An object travels North at 7 m/s for 3 s and then travels South at 9 m/s for  4 s. What are the object's average speed and velocity?

Jan 29, 2017

Average speed is $8 \frac{1}{7}$ $\frac{m}{s}$ and average velocity is $2 \frac{1}{7}$ $\frac{m}{s}$ south.

Explanation:

As the object travels North at $7$ $\frac{m}{s}$ for $3$ $s$,

the displacement is $7 \times 3 = 21$ $m$ north

then the object travels South at $9$ $\frac{m}{s}$ for $4$ $s$,

the displacement is $9 \times 4 = 36$ $m$ south

Hence, net displacement is $36 - 21 = 15$ $m$ south in $3 + 4 = 7$ $s$

As total distance covered is $21 + 36 = 57$ $m$ in $7$ seconds,

average speed is $\frac{57}{7} = 8 \frac{1}{7}$ $\frac{m}{s}$

But average velocity is $\frac{15}{7} = 2 \frac{1}{7}$ $\frac{m}{s}$ south.