A driver driving a truck at a constant speed of $20 m s^{- 1}$ suddenly saw a parked car ahead of him by $95 m$ . He could apply the brake after some time to produce retardation of $2.5 m s^{- 1}$ .An accident was just avoided, his reaction time is

0.5 s

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0.75 s

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0.8 s

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1 s

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Solution

The correct option is C $0.75 s$
$v_{r e l} = u_{r e l} + a_{r e l} t \Rightarrow 0 = 20 - 2.5 t \Rightarrow t = 8 s .$
Now,
$s = u t + \frac{1}{2} a t^{2}$
$s = (20 \times 8) - \frac{1}{2} \times 2.5 \times {(8)}^{2}$
$s = 160 - 80 s = 80 m$
Now, extra distance is $15 m$
Thus, reaction time $= \frac{15}{20} = 0.75 s$

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A driver driving a truck at a constant speed of 20 ms−1 suddenly saw a parked car ahead of him by 95 m. He could apply the brake after some time to produce retardation of 2.5 ms−1.An accident was just avoided, his reaction time is

The correct option is C 0.75 svrel=urel+arelt⇒0=20−2.5t⇒t=8s.Now,s=ut+12at2s=(20×8)−12×2.5×(8)2 s=160−80s=80 mNow, extra distance is 15 mThus, reaction time=1520=0.75 s

A driver driving a truck at a constant speed of $20 m s^{- 1}$ suddenly saw a parked car ahead of him by $95 m$ . He could apply the brake after some time to produce retardation of $2.5 m s^{- 1}$ .An accident was just avoided, his reaction time is