Mass, Weight and Energy
Mass is how much matter an object holds, and weight is the pull of gravity on it. Einstein's famous equation goes one step further and says that mass is itself a store of energy.
Where this comes from
I got curious about the physics I learned in secondary school. So I asked an AI assistant, Gemini, a string of simple questions. It began with one: what is mass? Each answer led to the next question, and this note follows that thread.
Mass and weight are different
Mass is how much matter is inside an object. Its unit is the kilogram. It never changes, whether the object is on Earth, on the Moon, or floating in space.
Weight is something else. It is the pull of gravity on that mass. Because it is a force, it is measured in newtons, not kilograms. The rule is simple: weight equals mass times gravity. On Earth, gravity is about ten meters per second, every second.
So a person with a mass of sixty kilograms weighs about six hundred newtons. In daily life we say “my weight is sixty kilos”. In physics, that sixty kilos is the mass.
The apple at the market
The same goes for one kilogram of apples at the market. The seller is measuring mass, the amount of apple you are buying, even though we say that they are weighing it.
Now take those apples to the Moon. The mass is still one kilogram, because nothing in the apples has gone missing. But the weight drops from about ten newtons to about one and a half. The Moon’s gravity is roughly one sixth of Earth’s, so the apples feel six times lighter in your hand.
The joule
The joule is the unit of energy, work and heat. One joule is the energy needed to lift a small apple, about one hundred grams, up by one meter. It is also one newton of force pushed through one meter.
Mass is energy
Then came Einstein’s equation: E equals m c squared. Here c is the speed of light in empty space, about three hundred thousand kilometers per second. Squared, it becomes an enormous number. So even a small mass holds a huge amount of energy.
In everyday life, we cannot turn a whole apple into energy. Burning wood or digesting food releases only a tiny part of it. Nuclear reactions do better. The Sun turns about four million tons of its mass into energy every second, although that is less than one percent of the fuel it burns. Only when matter meets antimatter does all of the mass turn into energy. But antimatter is very rare, and laboratories can only make a few atoms of it.
Why one rule fits everything
My last question was why an apple, a person and petrol, which are so different, all follow the same equation. The answer is that, deep down, they are built from the same things: protons, neutrons and electrons.
The equation does not care what the object is. It only cares how much mass there is. Even more surprising, most of the mass of a proton does not come from the tiny quarks inside it. It comes from the energy that holds the quarks together. So mass is, in a way, energy that has been locked in place.
What I want to remember
Mass is the amount of matter, and it never changes. Weight is a force that depends on gravity. The joule measures energy. And Einstein’s equation says that mass is a store of energy, which is why one rule works for apples, people and petrol alike.