Chapter 1: Physical Quantities and Measurement Physics Class 9 Short Questions and Answers

Q: What is the difference between base quantities and derive quantities? Give three examples in each case.
Ans:
Base Quantities:
The base quantities are those seven physical quantities which form the foundation for other physical quantities. These physical quantities are called base quantities.
Examples:
Length, Mass, Time
Derive Quantities:
Those physical quantities which are expressed in term of base quantities are called derive quantities.
Examples:
Volume, speed, Force

Q: Pick out the base unit and derive units in the following:
Joule, Newton, kilogramme, mole, ampere, metre, Kelvin, hertz, coulomb, and watt.
Ans:
BASE UNITS: Kilogramme, Mole, Ampere, Metre, Kelvin
DERIVE UNITS: Joule, Newton, Hertz, Coulomb, Watt

Q: Find base quantities involved in each of the following derive quantities.
(A) Speed
(B) Volume
(C) Force
(D) Work
Ans:
(A) Speed:
speed= distance/time = length/ time
So base quantities involved in speed are "length" and "time".
(B) Volume:
VOLUME = length × width × height
As width, and height basically indicate the lengths along the particular direction so base quantity involved in volume is " length".
(C) Force:
F= ma
·.· a = vf-vi /t
·.· v = d/ t
.·. Base quantities involved in force are "mass, length and time".
(D) Work:
W = F×d
·.· F= ma
·.· a = vf-vi /t
·.· v = d/ t
d is also length therefore:
Base quantities involved in Work are "mass, length and time".

Q: Estimate your age in seconds.
Ans: My age is exact 16 year 1 month and 3 days
One year= 365 days
1 month = 30 days
Therefore,
16 years, 1 month, 3 days = {(16×365)+(1×30)+3} =5873 days
and
1 day= 24 hours
Therefore
5873 days = 24 × 5873 hours
5873 days = 140952 hours

Also,

1 hour = 60×60 seconds
1 hour = 3600 seconds

Therefore,
140952 hours = 140952 × 3600 seconds
140952 hours = 507427200 seconds
140952 hours = 5.07 × 108 seconds
Therefore, there are 5.07 × 108 seconds in my age.

Q: What role SI units have played in the development of science.
Ans: SI units have played an important in the development of science. This system facilitates the students to understand different units and also to understand their mutual relationships. It became easier to convert one unit into other, which facilitate the scientific research. Using these, SI units we can also understand that what physical quantities are involved in any derived quantity.

Q: What is meant by vernier constant?
Ans: The difference between one small division on main scale of vernier calliper and one vernier scale division is 0.1mm and 0.01cm. This is called least count or vernier constant.

Q: What do you understand by the zero error of measuring instrument?
Ans: Zero error is basically the systematic error which exist in measuring instrument. This is the error in instrument due to which it shows measurement more or less than the actual measurement. This error can be recovered by adding or built acting the difference.

Q: Why the use of zero error is necessary in measuring instrument?
Ans: Knowledge of zero error is necessary for measurement. Otherwise our measurement would be inaccurate. Because this error effects all the measurement taken by the instrument.

Q: What is stopwatch? What is the least count of mechanical stopwatch you have in laboratories?
Ans: A stopwatch is an instrument which is used to measure the time interval of an event.
Least count of mechanical stopwatch we have in our lab is 0.1 second.

Q: Why we need to measure extremely small interval of times?
Ans: In scientific experiments, often we need to measure small intervals of time with perfect accuracy. For example
Time period of vibration of helical spring, time period of simple pendulum and in free fall experiments time taken in falling the bob etc. these time intervals must be measured with accuracy.

Q: What is meant by significant figures?
Ans: The significant figures are all tge accurately known digits and the one estimated digit. More significant figures means greater precision.
EXAMPLES:
0.03 has 1 significant figure
0.274 has 3 significant figures

Q: How is precision related to the significant in a measured quantity?
Ans: Significant figures play an important role in precise measurement. More significant figures means greater precision.
For Example:
If diameter of rod is measured by using ruler and it measures 5cm.
When same diameter is measured using vernier calliper it becomes 5'02cm.
In first case significant figure is one while in second case significant figures become 3. Number of significant figures are increased also we can see that measurement becomes more precise. Hence greater number of significant figures means greater precision.