Ask yourself a question: Which door is harder to open? A thick door made of aluminum or a same sized door made of iron? The obvious answer is the iron door. But why is it harder to open? Is it because it weighs more? No – because it has more mass.
The terms mass and weight are used interchangeably in everyday conversation, but technically they are very different. Mass is the measure of an object’s inertia, or resistance to having its position or motion chanced. The mass of an object depends on the number and kinds of atoms it is composed of. Weight, on the other hand, is the force with which an object is attracted to the center of the Earth, or other body, such as moon.
With the definition of mass in hand, we can return to the doors. An atom of iron has more mass than an atom of aluminum – its nucleus has more protons and neutrons (elementary particles that make up most of the mass of an atom) than does the aluminum atom’s. So amusing that both doors have the same number of atoms, but the iron door logically has greater mass than the aluminum one. That is, it is more resistant to change in position and thus is harder to open.
The weight of an object is calculated by multiplying its mass by the acceleration imposed on it by gravity. Whereas the mass of an object is constant, its weight varies according to the gravitational field in which the object is placed. To us, weight seems unchanging because the Earth’s gravitational field is, for the most part, constant. If we were to ferry an object from Earth to the Moon, we would find that its mass when measured on the Moon would be the same as it was on the Earth. But its weight would be only one-sixth what it was on Earth’s. It should not be surprising, then, that even a massive object under certain circumstances could be weightless. For example, as spaceship moves away from the Earth, the force with which it is attracted to the Earth – its weight – decreases. So, if the ship were to move into interstellar space, far away from the Earth or another heavenly body, it would weigh nothing at all.
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