Statics is the action of one body on another. A force is either a push or a pull. A force tends to move a body in the direction of its action. The action of a force is characterized by its magnitude, by the direction of its action, and by its point of application. Thus, force is a vector quantity, because its effect depends on the direction as well as on the magnitude of the action. Forces are classified as either contact or body forces. A contact force is produced by direct physical contact; an example is the force exerted on a
Orders of magnitude (mass) (the dalton). Astronomers use the mass of the sun (). Unlike other physical quantities, mass-energy does not have an "a priori" expected minimal quantity, as is the case with time or length, or an observed basic quantum as in the case of electric charge. Planck's law allows for the existence of photons with arbitrarily low energies. Consequently, there can only ever be an experimental lower bound on the mass of a supposedly massless particle; in the case of the photon, this confirmed lower bound is of the order of 3×10 eV = 10 kg. Orders of magnitude (mass) To help