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## Chapter: 11th Physics : UNIT 1 : Nature of Physical World and Measurement

Physics : Nature of Physical World and Measurement : Book Back Important Questions, Answers, Solutions : Short Questions and Answer

Nature of Physical World and Measurement | Physics

1. Briefly explain the types of physical quantities.

Physical quantities are classified into two type three are fundamental and derived quantities.

Fundamental base quantities are which cannot be expressed in terms of any other physical quantities. These are length, mass, time electric current, temperature, luminous intensity and amount of substance.

Quantities that can be expressed in terms of fundamental quantities are called derived quantities. For example area, volume, velocity, acceleration, force etc.

2. How will you measure the diameter of the Moon using parallax method?

Once the distance ‘D’ of a planet is determined the diameter ‘d’ angular size of the moon can be estimated by parallax method.

Two diametrically opposite points M and N of moon are viewed through telescope from a point A on the earth. The angle α between the two directions viewed is measured. Then by considering MN as arc of length d of a circle with centre at A and distance D as radius, we can write.

α = d / D

(or) d = α D 3. Write the rules for determining significant figures.

Rule for counting significant figures Rule and Example:

i. All non-zero digits are significant

Example : 1342 has four significant figures

ii. All zeros between two non zero digits are significant

Example : 2008 has four significant figures

iii. All zeros to the right of a non-zero digit but to the left of a decimal point are significant.

Example : 30700 has five significant figures

iv. a) The number without a decimal point, the terminal or trailing zero (s) are not significant.

b) All zero are significant if they come from a measurement.

Example : a) 30700 has three significant figures b) 30700 m has five significant figures.

v. If the number is less than 1, the zero (s) on the right of the decimal point but to left of the first non zero digit are not significant.

Example : 0.00345 has three significant figures

vi. All zero to the right of a decimal point and to the right of non-zero digit are significant.

Example : 40.00 has four significant figures and 0.030400 has five significant figures

vii. The number of significant figures does not depend on the system of units used

Example : 1.53 cm, 0.0153m, 0.0000153 km. all have three significant figures.

4. What are the limitations of dimensional analysis?

Limitations of Dimensional analysis

1. This method gives no information about the dimensionless constants in the formula like1, 2………… π, e, etc.

2. This method cannot decide whether the given quantity is a vector or a scalar.

3. This method is not suitable to derive relations involving trigonmetric, exponential and logarithmic functions.

4. It cannot be applied to an equation involving more than three physical quantities.

5. It can only check on whether a physical relation is dimensionally correct but not the correctness of the relation. For example using dimensional analysis, s = ut + 1/3at2 is dimensionally correct relation is s = ut ½ at2

5. Define precision and accuracy. Explain with one example.

Accuracy is a measure of the closeness of the measured value to the true value.

The precision of an instrument gives the minimum value that can be measured by it.

If a measurement is precise, that does not necessarily mean that it is accurate. How ever if the measurement is consistently accurate, it is also precise.

Example : Let the temperature of a refrigerator repeatedly measured by a thermometer be given as 10.4°C, 10.2°C, 10.3°C, 10.1°C, 10.2°C, 10.1°C, 10.1°C, 10.1°C. However if the real temperature inside the refrigerator is 90C, we say that the thermometer is not accurate but since all the measured value are close to 10°C, hence it is precise.

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