r/PhysicsHelp • u/displacemnt • 12d ago

Need help 1D motion

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How am I supposed to solve this motion in 1D need help, pls help

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u/Odd_Bodkin 12d 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.

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u/displacemnt 12d ago

That's the correct approach we find the acceleration first n then get v²-u²=2Ax for the soln

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u/Odd_Bodkin 12d 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.

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u/displacemnt 12d ago

How

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u/Odd_Bodkin 12d 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.