I’m assuming you and newme have the intended answer. It makes sense now that I see the logic in what you’ve done. The rest of the questions on the exam were pretty straightforward so I was a bit thrown by this one. As I said, my son lost just one point on this so this wasn’t the biggest issue he had on the test.
Thanks everyone for the efforts. I’m glad I’m not the only one that was confused by this.
One thing I would suggest for your son is that he do a “sanity check” on his answers. You write that he gives a final answer that’s a higher force than the weight of the block. Is that possible?
And when you do such a sanity check and find that it doesn’t make sense, try to find where your mistake is. And if you can’t find it, then write in your answer that you think you made a mistake but can’t find it, and why you think you made a mistake. Most teachers will grade more mercifully in such a case than where the student is completely oblivious (certainly I will).
On the specific question in this thread, I would assume the boxes to be weights in the middle of the ropes… But that makes this way too long and ugly a question for a test. At the high school level, it’s even pushing the limits of a good homework question. Yes, the students should be able to do it, but you can test their ability to do it with a question that doesn’t burn so much of their time.
If the boxes are assumed to be weights, the answer does pass the sanity check. But you are right, looking at an answer and asking if it makes sense is a good practice. As I said, he was given 4/5 on the problem so even if he did the sanity check and the pointed out that it doesn’t make sense, he likely wouldn’t have improved his score. In this case.
I have used that technique on a college level optics course. I was given points back because I did the sanity check, so I agree that is probably something most professors/teachers would reward.
Not with standard 11-th grade physicsland massless ropes and frictionless pulleys. A massless rope going over frictionless pulleys will have the same tension everywhere. A (massless) spring will have the same tension on both sides, too.
So my best interpretation of the diagram is that the boxes are weights attached to the rope (which is what the person who took the class seems to think, so that’s a pretty good confirmation). And I concur with newme’s solution.
It’s been a while, but this doesn’t seem too advanced for say AP physics, assuming the class has covered problems similar enough that the student isn’t trying to figure out what the boxes are supposed to mean, etc. You do have a system of three variables in three equations, but it’s degenerate with simple substitutions, so you don’t need advanced linear algebra to figure it out.