Question

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

• A. $0.5s$
• B. $0.75s$
• C. $0.8s$
• D. $1s$

Answer 1

Answer: (B)

$0.75s$

$v_{rel}=u_{rel}+a_{rel}t\Rightarrow 0=20-2.5t\Rightarrow t=8s.$
Now,
$s=ut+\frac{1}{2}a{t}^2$

$s=(20\times 8)-\frac{1}{2}\times 2.5\times {\left(8\right)}^2$

$s=160-80s=80m$

Now, extra distance is $15m$

Thus, reaction time$=\frac{15}{20}=0.75s$