A solid, liquid or gas of mass 1 kg will weigh 9.81 N due to the gravitational attraction of planet Earth. Since planet Earth is not a perfect sphere a mass of 1 kg will weigh slightly more at the poles and slightly less at the equator. However, for all practical purposes, the average weight of a mass of 1 kg is taken to be 9.81 N on planet Earth:
-The force of gravity attracts the mass of any body (solid, liquid or gas) towards the centre of planet Earth.
– The weight of a body is the force of gravity acting on that body.
– Therefore weight is a force measured in newtons.
– The factor 9.81 is the free-fall acceleration of a body due to the force gravity, it is 9.81 metres per second squared (m/s2).
– Since the weight of a body of mass 1 kg is approximately 9.81 N (average value) on planet Earth, the weight of that body will change on a planet having a different force of gravity or when travelling through space.

For example, the gravitational attraction of the Moon is approximately 1/6 that of planet Earth. Therefore a mass of 1 kg will weigh 1/6×9.81=1.64 N on the Moon.