I’m going to attempt to use real world examples to explain why I don’t trust kilograms when they are used to weigh things. I think Neil Armstrong (or, in the picture above, James B. Irwin) would agree with me.

First of all, pounds (pounds-force specifically) are a unit of weight (aka downward force due to gravity), and kilograms are a unit of mass, which is how much matter (such as protons and neutrons) is in something.

Let’s say Neil Armstrong took a normal digital bathroom scale (not a balance scale) to the moon, set it to display pounds, and weighed himself. Because the moon’s gravity is roughly 1/6th that of the earth’s, the scale would show that Mr. Armstrong weighs 1/6th of what he does on earth. However, if he switches the scale to display kilograms, it will incorrectly tell him that he is 1/6th of his actual mass. Neil still has all of his matter (assuming he didn’t poop any of it out).

Digital scales (and spring scales) generally work by measuring downward force. They can’t directly measure mass, because according to Newton’s second law, which is mass = force/acceleration (due to gravity), the scale also needs to know how strong gravity is. On earth where we know the acceleration of gravity, for all practical purposes one pound is equivalent to 0.45359237 kilograms. Scales use this number to convert pounds to kilograms, but this conversion doesn’t work on the moon.

The scale on the moon example probably isn’t a very good one. All it shows is that some types of scales can measure mass, and some measure weight. If you wanted to measure mass on the moon, you could use a balance scale like the one at the doctor. They work by comparing the weight of different masses to measure mass. A balance scale on the moon wouldn’t correctly measure weight.

But that’s beside the point. Scales that measure mass are still technically incorrect sometimes.

We don’t usually think about it, but air has mass. At sea level it has a density of about 1.19 grams per liter, and if the average human has a lung capacity of 6 liters, you gain about 7.14 grams of mass when you hold your breath. A scale won’t be able to detect this, just like a water balloon under water won’t feel any heavier than an empty balloon.

Therefore, when you hold your breath, a scale measuring kilograms will be inaccurate by about 7.14 grams, but a scale measuring pounds will still be correct because it only needs to measure downward force. Kilograms are absolutely untrustworthy.

In conclusion, eat all the McDonald’s quarter pounders you want because it’s a lot easier to lose pounds than it is to lose kilograms. Just go to the moon.