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A train 50m long passes over a bridge at a velocity of 30kmh1. If it takes 36s to cross the bridge, what will be the length of the bridge?
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Answer
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Hint: We are given with the velocity of the train and the time it takes to cross the bridge, so the distance the train travels will be the product of the velocity and time taken. However, remember to account for the length of the train when you take the distance travelled since the engine will have to travel an additional 50m so that the whole train crosses the bridge.

Formula Used:
Distance travelled d=velocity×time

Complete answer:
Let us take the length of the bridge to be L.
If the whole train has to cross the bridge, then the distance (d) covered by the engine for the whole train to cross the bridge will be d=(L+50)m=(L+0.05)km
Given that the velocity of the train is v=30kmh1 and it takes t=36s=363600=0.01h to travel, the distance travelled by the train is given by:
distance=velocity×timed=v×t
(L+0.05)=30×0.01
(L+0.05)=0.3
L=0.30.05=0.25km=250m
Therefore, the length of this bridge is 250m.

Note:
Instead of converting all the other parameters to make them consistent with the units of the speed, you can in fact just change the unit of speed to save time. To convert a speed from kmh1 to ms1 just multiply the speed in kmh1 by a factor of 518, and to convert ms1 to kmh1 multiple the speed by a factor of 185.