The value of the universal gravitational constant G is:

Isaac Newton's Universal Law of Gravitation, published in 1687 in "Principia Mathematica", states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres:

F = G m₁m₂ / r²

Here, G is the universal gravitational constant. Its value is about 6.67 × 10⁻¹¹ newton square metre per kilogram squared (N m² kg⁻²). Its dimensional formula is [M⁻¹ L³ T⁻²]. G is called "universal" because its value is the same everywhere in the universe, regardless of the masses, the medium between them or the time.

The value of G is extremely small. This is why the gravitational force between everyday objects, such as two people, is too weak to notice. It becomes significant only when at least one of the bodies is very massive, such as the Earth or the Sun.

The value of G was first measured in 1798 by the British scientist Henry Cavendish, using a torsion balance. In this experiment, he measured the tiny force of attraction between lead spheres. From this, he was able to calculate the mass and average density of the Earth; the experiment is often called "weighing the Earth".

It is important not to confuse G with g:

G is the universal gravitational constant; it is a constant scalar quantity.

g is the acceleration due to gravity at a place, about 9.8 m/s² on the Earth's surface. It varies from place to place: it is greater at the poles than at the equator, and it decreases with height above the Earth and with depth below the surface. The relation is g = GM/R².

Why not the other options?

• ✗9.8 m s⁻² is the value of g, the acceleration due to gravity.
• ✗1.6 × 10⁻¹⁹ C is the charge of an electron.
• ✗Good to know
• ✗6.67 × 10¹¹ has the wrong sign of the exponent.