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Non-moving objects
(i) The time taken by the train to cross a stationary object (e.g.) is (lamp post, pole, standing man) which is of negligible length is given by
t =
length of the train
speed of the train
 
Note:
If the object is of some length (e.g, bridge, a tunnel, platform, another train) then,
t =
length of (train + object)
speed of the train
 
Moving objects
(ii) If the object is moving and is of negligible length (e.g : a train crosses a running man) is given by
t =
length of the train
speed of (train - object)
 
(iii) If the object is moving and has some length then,
t =
length of (train + object)
speed of (train - object)
 
Note:
If the object is moving and direction put –ve sign before v.
(iv) Two trains crossing each other in both directions
Length of 1st train = L1
Length of 2nd train = L2
They are crossing each other in opposite directions, in t1 seconds.
They are crossing each other in same direction in t2 seconds then,
Speed of faster train =  
L1 + L2
2
 
1
t1
+
1
t2
   
 
Speed of slower train =  
L1 + L2
2
 
1
t1
-
1
t2
   
(v) If two trains start at the same time from points A and B towards each other and after crossing they take a and b seconds in reaching B and A respectively, then
A's speed : B's speed = b : a  
 
 
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