What is the formula for deceleration?

Question

On coming to a stop, a car leaves skid marks 92m long on the road. Assuming a decleration of 7.00m/s^2, estimate the speed of the car just before braking.

Answer 1

s = at^2/2 = vt/2 = v^2/(2a)
So v = sqrt(2as) = 35.89 m/s.

Answer 2

Deceleration is nothing but negative acceleration.
You can use the formula: S = v² - u²=2as
v = final vlocity=0 m/sec
u = initial velocity (at the time of applying the brakes)
a = -7 m/s²
s = 92 m
Substituting the values in the formula,
0² - u² = 2 x (-7) x 92
-u² = -(2x7x92) = -1288
u² = 1288
u = √1288 = 35.89 m/sec.......Ans
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