Question

An object travels North at `5 m/s` for `1 s` and then travels South at `3 m/s` for `3 s`. What are the object's average speed and velocity?

Answer 1

`Sav = 3.5 m/s`; `Vav = 1 m/s` South

Explanation:

Travel distance north is `5 m/s xx 1 s = 5m` (shorter)

Travel distance south is `3 m/s xx 3s = 9m` (longer)

Average speed is the total distance travelled divided by total time.

`Sav = (5m + 9m)/(1s + 3s) = (14m)/(4s) = 3.5 m/s`

Average velocity is the resultant change in velocity divided by the total time where the final velocity is South;

`Vav = -Vn + Vs`

Note that `Vn` is negative because its direction is opposite to the final velocity.

`Vav = (-5mn + 9ms)/(1s + 3s) = (4m)/(4s) = 1m/s South`

This scenario may occur in real life when a ping pong ball is returned from a serve.