Problems on Trains - Concept 1, Quantitative Aptitude Video Lecture | Quantitative Aptitude (Quant) - CAT

welcome to this edition of train for
aptitude in today's session we look at
the first concept and the problems
related to trains
let us now read the problem a train is
moving at 90 kilometers per hour and
crosses an electric pole in 9 seconds
what is the length of the train our
options are a 810 kilometers B 225
meters C 225 kilometers D 810 meters let
us now begin visualizing the problem at
hand
the first observation we need to make is
that the pole would have a negligible
width compared to the length of the
train that is passing by it so we could
easily ignore the additional distance in
consideration given that the electric
pole is stationary
its speed is zero kilometers per hour
the train is approaching the pole at a
speed of 90 kilometers per hour and
would completely cross it in 9 seconds
what is interesting to note here is also
that the unit of train speed and the
time to cross the pole are not uniform
whereas in kilometers per hour
and the other is in seconds this would
be the first hurdle we would need to
cross we would need to get the units
into meters per second let's now
visualize the next sequence the train
has passed by the electric pole given
that the electric pole is of negligible
width the total distance that is covered
by the train would be nothing but the
trains length itself what is interesting
is also that the relative speed that we
need to calculate the relative speed in
this case is only that of the train
moving since a polar stationary given
the information we already have we can
complete the calculations by merely
dividing the distance of the train by
the relative speeds
let us now get to calculating the answer
given that the speed of the Train is 90
km/h we would first need to convert it
to an to m/s the calculation would give
us 25 m/s as shown the relative speed of
the train is a next point that we need
to kick take care of the speed of the
train plus the speed of the pole the
speed of the pole in this case is zero
meters per second and therefore the
relative speed is 25 meters per second
to cross the pole it is mentioned the
train takes 9 seconds the total distance
that the train would have covered in
this exercise is that of its distance of
its length itself that is nothing but
the speed of a train into the time taken
which is 225 meters let us now
generalize the approach to the solution
the step 1 is nothing but making note of
the various informations that is
available in the problem like the speed
of the train the dimension of the train
or of the of any other object that is
mentioned the next step is to ensure
that the units of speed dimension time
are all consistent if they aren't then
we would need to make them consistent
this is a typical phase where most of
the mistakes could occur in problems of
this nature step 3 very often we do not
have a single object moving unlike in
this specific case there could be more
than one object moving so the relative
speed plays a very crucial role a simple
rule of the thumb that one could use in
calculating the relative motions or
speed is as follows what if the two
objects are moving in the same direction
the relative speed is nothing but the
between the two speeds second if the
objects are moving in opposite
directions one would need to add their
speeds as the last step we calculate the
distance travelled by the Train in the
given time at the calculated speed since
the size of the board is negligible this
would also be the length of the train
therefore we have the answer thank you
for watching
train for amplitude