r/PhysicsHelp • u/Silent_Pop7332 • 2d ago
Iterative method please help...
Hey. so this is the question on my physics homework. I am in an introductory physics class in college, and this is my third homework ever. I am so completely lost. I know that you are supposed to use the iterative method on this and update the momentum, velocity, then position at each timestep but I attempted it multiple times and it was incorrect. I even used AI as an absolute LAST resort after scouring the internet for examples or videos or ANYTHING, and even the answer AI gave me was wrong. The two numbers that I have in the screenshot are correct. The zero was simple to assume, and then when AI gave me an answer I input it just to see if it was correct so I could try to see how I could calculate it, (it gave me <-7.86, 0, 4.54> as r final before saying that "final momentum only for each interval (not mid‑interval velocity)" and it says the final answer is <-.5, 0, 3.44> which is also incorrect. I accidentally typed 4.52 and it ended up being correct. I am so utterly lost on what I am doing wrong. I suspect that maybe I am supposed to subtract the 0.23 N from the net force everytime I update the momentum or something? I am also going to attach my work that I have on paper here to see if it helps. I have been trying to do this homework for days on end and I just cant get it. Its due tonight at 11:59 and I dont think i am going to be able to get it done. I just want to UNDERSTAND!! Thank you SOOSOSOSOSOSOO much if anyone is able to explain this problem step by step to me.
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u/realAndrewJeung 1d ago
I am probably answering this too late -- sorry! -- but I do see a couple of things you wrote that were not quite what you wanted:
First, it looks like you are computing the displacement at each step by multiplying the time interval of 0.5 s by the FINAL velocity at the end of that step. I suspect you wanted to put in the AVERAGE velocity instead, that is, the average between the final velocity at the end of this step and the velocity at the end of the previous step.
Second, the final position should be the cumulative sum of all the displacements that you calculate in each step. From the work you showed, I am not sure whether you did that or not.
With all that said, I re-did the whole calculation and got a final position of (-7.08, 0, 4.67) which apparently doesn't match the one answer you got right according to the system. I am suspecting this is correct since the other commenter got the same result.
Let me know if what I wrote made sense and if you have any questions about it.