Initially you are driving at 55 mi/hr. If you come to rest in 7.5 s while traveling 450 ft, what is your average speed while stopping? (There are 5280 ft in one mi.)
A) 55 mi/hr
B) 0.016 ft/s
C) 120 ft/s
D) 60 ft/s

Question

Quizplus · Accepted Answer

To solve for the average speed while stopping, we need to convert the initial speed of 55 mi/hr to ft/s:
55 mi/hr * 5280 ft/mi * 1 hr/3600 s = 80.67 ft/s
Using the kinematic equation: 
d = vi*t + 0.5*a*t^2
where d = displacement (in ft), vi = initial velocity (in ft/s), t = time (in s), and a = acceleration (in ft/s^2)
We can solve for the acceleration:
450 ft = 80.67 ft/s * 7.5 s + 0.5*a*(7.5 s)^2
a = -32 ft/s^2 (negative because the direction is opposite to initial velocity)
Then, using the kinematic equation again, we can solve for the average speed while stopping:
v_avg = vi + a*t
v_avg = 80.67 ft/s + (-32 ft/s^2) * 7.5 s
v_avg = 60 ft/s

Initially You Are Driving at 55 Mi/hr