Physics Chapter 1 'Measurements' Short Questions and Answers Exercise for class 11

Q.1 Name several repetitive phenomenon occurring in nature which could serve as reasonable time standards.
Ans.
1) Natural periodic movements, e.g. stars at night.
2) Sunrise and sunset.
3) Human pulse rate.
4) Revolution of the moon around the earth.
5) Change of weathers.
6) The rotation of planets around the sun.
7) The rotation of earth about its own axis.
8) Lattice vibrations in a crystal.
9) Heart Beat.
10) Radioactive decay rate of some substances.

Q.2 Give the draw backs to use the period of a pendulum as a time standard.
Ans.

where, l = length of pendulum

g = gravitational acceleration

Time period of the pendulum depends upon g. since g varies at different places, so this cannot be a good standard.
Secondly, the pendulum should be free from frictional effects, and an ideal pendulum is difficult to construct.
Time period of the simple pendulum varies with Temperature.

Q.3 Why do we find it useful to have two units for the amount of substance, the kilogram and the mole?
Ans. The amount of a substance can be estimated by two ways:

By weighing for which the unit Kg is used.
By counting the Number of entities contained in the substance for which the unit is mole.

Mass is used for Macroscopic objects and Mole is used for Microscopic objects.

Q.4 Three students measured the length of a needle with a scale on which minimum division is 1mm and recorded as (i) 0.2145 m (ii) 0.21 m (iii) 0.214m which record is correct and why?
Ans. The correct record is 0.214m. Because the Least count of the scale is 1 mm = 1 / 1000 m = 0.001 m. So, the length can be accurately measured up to three decimals. Hence, 0.214m is although less precise but is more accurate.

Q. 5 An old saying is that “A chain is only as strong as its weakest link”. What analogous statement can you make regarding experimental data used in a computation?
Ans. Its analogous statement is "A result of experimental data is only as much accurate as its least accurate reading in experimental data."

Q.6 The period of simple pendulum is measured by a stop watch. What type of errors are possible in the time period?
Ans.
1) The least count of the timing device may result a zero error as a systematic error.
2) Timing uncertainty due to human reflections in observation as a personal error.
Q.7 Does a dimensional analysis give any information on constant of proportionality that may appear in an algebraic expression? Explain.
Ans. No, it does not give any information on constant of proportionality that may appear in an algebraic expression. We cannot determine numerical value of the constant of proportionality with dimensional analysis. It can be found experimentally or theoretically.
Q.8 Write the dimensions of (i) Pressure, (ii) Density
Ans.
(1)

PRESSURE=FORCE/AREA

P=F/A

.'. F=ma AND A=m2
NOW

P= ma/m2

P=[M][LT^-²] / [L²]

P= [ML^-¹T^-²]
⁽²⁾

^{DENSITY=MASS/VOLUME}
ρ= m/v

m=Kg , v=m3
NOW,

ρ= [M]/[L3]

ρ=[M¹L^-3]

Q.9 The wavelength λ of a wave depends on the speed v of the wave and its frequency f.
Knowing that
[λ] = [L], [v] = [L T-1 ] and [f] = [T]-1
Decide which of the following is correct,
f = v λ or f = v / λ
Ans.

Applying dimensional analysis
Dimensions of f = [T^-1] ….. (1)

Dimensions of v λ = [LT^-1] x [L] = [L² T^-1] …. (2)

Since [T^-1] ≠ [L² T^-1] So it is dimensionally not correct.

Now dimensions of f = [T^-1] ….. (3)
And dimensions of v / λ = [LT^-1] / [L] = [T^-1] ….. (4)
Since [T^-1] = [T^-1] So it is dimensionally correct.

Physics Chapter 1 'Measurements' Short Questions and Answers Exercise for class 11

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