Question

1) Explain how an object's average speed could be, say 60 m/s, while its average velocity is zero. Is this also the case for Instantaneous velocity and instantaneous speed why or why not?

2) Explain how to compare velocities as measured in two different frames of reference one of which is moving relative to the other

Answer 1

1) An object having average speed may have its average velocity to be zero, because average velocity is the ratio of total displacement to the total time while average speed is the ratio of total distance by total time. Now, displacement is a vector quantity and so if the object start from a point after end of journey again comes back to the starting point, its displacement is zero and so, average velocity is zero but total distance in such a case is not zero and as such, average speed is also not zero.

This is not the case with instantaneous speed and instantaneous velocity because in these quantities distance and displacement are defined for a particular instant of time not the whole time interval.