A 210 meter long train is moving due north at a of 25 m/s. a small bird is flying due south a little above the train with speed 5 m/s. The time taken by the bird to cross the train is

Answer

A, B, C

Explanation

[A] `:'` Distance `ge` Displacement
`:.` Average speed `ge` Average velocity
[B] `|vec(a)|+-0rArr Deltavec(v)+-0`
velocity can change by changing its direction
[C] Average velocity depends on displacement in time interval e.g. circular motion `rarr` after one reverage velocity but instantaneous velocity never becomes zero during motion.
[D] In a straight line motion , there must be reversal of the direction of velocity to reach the destination point for making displacement zero and hence instantneous velocity has to be zero at least once in a time interval.