Important Types questions on Trains Continues:-
Concept 3rd
:- Relative
speed Concept:-
Q13. A train running at the speed
of 25 km/hr takes 18 secs. To pass a platform. Next, it takes 12 seconds to
pass a man walking at 5 km/hr in the opposite direction. Find the length of the
train and that of the platform.
Sol. Analyse the given data
in the question:
Speed of the train = 25 km/hr.
Speed of the man = 5 km/hr.
Let the length of the train = l1
Length of the platform = l2
Distance travelled by train in passing the moving man = length of
the train (l1)
Distance travelled by train in passing the platform = length of
the {train (l1) + platform (l2)}
= (l1
+ l2) meters.
Time taken by the train to cross the man, moving in opposite
direction = 12 secs.
Relative speed while both are moving in opposite direction = 25 + 5
= 30
km/hr.
= 30 ×
(5/18) = (25/3) m/sec.
Time taken by the train to pass the moving man = Distance/
relative speed
12 = l1/(25/3)
l1 = 12 × (25/3) =100 m.
length of the train = l1 = 100 metres.
Time taken by the train to cross the platform = 18 secs.
Speed of the train = 25 km/hr = 25 × (5/18) = (125/18) m/sec.
Speed of the train = Distance/time
(125/18) = (l1 + l2)/18
(l1 + l2) = 125.
100 + l2
= 125.
l2
= 125 – 100 = 25 m
Length of the platform = 25 m
Hence the required length
of the train = 100 m and length of the platform = 25 m.
Concept 5th
:- Combination of two different types of Questions
Q14. A train passes a pole in
20 seconds and passes a platform 100 m long in 30 seconds. Find the length and
its speed.
Sol. Method 1st :- Analyse the given data in the
question:
Let the speed of the train = x m/sec.
Let the length of the train = L m
Time taken to cross the pole = 20 secs.
Speed = distance/time
x = L/20
L = 20x
Length of the platform = 100 m
Distance travelled by the train to cross the platform = (L + 100)
m
Time taken to cross the platform = 30 seconds.
Again speed = distance/time
x = (20x + 100)/30
30x = 20x +100
10x = 100
x = 10
m/sec
Now L = 20x = 20 × 10
L = 200m
Hence the
length of the train = 200 m and speed of the train = 10 m/sec.
Method 2nd
:- Concept :- In these type of questions the train covers its length in time t1
seconds and covers its length including the length of the platform in t2
seconds. This means it covers the length of the platform in (t2 – t1)
seconds. Thus we can calculate the speed of the train direct.
Sol. Time taken by the train
to cover its length (cross a pole) = 20 secs.
Time taken by the train to its length including platform (cross a
platform) = 30 secs.
Hence the actual time taken by the train to cover the length of
platform = 30 – 20 = 10 secs.
Length of the platform = distance travelled = 100 m
Speed of the train = distance / time.
Speed =
100/10 = 10 m/sec.
Length of the train = distance travelled in crossing the pole =
speed × time
Length of
the train = 20 × 10 = 200 m.
Hence the
length of the train = 200 m and speed of the train = 10 m/sec.
(Note you may say that this is so
long but dear this is the only way to explain the whole concept of the question
to you now its upto you that how fast and short handedly you can solve these
type of questions)
Hope
you have enjoyed the base concept of this chapter till now. More Conceptual
enjoyment with the Various Questions related to this chapter which are
frequently asked in the examinations will be presented to your for self study
on this blog soon, just keep visiting this blog regularly and writing me your
related queries through comment or forum.
More
types of questions will be followed soon on this blog........................
Best
of Luck for CGL Exams. To be continued soon........................
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