Starting with Archimedes with his principle of lift and the first mathematical description of gravity by Newton to Einstein’s theories of relativity and the research topics of our time based mostly on them, the subject of gravity offers a deep insight into physics. It can simultaneously take us into the depths of space and keep us on the surface of the earth with both feet on the ground.
This collection of questions about the physics of gravity is intended to give interested laymen basic insights into the fascinating subject of gravity.
For the more advanced reader there is corresponding literature recommended at the end of each article, which may include scientific publications from various journals (Nature, Science, Phys. Rev. Lett., etc.) depending on the topic.
Since this is a very broad and sometimes complicated subject area, we will add new information from time to time.
More about gravity
Gravity is the property of masses to attract each other. The force of attraction depends on the size of the masses and their distance. It decreases quadratically with distance. The first one to experimentally proof mass attraction with a torsion balance in 1798 was Cavendish.
The size of the earth’s gravitional pull or gravity acceleration g depends on the mass distribution in the earth.
The size of the earth’s gravitational pull or gravity acceleration g depends on the mass distribution in the earth. The more masses are below one up to the centre of the earth or the higher the density of this mass, the higher the gravitational pull. The local acceleration due to gravity can be measured with a gravimeter. From this, conclusions can also be drawn about the nature of the subsurface, e.g. cavities or ore deposits.
Gravitational acceleration depends on latitude and height above sea level. At sea level, at the equator g=9.780 m/s2, at the 45th parallel g=9.807 m/s2 and at the pole g=9.832 m/s2. With every additional height meter g decreases by about 3*10 -6 m/s2, as long as h is small against the earth radius.
At the equator the distance from the centre of the earth is greater and thus the gravity acceleration is lower than at the poles.
In addition, the centrifugal force counteracts the gravitational acceleration due to the earth’s rotation, so that this is lower at the equator. The effect is about 3 Gal.
The local absolute values of the acceleration due to gravity can be obtained from the Land Surveying Offices.
Tides are created by the rotation of the Earth and Moon around the common centre of mass. The difference forces between gravitational and centrifugal forces are decisive. These forces ensure that a flood mountain is formed on the Earth both on the side facing the Moon and on the side facing away from the Moon. Therefore, there are ebb and flow twice a day. In addition, the sun also exerts an influence on the tides, which results in spring tides by addition at new and full moon and nipp tides by subtraction at half moon.
The tidal forces can be measured with a gravimeter at any place on earth, independent of the tidal range of the oceans. They are in the range of 0.5 mGal to 1.1 mGal.
“Capital G” is the constant of proportionality in Newton’s law of gravit. It established a formal relationship between the masses, the distance and the resulting gravitational force. It is now assumed that G is not constant, but could depend on other, previously unknown factors.
Recognized CODATA value of 2010:
G = (6,67348 +/- 0,00080) 10 – 11 m³ kg-1 s-2. The accuracy of this value is only 0.15 %, which is very inaccurate for a fundamental constant.
“Small g” refers to the acceleration with which a freely falling body increases its velocity in the Earth’s gravitational field. Average value: G = 9.80665 m/s² (CODATA value of 2010).
‘Gal’ stands for Galileo and is another unit for the acceleration of gravity g, which is mainly used in geophysics. It is 1 Gal = 10 -2 m/s², thus g= 981 Gal. For the description of small effects mGal = 0.001 Gal is mostly used.
On earth, artificial weightlessness can only be achieved by letting a body fall freely. This is achieved in parabolic flights with airplanes or in drop towers. The achievable duration ranges from a few seconds (drop tower) to a few minutes (parabolic flight).
Gravitational anomalies: Are there places on our planet where the gravitational potential pulls a body (e.g. a car) up a mountain against all expectations?
Places like Roca di Papa in Italy or Karpacz Gorny in Poland are known for so-called “gravitational anomalies”. Only precise measurements could clarify what these phenomena are really about. The most obvious explanation is an optical illusion that makes you think that things are going uphill, although in reality they are going downhill. The fact that a nearby mountain changes the direction of gravity is practically impossible due to the much too small mass in relation to the earth.