A driver in a moving car applies the brakes. The car slows to a final speed of 3

Question

A driver in a moving car applies the brakes. The car slows to a final speed of 3.50 m/s over a distance of 40.0 m and a time interval of 8.00 s. The acceleration while braking is approximately constant.

(a) What is the car's original speed before braking?
6.5  m/s

(b) What is its acceleration during this time? (The car's initial velocity is in the positive direction. Indicate the direction with the sign of your answer.)

I got the correct answer for part A but can't seem to get part B correct. Help would be apprectiated!

Explanation / Answer

required equations are

v = u + at    ................... (1)

d = ut + 0.5*at^2 ............ (2)

now put eq1 value in eq2

d = (v - at)*t + 0.5*at^2

d = vt - 0.5*at^2

40 = 3.5*8 - 0.5*a*64

a = -12/32 = -0.375 m/s^2

put value of a in eq 1

v = u + at

3.5 = u - 0.375*8

u = 6.5 m/s

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