Question

An automobile travelling with a speed of $60km/h,$ 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

Answer 1

Answer: (D)

80 m

Answer 2

The stopping distance, $S \propto u^{2}$ $(\because\mspace{6mu} v^{2} = u^{2} - 2as)$

⇒ $\frac{S_{2}}{S_{1}} = \left( \frac{u_{2}}{u_{1}} \right)^{2} = \left( \frac{120}{60} \right)^{2} = 4$

⇒ $S_{2} = 4 \times S_{1} = 4 \times 20 = 80m$