I'm wondering how can we measure Earth's gravity without pendulum? of course we can do it by using Newton's laws of physics but the air friction might change the actual value also how physists measure distant planets gravity that are found by transit's (by detecting dip's in light intensity)? I know g=GM/(r^2)
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1$\begingroup$ I removed some comments which appeared to be answering the question, and their responses. Please keep in mind that answers should be posted as answers, not comments. $\endgroup$– David ZCommented Jun 10, 2018 at 20:28
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$\begingroup$ The elongation of a spring works too, as the equilibrium elongation under gravity can be done for a wide range of masses $\endgroup$– TriatticusCommented Jun 10, 2018 at 20:29
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1 Answer
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The conventional method for measuring the Earth's acceleration of gravity is that of measuring the trajectory of a free-falling body. The instrument that implements this principle is called the absolute free-fall gravimeter.
In this type of gravimeter, the free-falling body is usually a retroreflector (corner-cube) constituting an arm of an interferometer which measures the retroreflector trajectory. The retroreflector falls in a vacuum chamber, but to further reduce the residual friction, it is closed in a so-called drag-free chamber which follows the fall through a servo system, so that the retroreflector is surrounded by still air. The chamber and the servo system allow also for a smooth landing.
The acceleration of gravity is determined by fitting the trajectory to the theoretical equation of motion, which takes also into account the gravity gradient along the fall. The time reference is usually given by an atomic clock.
This kind of gravimeter is commercially available (see e.g. the FG5-X gravimeter).
answered Jun 10, 2018 at 21:03