Question

A bird fly east at $10\text{m/s}$ for $100\text{m}$. It then turns around flies at $20\text{m/s}$ for $15\text{s}$. Neglect time taken for turning, find its magnitude of average velocity.

• A. $8\text{m/s}$
• B. $16\text{m/s}$
• C. $9\text{m/s}$
• D. $17\text{m/s}$

Answer 1

The correct option is A $8\text{m/s}$


Let us take the $x$ axis to point east. A sketch of the path is shown in the figure. To find the required quantities, we need the total time interval.
The first part of the journey took
$\Delta t_1=\frac{100\text{m}}{10\text{m/s}}=10\text{s}$
and given, $\Delta t_2=15\text{s}$
Displacement in west direction $=15\times 20=300\text{m}$
Net displacement $=\Delta x=-200\text{m}$ (where negative sign indicates the west direction)
Hence the total time interval is
$\Delta t=\Delta t_1+\Delta t_2=25\text{s}$
$v_{avrg}=\frac{\Delta x}{\Delta t}$
$v_{avrg}=-\frac{200}{25}=-8\text{m/s}$
The negative sign means that $v_{avrg}$ is directed toward the west.