Fundamentals of Physics Extended (10th Edition)

Published by Wiley
ISBN 10: 1-11823-072-8
ISBN 13: 978-1-11823-072-5

Chapter 2 - Motion Along a Straight Line - Problems - Page 35: 40b

Answer

Neither strategy works.

Work Step by Step

We can convert $55~km/h$ to units of $m/s$: $(55~km/h)\times (\frac{1000~m}{1~km})\times (\frac{1~h}{3600~s}) = 15.28~m/s$ We can find the distance we would travel without braking in a time of $1.8~s$: $x = (15.28~m/s)(1.8~s) = 27.5~m$ Since the distance to the intersection is $32~m$, we should not continue driving at a constant speed of $55~km/h$, otherwise we would enter the intersection after the light turns red. We can consider the situation if we decide to brake. We can find the distance we travel in the $0.75~s$ before we react and apply the brake: $x = (15.28~m/s)(0.75~s) = 11.46~m$ We can find the distance required to stop after we apply the brake and decelerate: $v^2 = v_0^2+2ax$ $x = \frac{v^2 - v_0^2}{2a}$ $x = \frac{(0)^2 - (15.28~m/s)^2}{(2)(-5.18~m/s^2)}$ $x = 22.54~m$ After the light turns yellow, the total distance required to stop if we decide to apply the brake is $34~m$ Since the distance to the intersection is only $32~m$, we can not brake safely before reaching the intersection. Therefore, neither strategy works.
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