Gravitational acceleration

Gravitational acceleration is a specific type of movement in physics. It describes the steady gain in speed that an object experiences during free fall. This occurs within a vacuum, which is a space entirely free of air. Because there is no air, the object does not experience drag. Drag is the resistance that usually slows things down when they move through the atmosphere. In this state, the increase in speed is caused exclusively by gravitational attraction.

When objects are in the same gravitational field and in a vacuum, they all accelerate at the same rate. This remains true regardless of the mass or the composition of the bodies. Scientists who measure and analyze these specific rates use a field called gravimetry. On the surface of the Earth, the magnitude of gravity is not a single, simple number. It results from the combined effect of gravitation and centrifugal force. Centrifugal force is created by the rotation of the Earth. Because of this, the free fall acceleration varies depending on altitude, latitude, and longitude.

To understand the mechanism, we look to Newton's law of universal gravitation. This law states that a gravitational force exists between any two masses. The magnitude of this force is equal for each mass. The force is always aligned to draw the two masses toward each other. The strength of this attraction depends on the masses involved and the distance between them. If one object is much larger than the other, we treat the large object as a source. This source creates a gravitational field. The acceleration of a smaller sample mass depends only on the mass of the source and the distance to it. It does not depend on the mass of the smaller object itself.

Scientists use different models depending on how close an object is to a massive body. The "far-field" model works well for calculating the orbits of satellites. It treats large bodies, like planets, as point masses. This is possible because the distances between planets are much larger than the planets themselves. However, for more precision, scientists use "near-field" models. These models use the principle of superposition to account for how density is distributed through a body. This helps account for things like the bulge at Earth's equator or irregular mass concentrations on the Moon caused by meteor impacts.

Space missions have provided incredible data about these fields. In 2002, the Gravity Recovery and Climate Experiment, or GRACE, mission launched. It used two probes nicknamed "Tom" and "Jerry" in polar orbit around Earth. They measured changes in the distance between the probes to track the gravitational field. Between 2011 and 2012, the Gravity Recovery and Interior Laboratory mission studied the Moon. This mission used two probes named "Ebb" and "Flow." By orbiting the Moon, they helped scientists infer information about its physical makeup for future navigation.

Gravity values vary wildly across our solar system. The Sun has a massive acceleration of 27.9 m/s². In contrast, the Moon has a much lower acceleration of 1.65 m/s². The giant planets also show high values, such as Jupiter at 24.79 m/s². Even small bodies like Ceres have a tiny acceleration of 0.029 m/s². To visualize this, consider how long it takes to fall 100 meters. On Earth, it takes about 14.5 seconds. On the Sun, that same fall would take only 0.85 seconds. On Ceres, it would take 26.7 seconds.

Modern physics offers a different view through Einstein's theory of general relativity. In this theory, gravitation is not viewed as a force between bodies. Instead, it is an attribute of curved spacetime. Massive objects distort the geometry of spacetime in their vicinity. Other particles move along trajectories determined by this curvature. In this model, the gravitational force is considered a fictitious force. Objects in free fall are not actually accelerating in the traditional sense. Instead, they are traveling along straight lines, called geodesics, through curved spacetime.