We scientists don’t like to use weight to measure mass for one very important reason: your weight isn’t the same everywhere you go! Your weight, the number you get when you stand on a scale, is a measure of how hard gravity is pulling you onto the earth. But what if you’re on the moon, where gravity isn’t as strong? The scale will show a much smaller weight. Or what if, heaven forbid, you decide to weigh yourself while skydiving with a scale that’s falling alongside you? If you’re both falling at the same speed, the scale would say youweighed zero pounds!

No, that doesn’t work very well for scientists, who really need to be able to tell different objects apart no matter where they are or how fast they’re moving. That’s why we use mass, which is a measure of how much matter (the fundamental stuff that everything in the universe is made up of) is in an object. Your mass is always the same, whether you’re on the moon, on Mars, or flying in a spaceship.

Weight is the force of gravity acting on mass. The strength of gravity varies from place to place, so the weight of something can change depending on where you measure it, even if it has the exact same mass. In addition, gravity cannot be measured at all if you are falling or in orbit, because you and everything around you are being acted on by gravity to the exact same extent (i.e. your scale does not have a floor to push down on). As a result, something in orbit has no weight, even though it still has mass.

Mass is not quite the same thing as weight (although for most people we almost never get to experience the difference!). While mass is a fundamental property, weight is measured by the downward force something applies. This means that the strength of gravity (the force pulls us down towards the earth and keeps us from flying off into space every time we jump) will change an objects weight, but not its mass.

Everyone on earth experiences roughly the same amount of gravity. But if you go to a different planet, or even into outer space, the gravity can change a lot. If you weigh 85 pounds on earth, you would weigh only 14.1 pounds on the moon! In contrast, your mass would still be 38.6 kg no matter where you are.

Mass and weight are proportional (meaning they have a constant ratio between them and can be easily compared) but they do not measure the same property of a material. Mass is the property of matter that resists forces to move it, also referred to as inertia. Something that has more mass will require more energy to move, in all circumstances. Weight specifically refers to the force on an object’s mass due to gravity.

The two most common units to measure “heaviness” actually measure different properties: the metric kilogram is a unit of mass while the Imperial pound is a unit of weight (force). In most circumstances, mass and weight can be used interchangeably, since the force due to gravity can be considered constant on the surface of the Earth. They are separated by a constant factor, the acceleration due to gravity (~ 10 m/s² in the metric system).

Fun facts: the unit of force in the metric system is the Newton (named after the scientist, Sir Isaac Newton) and the unit of mass in the Imperial system is the slug… which may be why saying “pound” is more popular.

Measuring the weight of an object is the easiest way to obtain the mass, since the acceleration due to gravity is already known. You want to consider mass and weight separately if you are attempting to move an object against the pull of gravity or operating far away the surface of the Earth. On the moon, for example, objects weigh a tenth of what they do on Earth but still resist being moved to the same degree.