Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 4 - Kinematics in Two Dimensions - Exercises and Problems - Page 113: 55

Answer

$$68.7\;\rm m/s, \;21.3^\circ \tag{with vertical}$$

Work Step by Step

We know that the velocity of the rain relative to the ground is given by $$v_{RG}=v_{RC}+v_{CG}\tag 1$$ whereas $R\rightarrow$ rain, $G\rightarrow$ ground, $C\rightarrow$ car. We chose the North-south line to be our $x$-aixs at which the positive $x$-axis is in the direction of north. And we also chose up to be our positive $y$-direction. So, when the car is moving north at a constant speed of 25 m/s, its velocity vector relative to the ground is $$v_{CG}=(25)\hat i\tag 2$$ And the rain is making an angle of $38^\circ$ with vertical, which means its angle relative to our $+x$-axis is in the fourth quadrant, So, $$\theta_{RC}=270^\circ +38^\circ=\bf 308^\circ$$ Thus, its velocity vector relative to the car is given by $$v_{RC}=(v_{RC}\cos 308^\circ)\hat i+(v_{RC}\sin 308^\circ)\hat j\tag 3$$ From (1); $$ v_{RC}=v_{RG}-v_{CG}=v_{RG}\cos\theta\;\hat i+v_{RG}\sin\theta \;\hat j- (25)\hat i$$ $$ v_{RC}=(v_{RG}\cos\theta-25)\hat i+(v_{RG}\sin\theta )\hat j$$ We know that the angle in the first case is 308$^\circ$. Thus, $$\tan 308^\circ=\dfrac{v_{RG}\sin\theta }{v_{RG}\cos\theta-25}\tag 4$$ In the second case, when the car now moves south. $$ v_{RC}=(v_{RG}\cos\theta+25)\hat i+(v_{RG}\sin\theta )\hat j$$ We know that the angle in the second case is 270$^\circ$. Thus, $$\tan 270^\circ=\dfrac{v_{RG}\sin\theta }{v_{RG}\cos\theta+25}$$ hence, $$v_{RG}\cos\theta=-25 \tag 5$$ Plug into (4); $$\tan 308^\circ=\dfrac{v_{RG}\sin\theta }{-25-25} $$ Thus, $$v_{RG}\sin\theta=64\tag 6$$ Thus, the rain velocity magnitude is given by (see (5), and (6)) $$|\vec v_{RG}|=\sqrt{25^2+64^2}=\color{red}{\bf 68.7}\;\rm m/s$$ and its direction; $$\theta=\tan^{-1}\left[\dfrac{-64}{ 25}\right]=-68.7^\circ$$ which makes an angle of $21^\circ$ with the vertical
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