### Q.6:- A car moving along a straight highway with speed of 126 km h^{-1} is brought to a stop within a distance of 200 m. What is the retardation of the car (assumed uniform), and how long does it take for the car to stop ?

**Answer:-**

Initial velocity of the car, *u* = 126 km/h = 35 m/s

Final velocity of the car, *v* = 0

Distance covered by the car before coming to rest, *s* = 200 m

Retardation produced in the car = *a*

From third equation of motion, *a* can be calculated as:

v^{2} – u^{2} = 2as

(0)^{2} – (35)^{2} = 2 × a × 200

a = – 35 × 35 **/** 2 × 200 = – 3.06 ms^{-2}

From first equation of motion, time (*t*) taken by the car to stop can be obtained as:

v = u + at

t = (v – u) / a = (- 35) **/** (-3.06) = 11.44 s