Gravity

NASA lists Earth's mean surface gravity as 9.820 metres per second squared, with 9.780 at the equator and 9.832 at the poles.

The standard acceleration of gravity is defined as exactly 9.80665 metres per second squared.

Gravity is slightly stronger over places with more mass underground. NASA's GRACE spacecraft measured these variations and mapped them.

On the Moon, surface gravity is 1.62 metres per second squared.

Anything with mass has gravity, more mass means more gravity, and gravity weakens with distance.

The gravitational force between two masses follows the universal law of gravitation. It is an inverse square law, always attractive, and the two masses pull on each other with equal and opposite forces.

The Newtonian constant of gravitation is 6.674 30 x 10^-11 cubic metres per kilogram per second squared, with a standard uncertainty of 0.000 15 x 10^-11, in the 2022 CODATA values.

Gravity is the weakest of the four fundamental forces, yet it dominates the large-scale structure of galaxies and stars.

Einstein described gravity as a curve in space that wraps around an object such as a star or planet. Another object nearby is pulled into the curve.

In general relativity gravity is associated with a curvature of space-time, and changes in the arrangement of mass can produce gravitational waves.

Gravity also pulls on light. Shine a flashlight upwards and the light grows imperceptibly redder, a change scientists can measure.

Gravity Probe B, launched on 20 April 2004, tested the geodetic and frame-dragging effects of general relativity. It watched four near-perfectly spherical gyroscopes from a polar orbit 400 miles above the Earth.

At the end of the last Apollo 15 moonwalk, Commander David Scott dropped a geologic hammer and a feather together for the television cameras.

With no air resistance, the feather fell at the same rate as the hammer, as Galileo had concluded centuries earlier: objects released together fall at the same rate regardless of mass.

Inside a uniform-density Earth, gravity grows in proportion to the distance from the centre, so a falling traveller would oscillate through the centre like a mass on a spring.

Assuming uniform density and neglecting air friction, the traveller would pop up on the opposite side of the Earth after a little more than 42 minutes.

The traveller would be weightless at the centre, moving at about 7,900 metres per second.

With no friction there would be no energy loss, so the traveller would keep oscillating in and out of the tunnel.

Using the Earth's internal structure from seismic data, Alexander Klotz found that a fall along the diameter would take 38 minutes rather than 42.

The trip could not happen in the real world. One obstacle is the temperature of about 6,000 degrees in the core.