Mass

An astronaut on the Moon weighs one-sixth what she weighs on Earth. But if she tries to push a 200 kg lunar rover, it resists just as stubbornly as it would on Earth. Her weight changed. The rover's mass did not.

Mass is the measure of how much matter an object contains and how strongly it resists acceleration. It is an intrinsic property: it does not change with location, gravity, or temperature. Weight, by contrast, is the force that gravity exerts on a mass, and it varies depending on where you are. On Earth's surface, a 1 kg object weighs about 9.8 newtons. On the Moon, the same kilogram weighs about 1.6 newtons.

The distinction matters for energy because mass appears in two of the most important equations in physics. In kinetic energy (KE = ½mv²), mass sets the baseline for how much energy a moving object carries. In Einstein's equation (E = mc²), mass itself is a form of stored energy. That second equation explains why nuclear fuel is roughly a million times more energy-dense than chemical fuel: nuclear reactions convert a tiny fraction of mass directly into energy.

Worked Example

Two identical wind gusts hit two turbines. Turbine A has blades sweeping a 50 m radius circle. Turbine B sweeps a 100 m radius circle.

• Compare swept areas. Area scales with radius squared. Turbine B's swept area is (100/50)² = 4 times larger.
• More area means more air mass flowing through per second.

If both turbines face the same wind speed, how much more air mass does Turbine B capture per second?

Four times as much. Since power depends on the mass of air flowing through the rotor (among other factors), doubling the blade length quadruples the available energy. This is why modern turbines keep getting physically larger.

Mass connects to both kinetic energy and nuclear energy, two topics that build directly on this foundation.

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