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/ |
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