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  #1  
Old 08-26-2021, 10:46 AM
TigerHawk TigerHawk is online now
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Question Rocket Stability

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  #2  
Old 08-26-2021, 10:59 AM
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Originally Posted by TigerHawk

Nice money making scheme to sell to schools! As long as there has been model rocketry, the main companies have shown how do do swing tests without spending a nickel. lol

This item hasn't been in regular catalogs in the past that I know of, but they've always sold things through companies that provide items for schools like AC Supply, Nasco, etc. that weren't in regular catalogs. It may have been available before now through those sellers.
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Old 08-26-2021, 11:46 AM
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Default got a chuckle

from their product description, with separate mentions about "center of gravity" and "center of mass".....as though they are two different things in two different places

clearly, Estes does not use a technical editor to proof their advertising. kinda embarrassing?
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Old 08-26-2021, 12:03 PM
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That bit bugged me as well....and then I did a little research and found that that they are, in fact, two distinct things. If an object is in a uniform gravity field, they work out to be the same.

But if it's not, as when something is in orbit around a non-uniform body (say, the Earth or the Moon) then they are distinctly different things.

That said, to many of us, including you and me, it looks redundant.
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Old 08-26-2021, 12:39 PM
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Originally Posted by BEC
That bit bugged me as well....and then I did a little research and found that that they are, in fact, two distinct things. If an object is in a uniform gravity field, they work out to be the same.

But if it's not, as when something is in orbit around a non-uniform body (say, the Earth or the Moon) then they are distinctly different things.

That said, to many of us, including you and me, it looks redundant.

That's no different than saying something "weighs" 40 kilograms. Kilograms is mass, not weight. Weight is in Newtons in the metric system. I see scientists and college professors using weight and mass interchangeably all the time when referring to an object's mass.

We know that your mass doesn't change unless your name is Grendel and I rip your arm off, but your weight varies based on your proximity and the mass of the object you are attracted to. (Assuming we can magically change locations quicker than we can change our mass by eating, dieting, etc.)

If we are hanging out on earth and deal with local things, it's all good. We can interchange CG and CM. We can substitute mass for weight and it's not that big of a deal except for adolescent science students who have trouble understanding anything beyond smart phone apps and lust for the opposite sex.
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Old 08-26-2021, 01:25 PM
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Quote:
Originally Posted by tbzep
That's no different than saying something "weighs" 40 kilograms. Kilograms is mass, not weight. Weight is in Newtons in the metric system. I see scientists and college professors using weight and mass interchangeably all the time when referring to an object's mass.

We know that your mass doesn't change unless your name is Grendel and I rip your arm off, but your weight varies based on your proximity and the mass of the object you are attracted to. (Assuming we can magically change locations quicker than we can change our mass by eating, dieting, etc.)

If we are hanging out on earth and deal with local things, it's all good. We can interchange CG and CM. We can substitute mass for weight and it's not that big of a deal except for adolescent science students who have trouble understanding anything beyond smart phone apps and lust for the opposite sex.


This is all true...

But I wonder if this casual use of these terms interchangeably is a good thing. It's not quite as bad as one of my favorite pet peevesóconfusing accuracy and precisionóbut still...I think if it were up to me I'd be presenting the info and using "center of mass" and "mass" rather than "center of gravity" and "weight" except as a sidebar to make clear what the distinction between mass and weight is. But of course, generally it's not up to me. I haven't been in front of a classroom in quite awhile and don't expect to be again any time soon.

I did ping a contact I have at Estes who is involved in the website content a couple of days ago when I first saw this "Rocket Stability Kit" on their site and got a "thanks for the feedback" note earlier this morning.
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Old 08-26-2021, 01:37 PM
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Interchanging CG and CM for model rocketry falls under "Good Enuff, Ity'll doo" which is a far UNDER-utilized concept, virtually everywhere.
Objecting is Sheldonian.
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Old 08-26-2021, 02:22 PM
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Quote:
Originally Posted by BEC
This is all true...

But I wonder if this casual use of these terms interchangeably is a good thing. It's not quite as bad as one of my favorite pet peevesóconfusing accuracy and precisionóbut still...I think if it were up to me I'd be presenting the info and using "center of mass" and "mass" rather than "center of gravity" and "weight" except as a sidebar to make clear what the distinction between mass and weight is. But of course, generally it's not up to me. I haven't been in front of a classroom in quite awhile and don't expect to be again any time soon.

I did ping a contact I have at Estes who is involved in the website content a couple of days ago when I first saw this "Rocket Stability Kit" on their site and got a "thanks for the feedback" note earlier this morning.

I agree, mainly because the casual use of the terms causes confusion when you need to talk about them specifically. Everyone gets on scales and measures their "weight" in the metric countries in kilograms. Then when you want to teach about mass vs weight, regardless of age, the general public considers them to be the same thing. In some of my physics classes, us students used the proper terminology much better than our professors. My geology professors were much better about using the right terminology. Sheldon would not be happy.
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Old 08-26-2021, 03:29 PM
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Sheldon is a dweeb that would have been stuck in a trash can at BEST when I was in HS/College.
I wanted Leonard or Penny to backhand him HARD just once in the show.
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  #10  
Old 08-30-2021, 10:35 PM
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Default You're right

Quote:
Originally Posted by BEC
That bit bugged me as well....and then I did a little research and found that that they are, in fact, two distinct things......


After working a full career at three major aerospace companies, in the mass properties groups, I kinda knew that already. But we are not modelling the mass distribution of something the size of the ISS that is in orbit. Here on Earth, the mathematical difference between c.m. and c.g. on a model rocket is immeasurably small. Practical engineers ignore the splitting of such hairs. Yes, we used the terms interchangeably.

Build that same model rocket again, duplicating every stroke of balsa fin on sandpaper, every drop of glue, every spurt of paint, as best you can, and the c.m. location will still be miles different from the c.m. of your original rocket in comparison to the difference in c.m. and c.g.

My beef is not with you but with technically-oriented companies that have qualified resources available but do not use them, that confuse and misdirect young audiences who are already misled by internet "knowledge," and that seem to think any-old-idiot can write marketing copy. It's no wonder to me that we have such a crying need for STEM education.

But hey, I'm just an old crank....
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