Earth's gravity is measured using several methods, each providing unique insights into its strength and variations. The most common approaches involve observing how objects accelerate towards Earth or measuring subtle changes in the gravitational field itself.
Measuring Acceleration Due to Gravity (g)
One straightforward method involves measuring the acceleration due to gravity (often denoted as 'g'). This can be done in a couple of ways:
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Free Fall Experiment: Dropping an object from a known height and measuring the time it takes to reach the ground allows for the calculation of 'g' using the equations of motion. This simple technique provides a local measurement of gravitational acceleration. As noted in one source, "[t]o measure little g, scientists can use a variety of techniques, such as dropping a mass from a known height and measuring the time it takes to fall to the ground..."
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Pendulum Oscillation: A pendulum's period of oscillation is directly related to the local gravitational acceleration. By precisely measuring the period, we can calculate 'g'. This method is also suitable for local measurements.
The average gravitational acceleration near Earth's surface is approximately 9.8 meters per second squared (m/s²), or 32 feet per second squared (ft/s²). As stated by one source: "Near Earth's surface, the acceleration due to gravity, accurate to 2 significant figures, is 9.8 m/s2 (32 ft/s2)."
Measuring the Gravitational Constant (G) and the Earth's Gravitational Field
Beyond measuring 'g', scientists also measure the gravitational constant (G) and map Earth's overall gravitational field. These are more complex measurements:
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Gravimetry: This sophisticated technique involves using highly sensitive instruments to measure variations in the gravitational field. As explained by Wikipedia, "Gravimetry is the measurement of the strength of a gravitational field." These variations offer information about the Earth's mass distribution, density variations within the planet and subsurface features.
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Satellite Measurements: Satellites like GRACE (Gravity Recovery and Climate Experiment) utilize precise distance measurements between twin satellites to map variations in Earth's gravitational field. These changes relate to mass movements on or within Earth. As one source notes, "Gravity is the field around the Earth that can be measured by satellites. Changes in the gravity field are related to change or transportation of mass." GRACE's success is also highlighted: "An award-winning mission that's changed the way we study Earth's gravitational ..."
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Atom Interferometry: This advanced technique employs atom interferometers to measure the gravitational gradient – the rate at which the gravitational field changes over distance. A source describes this as: "We report the demonstration of an atom interferometer-based gravity gradiometer."
The current best estimate for the gravitational constant (G) is a very small number: 6.6743 × 10⁻¹¹ m³ kg⁻¹ s⁻². However, even this value has some uncertainty associated with it, as one source mentions: "The current best estimate for G is 6.6743×10−11 m3 kg−1 s−2, which is a very small number. However, not everyone agrees on this estimate because…"
In areas where Earth's gravitational forces are weaker, the mean sea level is lower. This illustrates the connection between gravity and the shape of the oceans. This is mentioned in a source stating, "In areas where the Earth's gravitational forces are weaker, the mean sea level will be lower. To measure the Earth's gravity field, geodesists use instruments…"
