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Motion In A Plane

Question
CBSEENPH11019948

Rain is falling vertically with a speed of 30 m s-1. A woman rides a bicycle with a speed of 10 m s-1 in the north to south direction. What is the direction in which she should hold her umbrella?

Solution
Let, 
vc = Velocity of the cyclist, and
vr = Velocity of falling rain 
The woman must hold her umbrella in the direction of the relative velocity (v) of the rain, in order to protect herself from the rain. 
straight v space equals space straight v subscript straight r space end subscript plus space left parenthesis negative straight v subscript straight c right parenthesis space

space space space equals space 30 space plus space left parenthesis negative 10 right parenthesis space

space space space equals space 20 space straight m divided by straight s space

tan space straight theta space equals fraction numerator space straight v subscript straight c over denominator space straight v subscript straight r space end subscript end fraction space equals fraction numerator space 10 space over denominator 30 end fraction
rightwards double arrow space straight theta space equals space tan to the power of negative 1 end exponent space open parentheses fraction numerator 1 space over denominator 3 end fraction close parentheses space
space space space space space space space space space equals tan to the power of negative 1 end exponent space left parenthesis 0.333 right parenthesis space almost equal to space 18 to the power of 0 space

Therefore, the woman must hold the umbrella towards the south, at an angle of 18o.