Gravitational constant

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The gravitational constant G is a fundamental physical constant which appears in Newton's law of universal gravitation and in Einstein's theory of general relativity.

In terms of SI units, the value of the gravitational constant is:

G = (6.674215 ± 0.000092) × 10⁻¹¹ N·m²/kg²

This means that two masses of 1 kilogram each, placed a distance of 1 meter apart, attract each other gravitationally with a force of about 6.67 × 10⁻¹¹ newton.

The uncertainty of 14 parts per million in its value puts the gravitational constant among the least precisely determined physical constants. Equally uncertain is therefore the mass of the Sun. The positions of the planets are known far more accurately, and also the product of G and the mass of the Sun. Therefore calculations in celestial mechanics are carried out using the unit of solar mass rather than the standard SI unit kilogram. In this case we use the Gaussian gravitational constant:

k = 0.01720209895 A³D^-2S^-1

G was first measured by Henry Cavendish (Philosophical Transactions 1798). He used a horizontal torsion beam with lead balls whose inertia (in relation to the torsion constant) he could tell by timing the beam's oscillation. Their faint attraction to other balls placed alongside the beam was detectable by the deflection it caused. See torsion bar experiment.

By combining the gravitational constant with Planck's constant and the speed of light in vacuum, it is possible to create a system of units known as Planck units. The gravitational constant, Planck's constant and the speed of light all take the numerical value 1 in this system.

External links

Measurement of NewtonÃÂÃÂs Constant Using a Torsion Balance with Angular Acceleration Feedback