Time taken by a train of length l meters to pass a pole or standing man or signal post is equal to the time taken by the train to cover l meters.
Time taken by a train of length l meters to pass a standing object of length b meters is the time taken by the train to cover (l + b) meters.
Distance covered by both the trains = l + b
If two trains or two bodies are moving in the same direction at u m/s and v m/s, then their
Relative Speed = (u - v) m / s
Distance covered = Sum of length of two trains
Time = Sum of length of two trains/Relative speed
If two trains or two bodies are moving in the opposite direction at u m/s and v m/s, then their
Relative speed = (u + v) m/s
then the time taken by the faster train to cross the slower train is
= Sum of length of two trains/Relative speed
To convert minutes into hour, we should divide the given minutes by 60.
If you want to convert the speed from km/hr to m/sec, you should multiply the speed by 5/18.
If you want to convert the speed from m/sec to km/hr, you should multiply the speed by 18/5.
Example 1 :
It took a train 50 seconds to pass a tunnel that was 1000 m long. It took the same train, traveling at the same speed, 75 seconds to pass a bridge that was 1625 m long. How long was the train ? At what speed was it travelling ?
Let x and l be the speed of the train and length of the train respectively.
Distance covered by the train when it crosses the tunnel of length 1000 m = l + 1000
Time taken = Distance / Speed
50 = (l + 1000) / x
x = (l + 1000) / 50 ------(1)
Distance covered by the train when it crosses the bridge of length (l + 1625)
75 = (l + 1625) / x
x = (l + 1625) / 75 ------(2)
(1) = (2)
(l + 1000) / 50 = (l + 1625) / 75
(l + 1000) / 2 = (l + 1625) / 3
3(l + 1000) = 2(l + 1625)
3l + 3000 = 2l + 3250
3l - 2l = 3250 - 3000
l = 250 m
By applying the value of l in (1)
x = (250 + 1000) / 50
x = 25 m/sec
To convert the speed from m/sec to km/hr, we get
x = 25⋅(18/5)
x = 90 km/hr
Hence the length of the train is 250 m, speed of the train is 50 km/hr.
Example 2 :
Two trains travelled towards each other on different tracks. Train A traveled at a speed of 54 km/hr and Train B travelled at 72 km/hr. A commuter aboard Train A recorded that Train B took 8 seconds to pass his window seat completely. How long was Train B ?
Speed of Train A = 54 km/hr
Speed of Train B = 72 km/hr
Converting the speed from km/hr to m/sec
Speed of train A = 54 ⋅ (5/18) = 15 m/sec
Speed of train B = 72 ⋅ (5/18) = 20 m/sec
Relative speed = 15 + 20
= 35 m/sec
Train B is taking 8 seconds to cross the window seat of train A.
Time = Distance covered by Train B / 35
Distance covered by train B = Length of train B
= 35 ⋅ 8
= 280 m
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