Question

A train travelling at 72 km/h crosses a platform in 30 sec and a man standing on the platform in 18 sec what is the length of platform?

A 240 m
B 360 m
C 420 m
D 600 m

Answer 1

Use v=x/t

Answer 2

You'll have to do some unit conversions first

Answer 3

(72*5)/18

Answer 4

As I said, you have to do unit conversions

Answer 5

Convert 72km/h to m/s

Answer 6

x=20m/s

Answer 7

And no, x represents the distance...the units would be meters.

Answer 8

i already made unit conversions
72km/hr=(72*5)/18 m/sec

Answer 9

Oh I see, yes that's good

Answer 10

So you have 72 km/h = 20 m/s, what's next?

Answer 11

calculate length of platform

Answer 12

Yeah, so x=vt

Answer 13

x=v*t
x=20m/s *30 sec

Answer 14

x=60 m/s

Answer 15

20*30 = 600...what's the units of distance?

Answer 16

The seconds get cancelled out

Answer 17

now what to do

Answer 18

meters

Answer 19

how to calculate length of platform

Answer 20

\(\large \begin{align} \color{black}{\normalsize \text{first u need to find the length of train }\hspace{.33em}\\~\\
\normalsize \text{let the length of train be }D_{train} \hspace{.33em}\\~\\
\normalsize \text{it will be speed times distance needed to cross the man }\hspace{.33em}\\~\\

S_{train}=\dfrac{D_{train}}{T_1} \hspace{.33em}\\~\\
72~~km/hr=\dfrac{D_{train}}{18~sec} \hspace{.33em}\\~\\
72\times 10^3~~m/hr=\dfrac{D_{train}}{18\times \dfrac{1}{60\times 60} ~~hr} \hspace{.33em}\\~\\
D_{train}=72\times 10^3 \times \dfrac{18}{60\times 60} ~~metres\hspace{.33em}\\~\\
D_{train}=360 ~~metres\hspace{.33em}\\~\\
\normalsize \text{first u need to find the length of platform }\hspace{.33em}\\~\\
\normalsize \text{let the length of platform be x }\hspace{.33em}\\~\\
\normalsize \text{now to cross the platform the train has to }\hspace{.33em}\\~\\
\normalsize \text{pass the length of train as well as the length of platform, hence }\hspace{.33em}\\~\\
S_{train}=\dfrac{x+D_{train}}{30~~sec} \hspace{.33em}\\~\\
72\times 10^3~~m/hr=\dfrac{x+360}{\dfrac{30}{60\times 60}~~hrs} \hspace{.33em}\\~\\
\normalsize \text{x will come }x=240~~metres \hspace{.33em}\\~\\
}\end{align}\)