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 106: 41

Answer

(a) The direction of motion is $25^\circ$ above the horizontal. The final velocity magnitude is $v_f=16.8\text{ m/s}$. (b) The ball has not reached the highest point.

Work Step by Step

We already calculated that the required time for the ball to reach the wall is $t=0.25\text{ s}$. (a) Now we need to calculate the final values of the ball's velocity components: $$v_{xf}=v_{x0}=v_0\cos32^\circ;\quad v_{yf}=v_{y0}-gt=v_0\sin32^\circ-gt.$$ Now, if $\theta$ is the angle between the ball's direction of motion and the horizontal, it holds: $$\tan\theta=\frac{v_{yf}}{v_{xf}}=\frac{v_0\sin32^\circ-gt}{v_0\cos32^\circ}$$ which yields $$\theta=\arctan\frac{v_0\sin32^\circ-gt}{v_0\cos32^\circ}=\arctan\frac{18\text{ m/s }\cdot\sin32^\circ-9.81\text{ m/s }\cdot0.25\text{ s}}{18\text{ m/s }\cdot\cos32^\circ}=25^\circ$$ and the direction is above the horizontal. The magnitude of the velocity is found to be: $$v_f=\sqrt{v_0^2-2gh}=\sqrt{18^2-2\cdot9.81\cdot2.1}\text{ m/s}=16.8\text{ m/s}.$$ (b) Since the direction of the ball's motion is still above the horizontal (the angle of the direction of motion with respect to the horizontal decreased from the initial $32^\circ$ down to $25^\circ$), the ball has not reached the highest point of its hypothetical parabolic trajectory. It will reach the highest point after it deflects off the wall.
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