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# A 150 m long train is travelling from West to East at a speed of 20 m/s. A bird is flying from East to West at a speed of 5 m/s above the train. How long will the bird take to cross the train ?

$(a)\;6s \quad (b)\;7.5s \quad (c)\;10s \quad (d)\;30s$

Can you answer this question?

Relative velocity of bird with respect to train
$=5-(-20)=25m/s$
Time taken by train $=\large\frac{150}{25}$$=6s$
$v_b=v_{\large bird}=5 m/s;\quad v_T=v_{\large train}=-20 m/s$
Relative velocity of bird with respect to train
$=v_b-v_T$
$=5-(-20)$
$=25 m/s$
The bird take 6s to cross the train
Hence a is the correct answer.
answered Jun 20, 2013 by
edited Mar 25, 2014