The shape of things

A central force is a force whose direction depends only on the distance between the point of application and a fixed point, known as central point. Hence, in a central force field, the force vector in every point is parallel to half-lines extending in every direction from the origin. For this reason, a spherically symmetric field is originated.
Gravitational attraction is a central force. Its formula is as follows:


The only vector capable of giving a direction to the force is r, the distance between the masses taken into consideration. So, for example, in the absence of other factors, from the gravitational collapse of a cloud of gas and dust, spherical bodies are created, such as the stars and planets. Actually, we know that during the formation of a celestial body, the clouds of gas and dust rotate around their centre; as a consequence, the objects produced are not perfectly spherical, but slightly flattened at the poles as a result of the centrifugal force. The Earth, for example, is not perfectly spherical but is a rotating geoid: the radius at the Equator is about 20 km less than the radius at the Poles.

Special reports

From the Multimedia section


  • 17 May 2011


    Neutrinos are extremely light neutral particles...

    The comet’s tail

    Comets are “dirty snowballs” that spend most of their life at the edges of the Solar System...

  • The definition of a dwarf planet

    Astronomy, like all scientific disciplines, is continuously evolving...

    The PP cycle

    Nuclear fusion within the stars begins with the different passages...

  • Children of the stars

    We are made of star dust. The iron in our blood, the oxygen we breathe...

    The Olbers paradox

    Why is the sky dark at night? What seems like an obvious question...