How Long is the Train?

Date: 05/21/99 at 11:18:33
From: benajmin kim
Subject: Length of train

Two trains are travelling in opposite directions along parallel 
tracks. Train A is travelling at 72kph and train B at 54kph. The 
driver of train A notices that it takes train B eight seconds to pass 
him. What is the length of train B?

Date: 05/21/99 at 12:19:16
From: Doctor Peterson
Subject: Re: Length of train

Hi, Benjamin.

Since the trains are going in opposite directions, to someone on train 
A it looks as if he is standing still and the other train is going 
past at 72+54 = 126 mph. Do you see why?

If it takes 8 seconds for the whole train to pass the driver, you can 
calculate how far an object going 126 mph will go in 8 seconds, and 
that is how long the train is.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/   

Date: 05/21/99 at 13:53:14
From: Doctor Anthony
Subject: Re: length of train

Since the trains are travelling in opposite directions they have a 
closing speed of  72 + 54 km/h  =  126 km/h = 126 x 1000/3600  m/sec

                                = 35 m/sec

You can think of the driver of train A as stationary and think of 
train B as passing him at 126 km/hr. ( = 35 m/sec)

      distance = speed x time

               =  35 x 8

               =  280 m

     So train B is 280 metres long.

- Doctor Anthony, The Math Forum
  http://mathforum.org/dr.math/