Mass and Weight

Mass and weight are two ideas from physics that sound like the same thing but are not. Mass is how much stuff, or matter, is inside an object. Weight is how hard gravity pulls on that object. In everyday life, people use the words as if they mean the same thing. Scientists keep them separate because they measure different things.

Think of a bowling ball. Its mass is all the matter packed inside it: the plastic, the heavy core, every tiny atom. That amount of stuff does not change, no matter where you take the ball. Its weight, though, depends on gravity. On Earth, the bowling ball might weigh 10 pounds. On the Moon, where gravity is much weaker, the same ball would weigh less than 2 pounds. Out in deep space, far from any planet, it would weigh almost nothing at all.

Mass is usually measured in grams or kilograms. Weight is measured in pounds or in a unit called newtons, named after Isaac Newton. Newton was the scientist who figured out, in the late 1600s, how gravity and motion work together. He showed that weight is really a force. It is the pull of a planet or moon on an object's mass.

This is why a bathroom scale is actually a weight scale, not a mass scale. It measures how hard Earth's gravity is tugging you down onto the springs inside it. A balance scale works differently. It compares one object to another, so it gives the same answer anywhere in the universe. That is why old-fashioned balance scales were used for trading gold and spices. Nobody could cheat by moving to a place with weaker gravity.

Mass also decides how hard something is to push. A shopping cart full of bricks is harder to get moving than an empty one, even if you tried to push them both in outer space where neither one has weight. More mass means more resistance to being pushed or stopped. Scientists call this resistance inertia.

So the next time someone says "I weigh 80 pounds," they are really saying something about Earth. Their mass belongs to them. Their weight belongs to the planet they are standing on. Move them to Mars, and the number on the scale would change, but the person would not.