Physical World and Measurement

Science helps in studying reality. Most investigations use some form of scientific method, which is a logical order of steps by which scientists and researchers conclude about the things around them. We discover physics from the fundamental learning of measuring. First, science starts by measuring physical quantities like length, time, temperature, heat, current etc. We measure each physical quantity by assigning a unit to it.

There are mainly two fields of science.

• Natural science: The science that we use to study the natural world, i.e., physics, chemistry, biology, etc.
• Social science: Study of human behaviour and society.   

As we are studying the physical world, let us see two main thrusts of physics.

1. Unification: In physics, many concepts and laws are proved after many attempts. These attempts are made to unify the fundamental forces of nature in terms of unification.
2. Reductionism: Attempts made to explain macroscopic concepts in terms of microscopic constituents. This process is known as reductionism.

There are two domains in physics.

• Macroscopic domain: Explains terrestrial, astronomical scales.
• Microscopic domain: Explains atomic, molecular and nuclear phenomena.

Some other classifications of physics are as follows:

1. Astrophysics: Study of planets, stars and their interaction.
2. Geophysics: Study of physics of the earth.
3. Biophysics: Study of physical laws relating to living organisms.

Link between technology and physics:

1. Aeroplane                   – Bernoulli’s principle in fluid dynamics.
2. Electron microscope – Wave nature of electrons.
3. Photocell                     – Photoelectric effect.
4. SONAR                         – Reflection of ultrasonic waves.
5. Steam engine – Law of thermodynamics.

Fundamental forces in nature

There are four fundamental forces in nature.

• Gravitational force.
• Weak Nuclear force.
• Strong Nuclear force.
• Electromagnetic force.

1. Theories that postulate the unification of strong and weak nuclear force and electromagnetic force are called grand unified theories.
2. Theories that add gravity to unify all four fundamental forces to a single force are called super unified theories.

What is electromagnetic force?

It is the force due to interaction between two moving charges. It is caused by the exchange of two photons between two charged particles. 

Units and dimensions

Physical quantities

All quantities measured directly and indirectly in terms of the laws of physics are called physical quantities.

Eg: mass, length, speed, force, etc.

Types of physical quantities

1.   Fundamental quantities are physical quantities that do not depend on any other fundamental or basic physical quantities. Eg: mass, length, time, luminous intensity.
2.   Derived quantities are physical quantities that depend on fundamental quantities. Eg: speed, acceleration, force, etc.

Unit

Unit is the standard unit for comparison.

Points to be noted for selecting a unit:

1. It should be suitable in size.
2. It should be defined accurately.
3. It should not change with time.
4. It should be universally acceptable.

Eg: To measure a distance, it is defined as 15 m.

Where 15 is a number and m is a unit for measuring distance.

If Q is a magnitude, then

Q=nu=n1u1=n2u2

Thus n is inversely proportional to u.

u1 and u2 are units and n1 and n2 are numerical values in two different systems of units.

Different types of unit systems

1. FPS (Foot-Pound-Second): The unit of length is foot, mass is pound and time is second.
2. CGS (Centimetre-gram-second): The unit of length is centimetre, mass is gram and time is second.
3. MKS (Metre-Kilogram-Second): The unit of length is metre, mass is kilogram and time is second.
4. SI (international system) units: The system of units employed by scientists and engineers worldwide is usually called the system of weights and measures, also called SI or the International System of Units since 1960. The advantages of the International System are: 
5. This method uses only 1 unit for one physical quantity, which suggests a rational system of units.
6. During this system, all the derived units are easily obtained from basic and supplementary units, which suggests it is a coherent system of units.
7. It is a system of weights and measures which suggests that multiples and submultiples will be expressed as powers of 10.

Measurement of basic quantities 

Measurement of length

Length is the distance between the 2 points. Metre is the SI unit of length. Large objects like the stars, galaxy, sun, moon, etc., constitute a macrocosm. Objects like molecules, atoms, electrons, bacteria, etc., and their distance constitute a microcosm.

(1) Measurement of small distances using:

Screw gauge: It is an instrument used for accurately measuring the dimension of an object up to a maximum of about 50 mm. The principle of this instrument is the magnification of linear motion using the circular motion of a screw. 

Vernier calliper: A measuring device used to measure linear dimensions and outer and inner diameters and depths.

(2) Measurement of large distances:

Triangulation method:

The formula for this method is

                tan (y) = height(h)/adjacent distance(x)

               =>h  = x tan y .

Parallax method:

Vast distances like the distance of a planet or star from the earth can be measured by the parallax method. Parallax means an apparent change in the object’s position with respect to the background when the object is seen in two different positions.

RADAR method:

Radio Detection And Ranging (RADAR) is used to measure the distance of a nearby planet or stars accurately. The formula for this method is

                D= v t/2

Where D is the distance travelled

t is the time taken and

v is the velocity of the radio wave.

Measurement of mass

Kilogram (Kg) is the International System unit of mass. For measuring large masses like planets, earth, etc., we use the  gravitational methods. To measure small masses like atoms, we use the mass spectrograph.

Errors in measurement

• Systematic errors:

These are inaccuracies that can be further created repeatedly in the same direction. Systematic errors can be classified as follows .

(a) Instrumental errors:

Instrumental errors occur due to faulty instruments – for instance, a metre scale with worn-out ends. 

b) Imperfections in experimental techniques:

These errors arise due to the limitations in experimental arrangements. For instance, using a calorimeter with no proper insulation can cause radiation losses and introduce errors.

(c) Personal errors:

When those who perform the experiment cause errors due to incorrect experiment arrangements or careless observations, personal errors occur.

(d) Errors due to external causes:

The change in external conditions during an experiment can cause errors. Eg: a change in temperature, humidity may affect the result.

(e) Least count errors:

The least count is the smallest value that an instrument can measure; an error due to this measurement is the least count error. The least count error can be avoided by using a high precision instrument for measurements.

2.Gross errors:

The errors due to the sheer carelessness of the observer are gross errors. Being careful and mentally alert can minimise such errors. 

Error analysis

1. Absolute error: The measurement between the true and measured value of a quantity is called absolute error.
2. Mean absolute error: The arithmetic mean of magnitude of absolute errors in all measurements is called mean absolute error.
3. Relative error: The mean absolute error and the mean value is known as relative error. This is also known as fractional error.
4. Percentage error: The relative error expressed in percentage is called percentage error.
   

Significant figures

The digits that are known reliably plus the first uncertain digits are known as significant figures or significant digits.

Eg: The number 0.0006032 has four significant figures.

Rules for counting significant figures:

1. All non-zero digits are significant.
2. All zeros between two non-zero digits are significant.
3. If the number is less than one, then zero(s) on the right of the decimal point but to the left of the first non-zero digit are non-significant.

Eg: 0.00345 has three significant figures.

4. All zeros to the right of the decimal point and the right of non-zero digits are significant.

Eg: 40.00 has four significant figures.

Dimensions

Conclusion

Evaluation of science begins with the fundamentals of counting. Humans start applying mathematics on scientific laws when they first start counting. Unit is the basic tool for counting physical quantities. So measurement is the basic and fundamental concept for understanding and applying science laws in real life. Some basic concepts of measurement and units are discussed in the article.