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: 47

Answer

(a) The initial direction is $32^\circ$ above the horizontal. (b) The time of the flight is $t_f=0.76\text{ s}$.

Work Step by Step

(a) The distance $D$ traveled is equal to the horizontal component of the velocity (which is constant during the motion) multiplied by the time of flight i.e. $$t_f=\frac{2v_0\sin\theta}{g}\Rightarrow D=v_xt_f=\frac{2v_0^2\sin\theta\cos\theta}{g}.$$ We can use the trigonometric identity that $$\sin2\theta=2\sin\theta\cos\theta$$ so the expression for distance becomes $$D=\frac{v_0^2}{g}\sin2\theta.$$ Now we find $$\sin2\theta=\frac{gD}{v_0^2}\Rightarrow2\theta=\arcsin\frac{gD}{v_0^2}\Rightarrow\theta=\frac{1}{2}\arcsin\frac{gD}{v_0^2},$$ and finally $$\theta=\frac{1}{2}\arcsin\frac{gD}{v_0^2}=\frac{1}{2}\arcsin\frac{9.81\text{ m/s}^2\cdot4.6\text{ m}}{7.1^2}=32^\circ$$ (b) The time of the flght is now readilly found: $$t_f=\frac{2v_0\sin\theta}{g}=\frac{2\cdot7.1\text{ m/s }\cdot\sin32^\circ}{9.81\text{ m/s}^2}=0.76\text{ s}.$$
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