Scientific Pet Peeve #1: Mass vs. Weight

Everyone has their pet peeves; unsurprisingly, at least a few of mine are science related.

Most people wouldn’t give a second thought to hearing someone say that something “weighs” twenty kilograms. I physically shudder whenever I hear or read such phrasing- and when I was a textbook editor, said shuddering was usually channeled straight into my red pen. Some of the first things you learn or teach in an intro physics class are the facts that mass is measured in kilograms, weight is measured in Newtons (or pounds), and mass and weight are not the same thing.

Mass is a measure of how much matter, how much stuff, is in an object. Weight is a measure of how hard Earth (or whichever astronomical body you happen to be on) is pulling on an object. These tend to be conflated because on Earth, 1 kilogram always weighs about 2.2 pounds. (A post about the mathematics of this- complete with whiteboard calculations!- is forthcoming.)

If you go to the Moon, you weigh less because the Moon doesn’t pull as hard on you as Earth does. However, you are made up of just as much matter, so your mass is unchanged. You’re easier to lift on the moon, since lifting something is just a matter of exerting a force equal to its weight. It might follow that you’d also be easier to push, but recall our discussion of Newton’s second law. The force needed to accelerate an object depends on its mass, not its weight. Assuming you were sitting on a surface where friction was negligible, such as an air hockey table or an ice rink, it would just be hard to push you on the moon.

Ice skating on the moon sounds awesome. Let’s go.


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