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Rain is falling vertically with a speed of $4ms^{-1}$.A man riding a bicycle is travelling at a speed of $3ms^{-1}$ in the north to south direction.In order to protect himself from rain,the man should hold his umbrella at an angle $\theta$ given by

$\begin{array}{1 1}\tan^{-1}\big(\large\frac{3}{4}\big)\normalsize \text{ in the north-west direction}\\\tan^{-1}\big(\large\frac{3}{4}\big)\normalsize\text{ in the south-west direction}\\\tan^{-1}\big(\large\frac{4}{3}\big)\normalsize\text{ in the north-east direction}\\\tan^{-1}\big(\large\frac{4}{3}\big)\normalsize\text{ in the south-east direction}\end{array} $

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1 Answer

$\tan^{-1}\big(\large\frac{3}{4}\big)$ in the south-west direction
Let $OR=v_r$ and $RM=v_m$ respectively represent the velocities of the rain and the man.Given $v_r=4ms^{-1}$ vertically downward from west to east direction.
$v_m=3ms^{-1}$ from north to south direction.In order to protect himself from rain,the man must hold his umbrella in the direction of the relative velocity $v_{rm}$,which is given by
Thus,we vector $v_{rm}$ is the resultant of vectors $v_r$ and $-v_m$.In the above figure,vector $RM'=-v_m$ from the parallelogram law of vector addition,the resultant velocity is given by vector OM'.If $\theta$ is the angle subtended by the resultant velocity $v_{rm}$ with the vertical,then from triangle ORM',we have
$\tan \theta=\large\frac{RM'}{OR}=\frac{V_m}{v_r}=\frac{3}{4}$
Hence the correct choice is $\tan^{-1}\big(\large\frac{3}{4}\big)$ in the south-west direction
answered Aug 14, 2014 by sreemathi.v

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