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

Options

A. Average speed of a particle in a given time is never less than the magnitude of the average velocity.
B. It is possible to have a situation in which `|(dvec(u))/(dt)| ne 0` but `d/(dt)|vec(u)|=0`
C. The average velocity of a particle is zero in a time interval. It is possible that the instantaneous velocity is never zero in the interval.
D. The average velocity of a particle moving on a straight line is zero in a time interval. It is possible that the instantaneous velocity is never zero in the interval. (infinite acceleration is not allowed)

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.