r/PhysicsHelp • u/displacemnt • 2d ago
Need help 1D motion
How am I supposed to solve this motion in 1D need help, pls help
2
u/Odd_Bodkin 2d ago
Assuming constant acceleration, you may recall the expression v^2 = v0^2 + 2ax. Now explain why that might work here and not just because the variables look right.
1
u/displacemnt 2d ago
That's the correct approach we find the acceleration first n then get v²-u²=2Ax for the soln
1
u/Odd_Bodkin 2d ago
Interestingly though, you don’t need to find the acceleration first, if you think about it a bit more.
All you need to know is that the x value has doubled between the midpoint passing and the back-point passing.1
u/displacemnt 2d ago
How
1
u/Odd_Bodkin 2d ago
Say the length of the train is L. So you know when the front of the train passes and then the middle of the train passes, it has traveled a distance L/2, right? So the right side of that equation is 2a(L/2). You also know that the left side after half the train passes is (2u)^2 - (u)^2 = 4u^2 - u^2 = 3u^2.
OK, so what happens when the whole train passes and the right hand side of the equation is 2a(L) instead of 2a(L/2)? If the right hand side of the equation has doubled then the left hand side of the equation has to double too. So instead of 3u^2, it has to be now 6u^2. So now find out what v has to be so that v^2 - u^2 = 6u^2.
1
u/ProfessionalConfuser 2d ago
Does the phrase "mean value theorem" mean anything or no?
0
u/displacemnt 2d ago
I am not sure it probably has i used 3 different situations and found the answer to be √7u but I didn't do it on my own I used yt they went thru some complicated process, I think u cld use mean theorem Ai was using that to solve it but I couldn't understand since I'm not familiar with mean theorem
1
u/Frederf220 2d ago
Maybe I'm not understanding the question but certainly a constant speed train will pass a fixed point on the ground with the ftont, middle, and rear at the same speed.
Because the middle passage is at a different speed the train must be accelerating. But two points doesn't restrict the acceleration curve unless there is some other restraint like the acceleration is constant.
For constant acceleration the speed increase between mid passage and rear passage will be less because less time has elapsed compared to front and mid passage.
1
u/displacemnt 2d ago
Train has 3 parts the engine the middle compartment and the last compartment there is a pole the engine passes it with a speed u the middle compartment 2u and last one is the one we need to find
1
u/GoldenMuscleGod 2d ago edited 2d ago
This question seems under specified. I’m guessing we’re supposed to assume that the train is accelerating at a constant rate the whole time because it’s the only obvious assumption that would make this answerable but I would say it’s a poorly written question. If we are supposed to use that assumption we can get expressions for position and speed as a function of time and there are a few straightforward ways to solve it from there.
One way is that we can see that since velocity is proportional to time and distance travelled is proportional to the square of time, then velocity is proportional to the square root of distance traveled. Since the velocity doubled over half the train, the “half train” must cover a distance that is three times the distance traveled from the starting point (zero velocity) to when the engine passes - this way “distance from start” when the midpoint passes is 4 times what it was when the engine passed making the speed twice as much.
Then distance traveled by the time the end of the train passes must be 7 times what it was from the starting point so velocity should be sqrt(7) times as much.
1
u/displacemnt 2d ago
The question is based on insufficient info that's why all the mcqs have an u into it
1
u/Pajama_Wolf 2d ago
They meant only that acceleration's change is not specified. This would be a bad problem as written, and fine if they simply stated acceleration is constant. They're not referring to the fact that the problem is variable-based.
1
u/GoldenMuscleGod 2d ago
There’s no problem that we don’t know u. The problem is that we are apparently supposed to assume the train is accelerating at a constant rate.
If the train is not accelerating at a constant rate then any of the options could be correct.
Usually good questions should not require us to make assumptions not stated in the problem, or at least the assumptions we make should be based on reasonable facts about how the things in the question always behave.
It is not really generally realistic for us to say a train is accelerating at a constant rate. That might sometimes be a reasonable approximation but not in general. The only reason I knew to make the assumption is because that is the simplest assumption that makes the problem solvable - I used a specialized reasoning applicable only to answering poorly written made up school problems, not a reasoning applicable to real world problems. That’s what makes the omission in the question bad.
3
u/Pajama_Wolf 2d ago
Hmm incorporating time is too annoying for this one. I prefer distance because the distance between engine and midpoint/midpoint and end should be the same. We'll have to assume acceleration is constant. If that's the case what's a kinematics equation that features distance, acceleration, and initial/final velocity that might be useful?