Physics question - Horizontal Kinematics!!?

Question

A driver of a car traveling at 17.8 m/s applies

the brakes, causing a uniform acceleration of

−2.7 m/s2.

a) How long does it take the car to acceler-

ate to a final speed of 14.2 m/s? Answer in

units of s.

006 (part 2 of 2) 10.0 points

b) How far has the car moved during the

braking period? Answer in units of m.

Answer 1

Let:

u be initial speed,

v be final speed,

a be acceleration,

t be time,

s be distance.

(a)

v = u + at

14.2 = 17.8 + (- 2.7)t

t = (17.8 - 14.2) / 2

= 1.8 s.

(b)

v^2 = u^2 + 2as

14.2^2 = 17.8^2 +  2 * (- 2.7)s

s = (17.8^2 - 14.2^2) / (2 * 2.7)

= 21.3 m.