Complex Numbers

Losing Weight Is Easy

Big Think offers up another way for people to annoy pedants, and vice-versa.

     It’s important that weight and mass are not the same  
Here on Earth, we commonly use terms like weight (in pounds) and mass (in kilograms) as though they’re interchangeable. They’re not.

By "we," I suppose they mostly mean "Americans." But even people in countries that use SI units will use kg as if it's a unit of weight. This is fine, as far as I'm concerned, because damn near 100% of us live on the Earth's surface. And while there are gravity variations due to latitude, elevation, or anomaly (such as the one from "Anomalies"  a couple of weeks ago), they mostly don't make much difference unless you're a scientist who requires greater precision.

Still, as a pedant, I think it's important to know the difference. Also, as a pedant, I think it's important to keep it to oneself if one does not wish to be uninvited from social gatherings for being a pedant.

Since this is my blog, I'm making an exception here.

Conventionally, here on the surface of the Earth, we can convert between the two using only a minimal amount of effort: 1 kilogram is 2.205 pounds, and vice versa, 2.205 pounds converts to 1 kilogram. Going back-and-forth requires only multiplication or division, which seems easy enough.

"Easy?" Have you met people? Some of them break out in hives if you ask them to add 10% to something.

Incidentally, I don't usually bother with the 0.005. For most practical purposes, 2.2 is close enough and much easier to do head-math with.

A kilogram is an example of mass, not of weight, while a pound is an example of a weight, not of a mass. It’s only here on the surface of the Earth, where we’re at rest relative to the rotating Earth, that these two concepts can rightfully be used interchangeably.

I mean, as long as we're being pedantic, it's a really stupendously big universe and it wouldn't surprise me if there are other planets with the same acceleration due to gravity as that of the Earth.

But for nearly 100% of the volume of the universe, weight is irrelevant and only mass matters (pun absolutely intended).

If you release an object from rest and allow it to fall, it falls straight down, accelerating at a constant rate. It gains speed directly proportional to the amount of time that it’s been falling, and the distance it covers is proportional to the amount of time squared that the object has been falling.

Again, if you're going to get technical about it, you'd add "without air resistance."

This phenomenon, however, appears to not depend on mass or weight. A light object will fall just as quickly as a heavy object, especially if air resistance isn’t a factor.

There it is.

Someone did that on the Moon,   incidentally. This proved two things: 1) Physics is right; 2) at least that particular Moon expedition wasn't faked on an Earthbound sound stage.

The article proceeds to go into a lengthy (and weighty) discussion of the differences between scales and balances, and how the former measures weight while the latter measures mass. It's remarkably math-light.

Then:

We can sum up the difference succinctly: your mass is an inherent quality of the atoms that make up your body, but your weight is dependent on how those atoms accelerate under the influence of all the factors and forces acting on it.

Which I suppose is the main point of the article; everything else exists to support it.

There are plenty of physics textbooks (and physics teachers) that ignore this difference, and simply state that your weight, W, always obeys the equation W = mg. This is incorrect; it is only true when you are at rest on the surface of the Earth.

There's a popular trick question: "Which weighs more, a pound of feathers or a pound of lead?" It's a trick for several reasons.  

We often talk about “watching our weight” or “trying to lose weight,” but if that was truly your goal, you could simply go to a higher elevation, move to a different planet, or even get into an elevator and wait for the door to close after you hit the “down” button.

Yes, "simply... move to a different planet."

Sounds like a good idea for other reasons, these days.