Moving along in my classes; stymied by learning pounds/ounces and kilograms don't measure the same thing!

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Ever since I quit dithering about The Right Way (For Me) to Take Notes, and decided to take my notes electronically this "term," I have been cranking, trying to get caught up in How Things Work (which is extremely elementary physics) and Roman Architecture.

The physics lectures rolled along, mostly, with me thinking "Ok.  Ok.  Ok.  Ok.  Hey, wait a minute....."  I have to keep stopping and cogitating and working things out on paper to convince myself that concepts make sense (like why we don't care about weight or mass when we are thinking about falling balls [near the surface of the Earth, discounting air resistance]).

It doesn't help that physics uses words that we all use (like velocity), but defines them in a different way (in physics, velocity and speed are NOT synonymous....).........

Unfortunately, when he got to the lecture about what happens with falling balls on the moon, rather than near the Earth's surface, he said something that threw me totally for a loop.  He tossed it in, pretty much as an aside, and did not go over it slowly.

Before I proceed -- weight, in physics, means pretty much what we all think it means, but mass is NOT equivalent to weight.  He did explain this difference carefully, and the part of his explanation that totally clicked for me was "Mass is the measure of resistance to acceleration."  (Well, it clicked for me because he'd already explained what acceleration was......) 

An object's mass is how hard it is to get it to move, or to stop it from moving.  The more massive an object, the harder it is to get it moving, and the harder it is to stop it from moving.  Think "rowboat" vs "ocean liner"..............  Their mass is all about how much work it takes to get them to move, or to stop moving.  (And their weight is just what you think it is, here near the surface of the Earth.)



Anyway.  Here's the thing that blew me away.

Did you know that pounds and ounces measure weight, but kilograms measure mass?

I had NO idea.  None whatever.  Really?  They measure totally different things???????

???????????????????????????????????

Apparently, they actually do.

This has really got my brain spinning in circles. 

How is it that I never knew this?  Totally different things!!!!!!!!!!!!!!!

WHY did they make two utterly different measuring schemes?  (I guess I can guess the answer to this one -- the people who invented the metric system thought it was better to measure mass than weight.  For whatever reason....)

And, finally, sort of combining the previous two, how is it that we can apparently treat pounds/ounces, and kilograms, as equivalent?

If I ask google to convert pounds to kilograms, I do not get a lecture on how those two are totally not equivalent.................... And yet................

I have tried asking in the class forum, but I have not gotten what seems to me to be an answer to this tricksy issue.  It seems that the people who understand this find it so elementary that they feel no need to discuss it?  Or something.  I don't know.

In any case, I'm entirely boggled by this, and have been stopped dead in my tracks, wanting to understand it.

He told us at the beginning of the class that IT IS NOT MAGIC, and that if we walk away taking anything on faith, he has failed.

Well -- I need an explanation, in small words, of why most people don't need to know or care that pounds and kilograms measure entirely different concepts.

I'm guessing that the answer is that both pounds/ounces and kilograms were defined right here, near the surface of the Earth, and that since what we want to measure with pounds or kilograms is here with us, near the surface of the earth, we can get away with pretending pounds and kilograms are measuring the same thing.

But I wish someone would confirm this, or refine it, or tell me what the real reason is.

Anyone?  Please?



Luckily, I guess, I am able to continue to pursue Roman Architecture while I am stuck in physics.  I am learning a lot about Roman ideas of design, and Roman construction techniques, and am enjoying all the nice pics of the Italian countryside....

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