## FSc Notes: Physics XI: Chapter 01 Measurements Exercise Short Questions:

**Question 1.1 Name several repetitive phenomenons’ occurring in nature which could serve as reasonable time standards?**

Answer 1.1Every type of natural phenomenon which is repetitive after same intervals of time acts as reasonable time standards for e.g.

Answer 1.1

- Rotation of earth around the sun in 365 days,
- Change of shadows of objects position of sun position of star,
- Moon revolve around earth in 28 to 29 days.

**Question 1.2 Give the draw backs to use time period of a pendulum as a time standards?**

Answer 1.2Drawbacks to use time period of a pendulum as a time standard

Answer 1.2

- Air friction can change the time period.
- Due to increase in temp length of pendulum
- Increase and hence the time period increases.
- Due to change in position from sea level gravitational acceleration changes hence time period changes.

**Question 1.3 Why do we find it useful to have two units for the amount of substance, the kilogram and the mole?**

Answer 1.3It is very useful to have two units for the amount of substance i.e. kg mole.

Answer 1.3

**Kilogram**is a**macro level unit**it only gives the amount of substance.**Mole**is a**micro level unit**one mole of substance contains the same no of molecules, no of radicals no of atoms etc.

**Question 1.4 Three students measured the length of a needle with a scale on which minimum**

**division is 1mm and recorded as (1)0.2145m (2).21m (3).214m**

**which is correct and why?**

Answer 1.4In these readings

Answer 1.4

**option (3).214m is correct**because the

**least count is 1m=.001mm**it measures accurately up to

**three decimal places**.

**Question 1.5 An old saying is that “A chain is as strong as its weakest link” what analogous statement can you make regarding experimental data used in computation?**

Answer 1.5The analogous statement is:

Answer 1.5

**A given data is as much accurate as its least accurate values.**

**Question 1.6 The period of a simple pendulum is measured by a stop watch what type of errors are possible in the time period?**

Answer 1.6Following errors is possible in the time period:

Answer 1.6

- Systematic error
- Random error
- Personal error
- Air friction.

**Two answers are possible here: Yes, we can find the units of constants by using dimensional analysis (e.g. G). If the constant is a number such as 2π and 6π, then dimensional analysis gives no information about the constant.**

Question 1.7 Does a dimensional analysis give any information on constant of proportionality that may appear in an algebraic expression? Explain.

Answer 1.7

Question 1.7 Does a dimensional analysis give any information on constant of proportionality that may appear in an algebraic expression? Explain.

Answer 1.7

**Question 1.8 Write the dimensions of (1) pressure and (2) density?**

Answer 1.8

Answer 1.8

**(1) Pressure: P = F/A= ma/A**

So [P] = [m] [a]/ [A] = [M] [LT-2]/ [L2]

[P]= [M1] [L1] [T-2] [L-2] = [M1] [L-1] [T-2]

**[P] = [ML-1T-2]**

**(2) Density: =m/v**

[d] = [M] / [L3] = [ML-3]

**[d] = [ML-3]**

**Question 1.9 The wavelength ƛ of a wave depends on the speed v of the wave and its**

frequency f. Knowing that [ƛ] = [L] [v] = [LT-1] [f] = [T-1].

Decide which of the following is correct. f=v ƛ or f=v/ ƛ.

Answer 1.9Given [ƛ] = [L] [v] = [LT-1] [f] = [ T-1]

frequency f. Knowing that [ƛ] = [L] [v] = [LT-1] [f] = [T-1].

Decide which of the following is correct. f=v ƛ or f=v/ ƛ.

Answer 1.9

**(i) f=v ƛ**

[T -1]= [LT-1] [L]

Or [T -1] = [L2T-1]

**As the dimension on both are not same so ( I ) is incorrect.**

**(ii) f=v/ ƛ**

[T -1] = [LT-1]/ [L]

[T -1] = [T-1]

As the dimension on both sides are same so formula (ii) is correct.

As the dimension on both sides are same so formula (ii) is correct.

very good thank you

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