Following are the list of important cosmic numbers that define the universe and are constant.

**Universal gravitational constant:**

The universal gravitational constant ‘G’ = 6.67300*10-11m3kg-1s-2. Universal constant is used in Newton’s law of gravitation and also in Albert Einstein’s theory of general relativity. It is used to determine the force acting between two masses.

The mathematical representation is given as:

F=(G*m1*m2)/(r2)

Where,

F: force acting between the two masses

G: gravitational constant

m1 and m2: masses of the two bodies

r : distance between the two masses

**Mach number:**

Mach number ‘M, is defined as a dimensionless value used in fluid flow dynamics where the significant factor is compressibility. The mathematical representation is given as:

M=(v)/(c)

Where,

M: Mach number

v: fluid flow speed in m.s-1

c: speed of sound in m.s-1

Depending upon the value of Mach number, speed can be classified as:

- If M<1, the flow speed is known as subsonic as the flow speed is less than the speed of the sound.
- If M∼1, the flow speed is known as transonic as the flow speed is approximately like the speed of the sound.
- If M>1, the flow speed is known as supersonic as the flow speed is greater than the speed of the sound.
- If M>>1, the flow speed is known as hypersonic as the flow speed is much greater than the speed of the sound.
- The speed of light:

The speed of the light ‘c’ is having an exact value of 299,792,458 metres per second by the definition of metre, which is defined as the length of the path traveled by light in vacuum during a time interval of 1/299792458 second.

The other values of speed are given as:

- Astronomical units per day: 173
- Miles per second: 186000
- Metres per second: 299792458
- Miles per hours: 671000000
- Kilometres per hour:1080000000
- From sun to earth (1 AU): 8.3min

**Absolute zero:**

According to the thermodynamic temperature scale, when entropy and enthalpy of a cooled ideal gas reaches a minimum value of 0, it is known as absolute zero. Lord Kelvin in the 19th century with his Kelvin temperature scale defined zero point as absolute zero. It is calculated by adding 273° to the ℃ temperature.

**Avogadro constant:**

Avogadro constant NA is defined as the number of molecules or atoms in a mole of a given substance is 6.022140857*1023 mol-1. It is named after scientist Amedeo Avogadro. Other values of NA based on units are:

- 6.022140856*1023 mol-1
- 2.73159734(12)*1026 (lb-mol)-1
- 1.707248434(77)*1025 (oz-mol)-1

These were a few important numbers of Physics. To know more important concepts of Physics like conservation of angular momentum**, **stress and strain, unit of capacitance, etc, visit BYJU’S.

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