Question

A person driving a car with a speed of 72 km/h suddenly sees a boy crossing the road. If the distance moved by car, before the person apply brakes is 5 m, the reaction time of the person is:
A. 0.5s
B. 0.1s
C. 10 s
D. 0.25s

Answer 1

Hint: This is a straight question where the formula of speed and velocity both can be used. Just used the formula and given data in the question. Put the value in the formula and get the answer. Basically, the meaning of reaction time is time spent at a distance in a given velocity.

Complete step by step answer:
To solve this type of question, first, understand what question you want to convey.
Explanation of question: a person who is driving a car with a speed of 72 Km/h, suddenly sees a boy while a boy is crossing a road. As soon as he sees the person, he applies the break. But within this time, the car gets crossed by 5 m.
We will see this question by a diagram, after that, we will solve this problem.
Now, what is reaction time? Time spent between object sees and Appling break by a driver.
Diagram:

Within distance A and B, the driver applies break.
Formula used:
Velocity =$=\dfrac{displacement}{time}$
Mathematically,
$v=\dfrac{d}{t}$
Where, v = velocity of car
d = displacement done by car is 5 m
t = time
Conversion of unit:
v=72 Km/hr
To convert into m/s:
v= 72 Km/hr=$72\times \dfrac{5}{18}$=20 m/s
Now find time:
$\begin{align}
  & t=\dfrac{d}{v} \\
 & t=\dfrac{5}{20} \\
 & t=\dfrac{1}{4} \\
 & \\
\end{align}$
t = 0.25 second or s
This implies that within 0.25 second, the driver sees the boy crossing the road and suddenly applies a break.
So, the correct answer is “Option D”.

Note:
Just apply a formula of velocity in this case, you can use the formula of speed also. Answers calculated by both the formulas will be the same. Speed is nothing but the distance cover by the given unit time.do not forget to mention the unit after the answer.