# An object travels North at  3 ms^-1 for 6 s and then travels South at  4  ms^-1 for  4  s. What are the object's average speed and velocity?

May 13, 2017

speed = 3.4 ms^-1

velocity = 0.2 ms^-1 North

#### Explanation:

The average speed is the total distance traveled divided by the total time.

In this case, the object travels $3 \times 6 = 18$ $m$ North and then $4 \times 4 = 16$ $m$ South. That means it travels a total of $18 + 16 = 34$ $m$ in $6 + 4 = 10$ $s$, so

speed = distance/time = $\frac{34}{10} = 3.4$ $m {s}^{-} 1$

The average velocity is the final displacement from the start point, divided by the total time. Since the object traveled $18$ $m$ North and then changed direction and traveled $16$ $m$ South, the displacement is $2$ $m$ North (remember, displacement and velocity are vector quantities, so we must specify direction).

velocity = displacement/time = $\frac{2}{10} = 0.2$ $m {s}^{-} 1$ North

(also, it really helps to draw a diagram)