# Physics

The brakes on your automobile are capable of slowing down your car at a rate of 4.2 m/s^2.

a) If you are going 145 km/h and suddenly see a state trooper, what is the minimum time in which you can get your car under the 90 km/h speed limit?

b) Graph x vs. t and v vs. t for such a deceleration.

I have tried this problem several different ways, but I still cannot get it. I'm pretty confused with this and it's all new to me, so help would be appreciated.

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1. a) 145 km/h = 40.28 m/s
You want to reduce the speed to 90 km/h = 25.00 m/s

The time T that it takes to decelerate to the lower speed at the fasted deceleration rate, a = 4.2 m/s^2, is given by

25 = 40.28 - a T
T = (40.28-25)/a = 3.64 s

b) Graph V = 40.28 - 4.2 t (in m/s)

If you want to graph V(t) with units of km/h, plot

V (km/h) = 145 - 15.12 t

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