可汗学院：物理学

Mass is how much stuff there is of something, how does something response to a specific force with Newton's second law.

Weight is the force of gravity on a mass or on an object.

The force of gravity between two objects is going to be a G which is F=G(m1*m2/r^2).

F equals G(m1/r^2) which the "m1" is the mass of the earth, the "r" is the distance from the center to surface that we can get the g=9.8m/s^22. So the gravitional field of earth is 9.8 m/s^2.

So the weight is the gravitional field of earth times the mass.

The unit of mass is "kg", the unit of weight is the unit of gravitional field which is "m/s^2" times "kg".

Then final unit of weight is kg*m/s^2 which is the unit of Newtons, force.

If we take the new weight on the Moon which we can get the different gravitional field of Moon than on Earth. But we can still multiply the mass with the gravitional field of moon.

Earth is not a perfect "ball", there is different weight in different places in the Earth.

England and the United States have different units in weight and mass. The mass is "slug", and the weight is "pound".