Q. 44.2( 11 Votes )

# A bus is moving with a speed 72 km/h can be stopped by brakes after at least 10 m. What will be the minimum stopping distance, if the same bus is moving at a speed of 144 km/h?

Answer :

We have to first find the retardation.

Using the third equation of motion: v^{2} – u^{2} = 2as

where: s = distance covered = 10 m

v = final velocity = 0 m/s (brakes are applied so that bus

comes to rest)

u = initial velocity = 72 km/h = 20 m/s

a = acceleration = ? m/s^{2}

v^{2} – u^{2} = 2as

0 – 20^{2} = 2 × a × 10

-400 = 20 × a

= a

a = -**20 m/s ^{2}**

Now, u = initial velocity = 144 km/h = 40 m/s

s = distance covered = ?? m

v = final velocity = 0 m/s ( brakes are applied so that bus comes to rest)

a = acceleration = -20 m/s^{2}

Using the third equation of motion: v^{2} – u^{2} = 2as

0 – 40^{2} = 2 × -20 × s

-1600 = -40 × s

= s

s = **40 m**

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