Question

Assertion: The speedometer of an automobile measures the instantaneous speed of the automobile.
Reason: The average velocity is equal to the total displacement divided by total time taken.
(A) Both the Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(C) Assertion is correct but Reason is incorrect.
(D) Both Assertion and Reason are incorrect.

Answer 1

Hint
The speedometer of an automobile is used to measure the instantaneous speed of the vehicle. And the average velocity is the ratio of the total displacement and the total time. So both the statements are correct but they are not related.

Complete step by step answer
The speedometer of a vehicle is a gauge that is attached to a vehicle and measures the instantaneous speed of that vehicle. It has no measure for the average speed or the average velocity of that vehicle.
The average velocity of a vehicle can be found by dividing the total displacement of the vehicle by the total time.
So if the initial position of any vehicle is ${S_i}$ and the final position is given by ${S_f}$, then the total displacement of the vehicle is,
$\Delta S = {S_f} - {S_i}$
If this displacement occurs in the time span of $\Delta t$, which is given by, $\Delta t = {t_2} - {t_1}$ where ${t_2}$ and ${t_1}$ are the final and initial time, then the average velocity can be found by,
${V_{avg}} = \dfrac{{\Delta S}}{{\Delta t}}$
That is equal to,
${V_{avg}} = \dfrac{{{S_f} - {S_i}}}{{\Delta t}}$
So the average velocity is the total displacement divided by the total time taken.
Therefore, both the statements given in the question are correct. But there is no relation between the assertion and the reason. So the reason is not a correct explanation of the assertion.
Therefore the option B. is correct.

Note
There are two types of speedometers present. One is the manual speedometer and the other is the electronic speedometer. In the manual speedometer, the wheel gear rotates the aluminum disc which in turn rotates the cable, whereas, in an electronic speedometer, a speed sensor is used.