quote:Original post by CobraA1
First, you say that you don''t know what a beam balance is, then you go on and keep talking about it anyway. From what I''m reading, you still think I''m using something akin to a bathroom scale. I''m not!

I think there''s a communication breakdown in this thread, . First, know that I''m not meaning to attack you personally. I did probably write that message in a way that it sounded like an attack, and I apologize for having written it that way.

I''m just trying (with Sandman, pouya, ddarius, JonnyQuest, and maybe a couple of others here who have a correct understanding of this issue) to shed some light on the difference between mass and weight. I feel like you still don''t quite understand the difference. Read the posts from the above people, because they''re fairly good and accurate. Look in any elementary physics book and you''ll see that those folks and myself are correct.

I know exactly what you meant by "beam balance." You unfortunately did not follow my example very well. It needed a picture. I wasn''t talking about bathroom scale.

By "beam balance" you meant a beam, with a fulcrum in the middle, and two objects on either side. A device that exploits the simple lever to achieve balance. If the lever-arm times the weight of the objects is the same on both sides, then they are in balance.

When I questioned what you meant by "beam balance," I was meaning to identify a breakdown in your understanding of the problem. You were talking about measuring the "mass" with a balance, when in fact balances cannot measure mass. Balances measure forces and mass is not a force! There are very few ways to actually measure mass. In fact, it can''t be measured directly using a balance (or a scale). It can be *implied* by measuring the gravitational constant using a very special balance (an *extremely* precise torsional balance, as invented by Henry Cavendish in the late 1700''s).

quote:Original post by CobraA1
First, it''s called a balance for a good reason. You have two sides, one where you place the unknown mass, and the other where you place known masses. If it tips in the direction of the unknown mass, you need to place more known masses on the other side, and if it tips toward the known masses, you need to take some off. Once it is level, you have equal masses on both sides, and you count the known masses and you know the mass of the unknown mass, regardless of gravity!!!

You keep saying the masses balance each other. Well, on a single planet where the gravitational acceleration is constant you can make that generalization. But it simply is *NOT* the mass values that cause the balance to be in balance. It is the weights (and lever arms). You''re statement is just not technically correct. Replace "mass" with "weight" in your paragraph and you''re closer to being correct. But if you leave in "mass" then you''re wrong, strange as it may seem. I know it seems that you are right. Since, for your experiment, both "masses" are on the same planet, adding more mass just happens to correspond to adding more weight. But in fact, what is physically happening is that the torques due to the weights are balancing out. Vector forces and torques are balancing. The amount of mass is just a property that leads to the production of the forces that cause the balance.

quote:Original post by CobraA1
BTW, the bathroom scale is technically not a balance.

Sure it is. There are forces in balance. If you step on a scale, why doesn''t the scale collapse and get crushed on the floor due to your weight? Because a stiff spring and the structural integrity of the scale push back up on your weight, putting the forces in balance. The word "balance" does not imply specifically a fulcrum or beam balance. A beam balance needs a balance of torques. A bathroom scale needs a balance of forces. Both are balances of vector properties created by gravitational fields on masses.

I think you did not follow my mind experiment properly. My example is just like your beam balance, but with only one object. I''m going to attempt to draw a text picture of what I was meaning to describe:

                                    ^                              spring_force                                    |  object                            |*******************BEAM*********************                    ^^                   ^^^^                  ^^^^^^ 

See, I have a fulcrum in the middle, just like you were expecting! And as I described, the object whose *WEIGHT* is a force vector that that acts downward to balance (!) the spring force is only on one side. The spring force is on the other side, but there is no object there. This example is something you can take from planet to planet and get entirely different results, and it is that behavior that proves that weight and not mass value causes the balance.

quote:Original post by CobraA1
No, SiCrane didn''t. There is a difference between a spring scale and a balance. I''m surprised a "Senior Scientist" didn''t know this.

SiCrane made the same mistake you did.

Let''s not start up with these personal attacks. It won''t accomplish anything.

As for my title, it is accurate. I''m confident that I know just a wee little bit more about elementary physics, even rather advanced physics, than you are imagining right now, .



Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

quote:As to where they got the definition of the kilogram:
A gram is defined as the mass of 1 cubic centimeter of water.

ISTR this is only partially true. The original definition of a gram is like this, but the lump of platinum that represents the kilogram is *not quite* the same mass as 1000 cubic cm of water. So since the acceptance of the standard kilogram the definition of the gram has changed.

I didn''t read this entire thread, but I''ll answer in reference to Afterlife''s main question. All I have to say is why is this like the hardest concept to grasp for crying out loud? I remember being like the only kid in my eigth grade physical science class that understood what my teacher was talking about when he differentiated mass and weight! Then I get to physics in senior year high school - and there are still kids who can''t understand why your weight changes and your mass doesn''t!! Mass is the amount of matter in your body, weight is just the amount of force with which gravity is pulling you down. Simple!! I''m sorry, I really am - no offense to Afterlife or anything but I so can''t understand why everyone in the world can''t get this.

==============================
"Need more eeenput..."

- #5, "Short Circuit"
==============================

Actually I did understand the difference between mass and weight. I''m a finn so I knew how to translate those two into different words (Wow! ). But guess I was so stupid to think that my physics book was saying that a normal scale shows the same figure of kilos on both Moon and Earth. Just couldn''t buy that and actually I was correct from the scratch.

Ah. Well, then my rants are no longer directed towards you, Afterlife. Now, the rest of you simpletons in the world... *narrows his eyse*

==============================
"Need more eeenput..."

- #5, "Short Circuit"
==============================

quote:

I think there's a communication breakdown in this thread, .

You're right. I wasn't thinking.

quote:First, know that I'm not meaning to attack you personally. I did probably write that message in a way that it sounded like an attack, and I apologize for having written it that way.

I'm just trying (with Sandman, pouya, ddarius, JonnyQuest, and maybe a couple of others here who have a correct understanding of this issue) to shed some light on the difference between mass and weight. I feel like you still don't quite understand the difference. Read the posts from the above people, because they're fairly good and accurate. Look in any elementary physics book and you'll see that those folks and myself are correct.

I know exactly what you meant by "beam balance." You unfortunately did not follow my example very well. It needed a picture. I wasn't talking about bathroom scale.

By "beam balance" you meant a beam, with a fulcrum in the middle, and two objects on either side. A device that exploits the simple lever to achieve balance. If the lever-arm times the weight of the objects is the same on both sides, then they are in balance.

You're right, it is actually the force of gravity between the objects and the planet (the weight) that makes it work.

quote:
When I questioned what you meant by "beam balance," I was meaning to identify a breakdown in your understanding of the problem. You were talking about measuring the "mass" with a balance, when in fact balances cannot measure mass. Balances measure forces and mass is not a force! There are very few ways to actually measure mass. In fact, it can't be measured directly using a balance (or a scale). It can be *implied* by measuring the gravitational constant using a very special balance (an *extremely* precise torsional balance, as invented by Henry Cavendish in the late 1700's).

Original post by CobraA1
First, it's called a balance for a good reason. You have two sides, one where you place the unknown mass, and the other where you place known masses. If it tips in the direction of the unknown mass, you need to place more known masses on the other side, and if it tips toward the known masses, you need to take some off. Once it is level, you have equal masses on both sides, and you count the known masses and you know the mass of the unknown mass, regardless of gravity!!!

You keep saying the masses balance each other. Well, on a single planet where the gravitational acceleration is constant you can make that generalization.

This is exactly why these balances work.

quote: But it simply is *NOT* the mass values that cause the balance to be in balance. It is the weights (and lever arms). You're statement is just not technically correct. Replace "mass" with "weight" in your paragraph and you're closer to being correct. But if you leave in "mass" then you're wrong, strange as it may seem. I know it seems that you are right. Since, for your experiment, both "masses" are on the same planet, adding more mass just happens to correspond to adding more weight. But in fact, what is physically happening is that the torques due to the weights are balancing out. Vector forces and torques are balancing. The amount of mass is just a property that leads to the production of the forces that cause the balance.

Original post by CobraA1
BTW, the bathroom scale is technically not a balance.


Sure it is. There are forces in balance. If you step on a scale, why doesn't the scale collapse and get crushed on the floor due to your weight? Because a stiff spring and the structural integrity of the scale push back up on your weight, putting the forces in balance. The word "balance" does not imply specifically a fulcrum or beam balance. A beam balance needs a balance of torques. A bathroom scale needs a balance of forces. Both are balances of vector properties created by gravitational fields on masses.

I think you did not follow my mind experiment properly. My example is just like your beam balance, but with only one object. I'm going to attempt to draw a text picture of what I was meaning to describe:

                                    ^                              spring_force                                    |  object                            |*******************BEAM*********************                    ^^                   ^^^^                  ^^^^^^                    

<

See, I have a fulcrum in the middle, just like you were expecting! And as I described, the object whose *WEIGHT* is a force vector that that acts downward to balance (!) the spring force is only on one side. The spring force is on the other side, but there is no object there. This example is something you can take from planet to planet and get entirely different results, and it is that behavior that proves that weight and not mass value causes the balance.

Original post by CobraA1
No, SiCrane didn't. There is a difference between a spring scale and a balance. I'm surprised a "Senior Scientist" didn't know this.

SiCrane made the same mistake you did.

Let's not start up with these personal attacks. It won't accomplish anything.

As for my title, it is accurate. I'm confident that I know just a wee little bit more about elementary physics, even rather advanced physics, than you are imagining right now, .



Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

I guess you're right. I was thinking something like this:


(known mass)                    (unknown mass)

  object                            object

*******************BEAM*********************
                   ^^

                  ^^^^

                 ^^^^^^

You're right, it's the force of gravity being the same on either side, and the result being that the torques on the lever arms cancelling out. I should've thought more before I spoke. And thanks for the explanation of the bathroom scale.


Edited by - CobraA1 on November 15, 2000 10:53:55 AM

Gee I hope the admins of this board wont mind these sciense talks I seem to cause every now and then. Can''t help it, I just love sciense (but I''m not good at it, which is too bad. Sinse, with my enthusiasm I could have gotten to be a nobel winner or something).

I now declare that trying to format anything correctly on this forum is futile, or nearly so. I''ve tried practially every tag in HTML. Any suggestions? doesn''t cut it.


"Whoever performs only his duty is not doing his duty." --Bahya ibn Pakuda

quote:Original post by Sandman
ISTR this is only partially true. The original definition of a gram is like this, but the lump of platinum that represents the kilogram is *not quite* the same mass as 1000 cubic cm of water. So since the acceptance of the standard kilogram the definition of the gram has changed.

Actually, the perfect kilogram is a platinum-iridium alloy. However, it is now defined as the mass of a litre of water (try it, it works). However, as we know, if we heat up water it increases in volume. So the density will not be constant, so I think it is at room temperature and pressure. The measures are so arranged that the only ''artificial'' measure needed (ie it is arbitrary) is the second, which is some number of caesium vapour atom vibrations or something.


Please state the nature of the debugging emergency.

sharewaregames.20m.com

This is almost as good as the "MP3 beating compression" thread.
Anybody remember that one? That was before I even registered this username.


Please state the nature of the debugging emergency.

sharewaregames.20m.com