Measurements Are Often Full of Lies—and That's OK

Recently in my physics lab, students were having trouble with the difference between mass and weight . I had ready answers—it’s a topic I’ve gone over hundreds of times. But after I explained that mass is what matters in an acceleration problem, a student asked about the digital balance we were using in the lab. “Does this measure mass or weight?”

Uh-oh. I was busted. I told the student that the instrument reports in units of grams, which is a unit of mass, so … it measures mass. In other words, I lied. Later I confessed to some other educators on Twitter.

Really, we shouldn't blame students for their confusion on this issue. For one thing, even those who know better commonly use the words weight and mass interchangeably. Second, it's not always completely clear what our lab instruments really measure.

Interestingly, people often approach the concept of mass with caution, but they always think they know what weight is. In truth, mass is the more straightforward concept (at least in theory). At least it’s an actual property of objects, whereas objects don’t really “have” a weight—-or they have weight only in relation to other objects. Totally confused now? Good! That’s the first stage of learning .

Mass vs. Weight

Let’s start with definitions:

Mass measures how much "stuff" is in an object. If everything were made of 2 x 2 Lego blocks, we could measure the mass of something by counting how many blocks are in it. If everything is made of electrons, protons, and neutrons, we “simply” count up the amount of that stuff. The basic unit for mass is the kilogram .

Weight is a measure of the downward force an object exerts on whatever is supporting it. All objects that have mass interact with other massy objects—that’s called gravitational interaction. If I am standing on the Earth, the magnitude of the attractive force between me and Earth is “my weight.” But it’s really a measure of how much the Earth wants me.

But if mass and weight are different, why are the terms used interchangeably? The reason is that in everyday life, as a practical matter, if you know one you know the other. Here’s the relationship between weight (W) in newtons and mass (m) in kilograms: