Why is the pound, ton, etc. not listed under units of weight?
The weight of an object, when it is resting on Earth s surface, is the force with which it presses against the surface. When you hold the object, you feel that weight as a downward force against your hands. That force depends not only on how much material is in the object, but on the local gravitational field, and your motion (or lack thereof) at the time. The mass of the object is the measure of the amount of matter in it, independently of the forces acting on it. The above-mentioned units are conventionally units of mass, since they refer to the actual amount of matter. For example, when you buy a pound of butter, you buy a certain amount of butter, not just the quantity that happens to exert a certain force under local conditions. (Selling material by weight, rather than mass (as with balances with springs instead of poises) is illegal almost everywhere.) The weight, when the object is resting on Earth s surface, is the mass, multiplied by Earth s surface gravity. The Universal Unit
