Physics Technology Update (4th Edition)

Published by Pearson
ISBN 10: 0-32190-308-0
ISBN 13: 978-0-32190-308-2

Chapter 4 - Two-Dimensional Kinematics - Problems and Conceptual Exercises - Page 107: 48

Answer

(a) The initial velocity is directed at $44.5^\circ$ above the horizontal. (b) The initial speed is $30\text{ m/s}$.

Work Step by Step

(a) The horizontal distance traveled is given by the product of the horizontal component of the velocity and the time of the flight $$D=v_0\cos\theta\cdot t_f$$ The time of the flight is given by $$t_f=\frac{2v_0\sin\theta}{g}.$$ Dividing previous two equations we obtain $$\frac{D}{t_f}=\frac{v_0t_f\cos\theta}{\frac{2v_0\sin\theta}{g}}=\frac{gt_f\cos\theta}{2\sin\theta}=\frac{gt_f}{2\tan\theta}.$$ This further gives $$\tan\theta=\frac{gt_f^2}{2D}\Rightarrow\theta=\arctan\frac{gt_f^2}{2D}=\arctan\frac{9.81\text{ m/s}^2\cdot(4.30\text{ s})^2}{2\cdot92.2\text{ m}}=44.5^\circ,$$ so the initial velocity is directed $44.5^\circ$ above the horizontal. (b) From the equation for $D$ we get $$v_0=\frac{D}{t_f\cos\theta}=\frac{92.2\text{ m}}{4.30\text{ s }\cdot\cos44.5^\circ}=30\text{ m/s}.$$
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