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Are Mass and Weight the Same Thing?

Is your mass the same as your weight? If not, what's the relationship between these two ideas? And why do so many people get them confused? Keep on reading to find out!

By
Jason Marshall, PhD,
August 9, 2014
Episode #207

Page 1 of 2

Balance ScaleIn everyday life we use words like "variable," "line," "theory," and "force" (to name just a few) rather loosely, but in math and science, these words have very specific and well-defined meanings.

In truth, the differences between the precise mathematical meanings and the everyday colloqiual uses of such words don't have a huge impact on your day-to-day life. But even so, they are nonetheless interesting things to talk about. In fact, some of the squiggly little nuances that pop-up in those discussions turn out to be extremely interesting!

Which is exactly why we're about to embrace our intrepid spirit of curiousity and ponder just such a topic. Specifically, today we're going to talk about weight and mass. Are they the same thing? If so, why do we have two words for one idea? If not, how are they different? And why are people always getting them mixed up?

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What Is Mass?

Is your weight the same as your mass? In short (spoiler alert), no.

To see why this is, let's take a step back and think about what it is that we're trying to figure out when we "weigh" something. For example, why does the person at the farmers' market weigh your bag of peaches before charging you? Obviously, they want to figure how much peach-stuff you have in your bag. I don't mean the number of peaches you have (because peaches can be big or small), I mean the overall amount of peach matter you've bagged up.

Mass is a measure of how much stuff something contains.

This amount of matter is what we call the "mass" of the peaches—it's equal to the combined mass of all their atoms, and is measured in units of grams or kilograms.

As long as you don't take a bite out of any of your peaches (or do anything else to them to alter how much matter they contain), their mass will not change. Putting them in your car won't change their mass, singing to them won't change their mass, even moving them to another planet won't change their mass.

Why? Because mass is a property of something—it's a measure of how much stuff it contains. And that doesn't spontaneously change.

What Is Weight?

While moving your peaches to another planet won't change their mass, it turns out it will change their weight. And that's because the weight of an object is a measure of how hard gravity is pulling on it. When you stand on a scale on Earth, you feel your feet pushing down on the scale and you see the scale respond to that pressure. But why are your feet pushing down on the scale?

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