in this Atwood machine digram

atwood machine digram

y1 is the amount that m1 gets displaced and y2 is the amount that m2 gets displaced the lecturer then wrote $y1=-y2$ , I am trying to understand what did he do to get this, when I tried to write a relation I took y1 and y2 as displacement from the ground the sum of this should give a constant lets name it h $$y1+y2 =h $$ because the length of the rope is a constant if one goes up the other goes down I can then say $y1=h-y2 $ when $y1=0$ mass1 is on the ground and mass 2 distance from the ground is h

in other cases y2 and y1 I can play around with them but now I have a bit of a visual idea of the amount so this image does fit my process the best

enter image description here

now what I could deduce is that the lecturer took an x axis where the two masses line up so I went and lifted up the axis to the middle
I checked by saying when y1=0 y2=0 so they should both be on the x axis

so is this simply what the lecturer did , if yes can you share how would you analyse the situation, or possibly a different problem when getting stuck thanks


1 Answer 1


You are right in that the lecturer chose a reference such that $h=0$. Which is good sense, since this choice simplifies the problem; no need to have an $h$ floating around everywhere. When stuck, it always a good idea to do as you did, and monkey around; try one's own coordinates, test the one's used by the lecturer, etc.

A favorite approach of mine is to think about the problem right before going to bed for the night, and when arising in the morning: work on the problem. I am amazed at how often the solution is forthcoming under these circumstances. I am told that there is good scientific evidence to support this strategy.

I also think it is important to resist the urge to get help right away, either from individuals or solutions manuals. Yesterday, I broke my own rule and came here to Stack Exchange right away; if I had played around with the problem a bit (means actually doing calculations) then I would have no doubt gotten around my hang up. When we get help too easily, we deprive ourselves crucial experience in problem solving; there is no royal road to becoming a problem solver. However, it is necessary to strike a balance, one can't always accomplish everything oneself, at least not without undue exertion; when pressed for time it is not a bad idea to get some help so as to expedite the learning process.

  • $\begingroup$ thanks for the insight I just noticed how badly phrased the question at the end was yet you managed to answer what I wanted lol ,I will just leave the question without accepting an answer for a bit so that there is a possibility to receive new answers , and then accept one $\endgroup$
    – lodo
    Feb 27 at 14:40
  • $\begingroup$ @lodo Thats a good idea, its always best to wait a bit, you never know what interesting things others will contribute. $\endgroup$ Feb 27 at 14:43
  • $\begingroup$ may I ask If there is a way you would recommend to learn physics I would like to gain some insight from people who have more knowledge in the field , I am currently reading Berkeley physics book which really feels like a gem , I also noticed how I rarely solve problems outside of class because of the number of courses I have this semester and how the uni made the semester shorter yet with same amount of materials and I am trying to fix this since application of knowledge is a must but $\endgroup$
    – lodo
    Feb 27 at 14:49
  • 1
    $\begingroup$ You are absolutely correct in your conviction to solve more problems; I cannot stress how important this is to mastering physics, the more you do, the better you will become. If it is possible not to over pressure yourself with a class-load that is too heavy, then reduce the number of classes so that you have more time to devote to physics. Use more than one textbook, if you can use three or four textbooks at the same time, then do so, the insight that is gained from reading multiple authors is inestimable. This last technique has probably helped me more than any one thing. $\endgroup$ Feb 27 at 15:22
  • $\begingroup$ I appreciate the suggestions I already Brought a couple of books home from the library thinking it may help me learn and see a couple of new perspectives but I focused only on one in the end ,I will try using them simultaneously , and I certainly would have reduced the number of classes since my goal is to learn and understand to the best of my ability not just to pass courses or get a degree yet for the first year I need to try and handle the pressure since I want to switch to physics+another major and I need to finish all the mandatory courses to do so ,thank u helped me clear my mind $\endgroup$
    – lodo
    Feb 27 at 16:05

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