Physics
posted by Amber .
A police radar has an effective range of 1.0 km, and a motorist's radar detector has a range of 1.9 km. The motorist is going 100km/h in a 80km/h zone when the radar detector beeps. At what rate must the motorist decelerate to avoid a speeding ticket?

Find: a
Given:
s = (1.91.0)[km]
u = 100[km/h]
v = 80[km/h]
Using two equations:
<1>: s = u t + ½ a t²
<2>: v = u + a t
Rearrange <2> to get t.
t = (vu)/a
Substitute this into <1> to get a.
s = u(vu)/a + (vu)²/(2a)
Rearrange
.: a = (v²u²)/(2s)
Calculate using the given values.
I suggest converting from [km/h²] to [m/s²] 
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