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Speed , train length

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by Patrick_GMATFix » Mon Jun 21, 2010 1:45 pm
Hi Francois,

When the train overtakes the bike, the head of the train is x meters from the head of the bike, where x is the length of the train. So the question is, 'what is x?'. We know that the train is 36km/h faster , so every 3600 seconds (each hour), the head of the train gains an advantage 36,000 meters (36 km).

We know that the train takes 40 seconds to overtake the bike. The value of x (the length of the train = distance that the head of the train must gain over the head of the bike for the train to overtake the bike) is equal to the advantage that is accrued in 40 seconds. Set up a proportion:

36,000meters / 3,600seconds = x meters/ 40seconds. Solve to find that x = 400 meters

-Patrick

*Edit: fixed difference between speed is 36km/hr, not 60km/hr
Last edited by Patrick_GMATFix on Mon Jun 21, 2010 1:52 pm, edited 2 times in total.
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by francoisph » Mon Jun 21, 2010 1:47 pm
OA is 400 meters mate!
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by Patrick_GMATFix » Mon Jun 21, 2010 1:50 pm
Thanks Francois. I'll fix my answer.

The mistake I made is:
We know that the train is 60km/h faster
. I misread the original question and took 40km/hr as the speed of the bike. The bike actually goes at 64km/hr, so the difference in speed is actually 36km/h, not 60. Careless mistake
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by amising6 » Mon Jun 21, 2010 7:08 pm
francoisph wrote:A train traveling at 100 kmph overtakes a motorbike traveling at 64 kmph in 40 seconds. What is the length of the train in meters?

I really didnt get this one!
now train travelling at 100 kmph will overtake motorbike travelling 64 kmph so assuming they are travelling in same direction
relative speed 100-64=36kmph i.e 36*5/18=10 m/sec (now since train is trying to overtake motorbike so its travelling extra distance at the rate of 10 m/sec)
now overtake has been done in 40 sec so extra distance travelled by train will be equal to the length of train 1.e 40*10=400 m
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by Testluv » Mon Jun 21, 2010 7:17 pm
Here's a good takeaway for 2 moving-object problems:

--if the objects travel in the same direction, subtract the rates
--if the objects travel in opposite directions (meeting or separating), add the rates.
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by francoisph » Tue Jun 22, 2010 5:27 am
thks guys

yeah it is relative speed concept here

relative speed 100- 64= 36
36*40 sec *5/18 = 400m
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by outreach » Tue Jun 22, 2010 10:01 am
trains are travelling in same direction hence the relative speed will be 100-64=36 km/h

1km/h=5/18 m/s
36km/h=x

x=10m/s

when calculating sped of train we can ignore the lenght of bike

length of train=time*speed=40*10=400m/s
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