Question

All the following statements about instantaneous speed are false except
A. Instantaneous speed is, when an object moves with the same speed throughout the journey.
B. Instantaneous speed is measured in \[{\text{c}}{{\text{m}}^{\text{3}}} \cdot {{\text{h}}^{{\text{ - 1}}}}\].
C. Instantaneous speed is the average speed over a certain distance.
D. Instantaneous speed is the speed at any instant of time during the journey.

Answer 1

Hint: Remind the basics of instantaneous speed. Check the unit of the instantaneous speed, the relation of the instantaneous speed with the average speed and whether the instantaneous speed of an object remains the same all the time.

Complete step by step answer:
When an object moves with a constant speed throughout the journey, then the speed of the particle remains the same and uniform all the time. Such an object is said to have an uniform speed and not an instantaneous speed.
The instantaneous speed of an object is constant only in uniform circular motion.
Hence, the statement given in option A is false.
The instantaneous speed or any speed of an object has various units which are meter per second (\[{\text{m}} \cdot {{\text{s}}^{{\text{ - 1}}}}\]), centimeter per second (\[{\text{cm}} \cdot {{\text{s}}^{{\text{ - 1}}}}\]), kilometer per hour (\[{\text{km}} \cdot {{\text{h}}^{{\text{ - 1}}}}\]), etc.
But \[{\text{c}}{{\text{m}}^{\text{3}}} \cdot {{\text{h}}^{{\text{ - 1}}}}\] is not the unit of any type of speeds and hence, not the unit of instantaneous speed also.
Therefore, instantaneous speed is not measured in \[{\text{c}}{{\text{m}}^{\text{3}}} \cdot {{\text{h}}^{{\text{ - 1}}}}\].
Hence, the statement given in option B is false.
The time duration for which the instantaneous speed is calculated is very small and reaches nearly to zero.
Hence, the instantaneous speed is defined as the limit of the average speed and not the average speed over a certain distance.
Hence, the statement given in the option C is false.
The instantaneous speed is the speed of an object at any particular instant throughout the motion of the object or during the journey of the object.
Hence, the statement given in option D is the only correct statement about instantaneous speed.

So, the correct answer is “Option D”.

Note:
The instantaneous speed of any object cannot be measured accurately and instantaneously as for any instrument to measure the speed needs some time.
The instantaneous speed of an object is constant only in uniform circular motion.