Speed of car = 18 km h^{-1}

(i) To convert speed to m s^{-1}

18 km h^{-1} = 18\ km\over 1\ h

= 18 \times 1000\ m\over 60 \times 60\ s

= 18 \times 10\ m\over 6 \times 6\ s

= 5 m s<sup>-1</sup>

Hence, **18 km h ^{-1} is equal to 5 m s^{-1}.**

(ii) As the car is stopped, the final velocity = 0

Initial velocity (u) = 18 km h^{-1} = 5 m s^{-1}

Time (t) = 5 s

Acceleration (a) = v-u\over t

= 0-5\over 5

= -1 m s^{-2}

Hence, acceleration of the car is -1 m s^{-2}. Negative sign shows that the velocity decreases with time, so **retardation is 1 m s ^{-2}.**

(iii) Time (t) = 2 s

acceleration (a) = -1 m s^{-2}

Initial velocity (u) = 5 m s^{-1}

Substituting the values in the formula for acceleration, we get,

Acceleration (a) = v-u\over t

or, -1 = v-5\over 2

or, -2 = v – 5

or, v – 5 = -2

or, v = 3 m s^{-1}

Hence, the **speed of the car after 2 s of applying the brakes = 3 m s ^{-1}.**