Question

a driver driving a truck at a constant speed of 20 metre per second suddenly saw a parked car ahead of him by 95km. he could apply the brake after sometime to produce retardation of 2.5 metre per second square. an accident was just avoided. his reaction time is​

Answer 1

Given that,

Velocity = 20 m/s

Retardation = 2.5 m/s²

Distance = 95 km

We need to calculate the time

Using first equation of motion

$v=u-at$

Where, v = final velocity

u = initial velocity

a = deceleration

t = time

Put the value into the formula

$0=20-2.5t$

$t=8\ sec$

We need to calculate the distance

Using second equation of motion

$s=ut-\dfrac{1}{2}at^2$

Put the value into the formula

$s=20\times8-\dfrac{1}{2}\times2.5\times8^2$

$s=80\ m$

We need to calculate the extra distance

Using formula for distance

$D=d-s$

Put the value into the formula

$D=95-80$

$D=15\ m$

We need to calculate the reaction time

Using formula of velocity

$v=\dfrac{D}{t}$

$t=\dfrac{D}{v}$

Put the value into the formula

$t=\dfrac{15}{20}$

$t=0.75\ sec$

Hence, The reaction time is 0.75 sec.