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 6 - Dynamics I: Motion Along a Line - Exercises and Problems - Page 163: 47

Answer

$1.90\times10^5\;\rm N$

Work Step by Step

In this case, the net force exerted on the jet plane in the $x$-direction is given by $$\sum F_x=F_{Thrust}-f_r=ma_x$$ Air resistance is negligible but not the rolling friction as the author told us. Solving for the thrust force; $$F_{Thrust}= ma_x+f_r$$ Rolling friction is given by $f_r=\mu_rF_n$ $$F_{Thrust}=ma_x+\mu_r F_n \tag 1$$ In this case, the net force exerted on the jet plane in the $y$-direction is given by $$\sum F_y=F_{n}-mg=ma_y=m(0)=0$$ Thus, $$F_n=mg$$ Plugging into (1); $$F_{Thrust}=ma_x+\mu_r mg $$ $$F_{Thrust}=m\left[a_x+\mu_r g\right] \tag 2 $$ Now we need to find the plane's acceleration, we know it starts from rest and reaches a speed of 82 m/s during a time interval of 35 s. Thus, $$v_{x}=\overbrace{v_{ix}}^{0}+a_xt$$ Thus, $$a_x=\dfrac{v_x}{t} $$Plugging into (2); $$F_{Thrust}=m\left[\dfrac{v_x}{t} +\mu_r g\right] $$ Plugging the known; $$F_{Thrust}=75000\left[\dfrac{82}{35} +(0.02\cdot 9.8) \right] $$ $$F_{Thrust}=\color{red}{\bf 1.90\times10^5}\;\rm N$$
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