JEE Main · 2004 · 24 Apr 2004 · Offline · Kinematics

An automobile travelling with speed of

60 km / h

, can brake to stop within a distance of 20 m. If the car is going twice as fast, i.e

120 km / h

, the stopping distance will be

(A)20 m
(B)40 m
(C)60 m
(D)80 m✓ Correct

Answer: (D)

Solution

On the same surface the braking deceleration is unchanged, and

v^{2} = u^{2} - 2 a s

with the final speed zero gives a stopping distance

s = \frac{u ^{2}}{2 a}

that grows as the square of the initial speed. Raising the speed from

60

120

km/h doubles it, so the stopping distance increases by a factor of

2^{2} = 4

, giving

4 \times 20 = 80

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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