Question

A vehicle was moving at a speed of 90 km/h. On seeing a baby 20 m ahead on the road, the driver jammed on the brakes and it came to rest at a distance of 15 m. What is its retardation and how long does it take to come at rest?

Answer 1

v = 0

u = 90 km/hr = 20m/s

s = 15 m

a = ?

by using third equation of motion

v² = u² + 2as

0² = 20² + 2(a)15

0 = 400 + 30a

-400= 30a

a = -400/30

a = - 13.33 m/s² (approx)

Answer 2

Answer:

acceleration= -20.8 m/s^2

Explanation:

Given

→U = 90 km/hr = 90 x 5/18 m/s = 25 m/s

→distance travelled till velocity becomes zero = 15m

→V= 0 m/s (stopped)

V^2 - U^2 = 2as

(0)^2 - (25)^2 = 2(a)15

- 625 = 30(a)

a= -625/30= -20.8m/s^2. (minus shows retardation/acceleration is decreasing)

v=u+at

0 = 25 - 20.8 (t)

20.8(t)= 25

t = 25/20.8

t=1.2 seconds