Fig. 5-1 Weight is the only force acting on a free-falling object (Neglect air resistance).

Mass is a measure of inertia. It also represents the quantity of matter in an object. Therefore, the mass of an object is independent of the place we measure it. Mass is a scalar and the SI unit of mass is kilogram.

Weight is the gravitational attractive force acting on the object by the Earth. Therefore, weight is a vector which always points vertically downward, and its SI unit is Newton. Any object on the surface of the Earth falls with the same acceleration due to gravity (neglect air resistance). The only force acting on the object is its own weight (Fig. 5-1).

Since by Newton's Second Law, the resultant force acting on the object is equal to the product of its mass and its acceleration , we have

Fig. 5-2 We should talk about the mass of a pack of rice.

Therefore, strictly speaking it is wrong to say that the weight of a pack of rice in supermarket is 5 kg or 8 kg. We should talk about the mass of the pack of rice. The confusion on the concepts of mass and weight arises from the fact that the acceleration due to gravity is more or less constant in any place on the Earth's surface, thus the weight of an object is directly proportional to its mass.

In space, the acceleration due to gravity depends on the strength of gravity in the place concerned and would therefore vary from place to place. For example, the acceleration due to gravity on the surface of the Moon is only about one-sixth that on the Earth. In deep space where the gravitational force of stars and planets is negligible, the acceleration due to gravity is practically zero. Fig. 5-3 shows the mass and weight of an 2 kg object at different places.

Note that the weight varies from place to place but the mass is always constant.

Fig. 5-3 The mass and weight of an 2 kg object at different places.