# FAQ – Gravitation

### What is gravity?

### What is the difference between G and g?

The big G defined by Newton is called the universal constant of gravitation or also ‘big-G’.

This constant provides the formal relationship between the masses and the distance resulting in gravity force. Meanwhile, it is assumed that G is not constant, but may depend on other, so far unknown factors.

Recognized CODATA value 2010 is:

G = (6.67384 ± 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 in which a free-falling body increases its speed in the gravitational field of the earth. Average rating: g = 9.80665 m/s2 CODATA value 2010.

### What does the unit Gal stand for?

Gal stands for Galileo it is another unit for the acceleration of gravity, mainly used in geophysics. 1 Gal = 10 -2 m/s2, i.e. g = 981 Gal. mGal = 0.001 Gal is mostly used to describe small effects.

### Why is there on earth different gravity, what does it depend upon, and where is gravity greatest?

The size of gravitational force, resp. gravitational acceleration g, depends on the mass distribution in the earth system. The greater the mass at the center of earth, or the higher mass density is, the greater the attraction.The local gravitational acceleration on earth due to gravity can be measured with a gravimeter. Thus enabeling conclusions on the nature of the subsurface, e.g. detecting hallow spaces or deposits of ore.

The gravitational acceleration depends on the geographical latitude and the altitude above sea level. At sea level at the equator g = 9.780 m/s2, on the 45th latitude g = 9.807 m/s2 and at the pole g = 9.832 m/s2. For 1m height “g” decreases by approx. 3 × 10 -6 m/s2, as long as h is small compared to earth’s radius.

At the equator, the distance from earth’s center is greater and thus the gravitational acceleration is lower than at the poles.

In addition the centrifugal force counteracts due to earth rotation toward gravitational acceleration, therefore having a lower effect at the equator. The effect amounts to about 3 gal.

The absolute values of the local acceleration due to gravity can be obtained at land survey offices.

### What is the relationship between the moon and tides?

Tides are caused by rotation of earth and moon around their joint center of mass. Determining factors are the differences between gravitational forces and centrifugal forces.These force differences cause on earth a “pile up” of tidal bulges, both on the side facing the moon and on the opposite side of the moon.

Therefore high and low tides occure twice a day.In addition tides are also influenced by the sun.

The sun excerts its influence on tides, whereas by addition during new moon and full moon flash floods and by subtraction during half moon neap tides occur.The tidal force can be detected with a gravimeter at every location on earth independent of the tide range of the sea. They are in order of 0.5mgal to 1.1mgal.

### Has it been possible up to now to create an artificial weightless environment?

To this point artificial antigravity (or weightlessness) on earth, can only be achieved by letting an object drop in a free-fall. Weightlessness can be experienced in parabolic flights with an aircraft for a short period of time such as a few minutes or in drop towers for a few seconds.

Are there any places on our planet where the gravitational potential can pull an object (e.g. automobile ) up the hill on a mountain, contrary to all expectation ? …

Well known places for so-called “gravity anomalies” are places like Roca di Papa in Italy or Karpacz Gorny in Poland.

Only precise measurements uncovered what this phenomenon is all about. The most obvious explanation is an optical illusion that pretends that the object moves uphill, although in reality it is going downhill. The fact that a nearby mountain changes the direction of gravity is practically impossible because it’s mass is much too small compared to earth’s mass.