Relative speed : Two trains crossing each other

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Discussion Overview

The discussion revolves around a problem involving two trains crossing each other, focusing on the relevance of their lengths and the concept of relative speed. Participants explore the formula for calculating the time taken for the trains to completely pass each other, as well as the implications of initial separation and the distinction between overtaking and crossing.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants question the relevance of the lengths of the trains in the formula for calculating crossing time.
  • Others argue that the initial separation of the trains is not necessary for determining the crossing time, as it only depends on the lengths and relative speeds.
  • A participant suggests that visualizing the crossing can aid in understanding the problem.
  • There is a discussion about the difference between overtaking and crossing, with some asserting it relates to direction.
  • One participant expresses confusion about the relationship between closing speed and duration of crossing.
  • Another participant emphasizes the importance of understanding the underlying concepts rather than relying solely on hints for answers.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of initial separation and the relevance of train lengths, indicating a lack of consensus on these points. The distinction between overtaking and crossing also remains a topic of contention.

Contextual Notes

Some participants highlight the need for additional context in the problem statement, such as initial separation, to enhance understanding and critical thinking skills.

Who May Find This Useful

This discussion may be useful for students grappling with concepts of relative speed, crossing time calculations, and the distinctions between different types of motion in physics.

gracy
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Two trains having lengths of 120 m and 100 m are running in the opposite directions with velocities 40 km/h and 50 km/h . In what time they will completely cross each other? In the solution it's given that
Time taken by the two trains to cross each other = sum of length of trains/relative speed of trains
I fail to understand the above formula . How are the lengths of trains relevant?
 
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gracy said:
How are the lengths of trains relevant?

You don't understand why the length of a train is relevant to how long it takes it to pass something?

 
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Should not the question mention what their (the two train's) initial separation was?
 
gracy said:
Should not the question mention what their (the two train's) initial separation was?

it's not relevant as the timing of the crossing only involves the period that it takes to cross ( pass ) each other. it doesn't matter if they started 10 or 100 km apart
draw a pic looking down on the 2 trains as they start to cross and one below it when they finish the passing

you didn't answer AT's query as to why the length of each train is important
 
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gracy said:
Should not the question mention what their (the two train's) initial separation was?

It should, along with some other irrelevant info, to train the student's filtering skills.
 
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gracy said:
crossing.png

So how far are the ends apart, when the fronts meet? And how fast is that distance reducing?
 
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A.T. said:
So how far are the ends apart, when the fronts meet?
Equal to sum of the two train's lengths
A.T. said:
And how fast is that distance reducing?
I don't know how to answer this.
 
A.T. said:
And how fast is that distance reducing?
I think it is same as asking time taken by trains to cross each other
 
  • #10
A similar type of problem I have come across is as follows
Two train each of length 100 m are running on parallel tracks. One overtakes the other in 20 seconds and one crosses the other in 10 second. Calculate the velocities of two trains. I just want to ask what's the difference between overtaking and crossing?
 
  • #11
gracy said:
I think it is same as asking time taken by trains to cross each other
No, closing speed is not the same as duration.
 
  • #12
gracy said:
I just want to ask what's the difference between overtaking and crossing?
Direction.
 
  • #13
A.T. said:
Direction.
Could you please explain a bit more?
 
  • #14
gracy said:
Time taken by the two trains to cross each other = sum of length of trains/relative speed of trains
I fail to understand the above formula .
Time = distance/ speed
Time taken by the two trains to cross each other = distance to cross/ relative speed (as both are moving)
Distance to cross =sum of length of trains
Therefore
Time taken by the two trains to cross each other = sum of length of trains/relative speed of trains
Right?
 
  • #15
gracy said:
Time = distance/ speed
Time taken by the two trains to cross each other = distance to cross/ relative speed (as both are moving)
Distance to cross =sum of length of trains
Therefore
Time taken by the two trains to cross each other = sum of length of trains/relative speed of trains
Right?
Yes.
 
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  • #16
Could you please elaborate your post #12. I'll be extremely thankful.
 
  • #17
Gracy, I don't think PhysicsForums is helping you. You've asked us about various topics in the past, and now it's clear you haven't learned them.

The problem is that you immediately jump to asking a question here without having put much work into it, and when you are guided by someone towards the answer, you don't try and work it out for yourself, but instead ask for another hint. And another. And another. Eventually, you have been hinted all the way to the answer. Well, you've gotten the answer, but you haven't really learned.

You're going to have to decide if you want to learn or not. If you want to learn, you are going to have to spend more time thinking and working on your own. Actually, I think you have decided - you've decided that so long as you have access to PF, you don't need to learn: you can count on people hinting you all the way to an answer.
 
  • #18
gracy said:
Could you please elaborate your post #12. I'll be extremely thankful.
Do you drive any vehicle?
 
  • #19
This should have been posted in a homework forum.

Thread closed.
 

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