string or pulley. 
If it was on a treadmill would that change anything?