Question

Rain is falling vertically with a speed of 30 $m{s}^{-1}$. A woman rides a bicycle with a speed of 12 $m{s}^{-1}$ in east to west direction. In which direction she should hold her umbrella?

• A. At an angle of tan${}^{-1}$(2/5) with the vertical towards the east.
• B. At angle of tan${}^{-1}$(2/5) with the vertical towards the west.
• C. At angle of tan${}^{-1}$ (5/2) with the vertical towards the east.
• D. At angle of tan${}^{-1}$ (5/2) with the vertical towards the west.

Answer 1

Answer: (B)

At angle of tan${}^{-1}$(2/5) with the vertical towards the west.

In the figure, ${\overrightarrow{v}}_r$ represents the velocity of rain and ${\overrightarrow{v}}_b$ the velocity of the bicycle, the woman is riding. To protect herself from rain, the woman should hold her umbrella in the direction of the relative velocity of rain with respect to the bicycle ${\overrightarrow{v}}_{rb}$. From figure,

tan$\theta =\frac{v_b}{v_r}=\frac{12}{30}=\frac{2}{5}or\theta =ta{n}^{-1}\left(\frac{2}{5}\right)$

Therefore, the woman should hold her umbrella at an angle of $ta{n}^{-1}\left(\frac{2}{5}\right)$with the vertical towards the west.