Relative speed : Two trains crossing each other

In summary, two trains with lengths of 120 m and 100 m are traveling in opposite directions with velocities of 40 km/h and 50 km/h. The formula for calculating the time it takes for the trains to completely cross each other is given as the sum of the length of the trains divided by the relative speed of the trains. This formula is based on the concept that the distance to be crossed is equal to the sum of the two train's lengths. The difference between overtaking and crossing is the direction of the trains. To solve similar problems, one must understand the concept of relative speed and the formula for calculating time, distance, and speed.
  • #1
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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  • #2
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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  • #3
Should not the question mention what their (the two train's) initial separation was?
 
  • #4
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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  • #5
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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  • #7
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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  • #8
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.
 
  • #9
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.
 

What is relative speed and how does it apply to two trains crossing each other?

Relative speed is the measurement of the speed of one object in relation to another object. In the case of two trains crossing each other, it is the speed at which one train is moving in relation to the other train.

How is relative speed calculated for two trains crossing each other?

The relative speed between two trains crossing each other is calculated by adding the speeds of both trains together. For example, if one train is moving at 50 mph and the other train is moving at 60 mph, the relative speed between them would be 110 mph.

What factors can affect the relative speed of two trains crossing each other?

The relative speed of two trains crossing each other can be affected by the speed of each train, the direction they are traveling, and any external forces such as wind or inclines on the tracks.

Is there a maximum relative speed for two trains crossing each other?

There is no maximum relative speed for two trains crossing each other. As long as the trains are moving at a safe speed and can stop or slow down if necessary, they can cross each other at any relative speed.

Why is it important to consider relative speed when two trains are crossing each other?

Considering relative speed when two trains are crossing each other is important for safety reasons. It allows train operators to anticipate the speed at which the other train is approaching and make any necessary adjustments to ensure a safe crossing.

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