Question

An object travels North at `9 m/s` for `4 s` and then travels South at `3 m/s` for `1 s`. What are the object's average speed and velocity?

Answer 1

`"average speed" = 7.8 m/s` and `"average velocity" = 6.6 m/s`

Explanation:

While going North, the object goes `9 m/cancel(s) * 4 cancel(s) =36 m`.

While going South, the object goes `3 m/cancel(s) * 1 cancel(s) =3 m`.

For average speed , direction does not matter, just total distance and total time.

`"average speed" = "total distance"/"total time" = (36 m+3 m)/(4 s + 1 s) = 7.8 m/s`

For average velocity, direction does matter, so we need the resultant of all displacements (which are vectors) and total time.

`"average velocity" = "net displacement"/"total time" = (36 m-3 m)/(4 s + 1 s) = 6.6 m/s` to the North.

I hope this helps,
Steve