The gravitational force or also referred to as weight, is caused by the action of the gravity field strength on a body. It is directed downward and is in the rotating frame of a celestial body such as the Earth to the gravitational force and a small correction together, mainly due to the centrifugal force.

The weight of such other forces in the unit Newton is measured (N).

Amount, direction and point of

The weight is calculated as the product of the mass with the gravitational acceleration (even acceleration due to gravity or spatial factor):

Apart from minor irregularities, the weight force is always directed towards the center of the celestial body out on which one is because the gravitational field is a radial field to a good approximation. In most applications, but you can also reach a sufficient accuracy, if you look at the gravity field of a homogeneous field, namely when all dimensions are much smaller than the radius of the celestial body. In this case, the weight is anywhere vertically downward, which, inter alia, is exploited in plumb.

The trajectory of a moving rigid body runs exactly as handles the entire weight of the center of gravity of the body on. Is she the only force that acts on him, he is in a state of free fall. Since the inertia also depends on the mass as the weight of the body is the acceleration of all freely falling bodies, the gravitational acceleration, the same.

Gravitational force on earth

On the surface one can use the approximate value for the gravitational acceleration.

If you want to, however, determine the force of gravity on Earth precisely, the spatial dependence of the gravitational acceleration () to be considered by gravity formulas, for example, by the formula of Somigliana. For this spatial dependence, there are several reasons:

The first cause depends on the latitude of the site; the second because the Earth flattening also, as well as on the level of the site.

Weight and mass

In everyday language is often spoken of the weight of a body, without distinguishing whether so its mass or its weight is meant. Nevertheless, it is very different physical terms:

  • The mass is a measure of how much a body is generally influenced by gravitational fields, and how much he resists acceleration ( inertia).
  • The weight force, however, indicates how strongly a body becomes concrete, attracted by the Earth or the celestial body on which it is located.

The mass is therefore an intrinsic property of the body, while the gravitational force is the result of an external influence on the body.

Consequently, the mass of a body is independent of its whereabouts ( Earth, Moon, Zero Gravity, ... ) always the same, while the force acting on it weight force of the gravitational acceleration depends ( on the moon is the weight of only about one-sixth of that on Earth, ie the weight of a body of mass 100 kg on the moon is approximately equal to that which 16.5 kg acts on the earth on a body of mass, in weightlessness can feel no weight ):

Until 1960, it was customary to specify the force in the unit kilogram-force ( kgf ). This was defined as the weight of the earth, as measured in kiloponds, had the same measure as the mass in kilograms (). After that, the kilogram-force ( daN ≈ 1 1 kp = 9.80665 N) was in the SI system of units by the unit Newton replaced. Since then, the mass and the weight of metrics, which is approximated by the above Factor 9.81 differ.


Instruments for direct determination of a weight force dynamometer, for example, spring scales. However, the static buoyancy distort the result, which is particularly noticeable at low density bodies felt.

Indirectly, one can determine the weight by weighing and subsequent conversion of the weighing result. For a closer look at the workings of a scale, it is found that the actual expense recognized directly measured quantity is already falsified by the buoyancy force of gravity, even if a mass is displayed as a weight value. So, for example, compares a simple beam balance the forces that exert the two masses on their respective scales.