# Chemistry Question re: mass and weight

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Ok, I'm feeling really stupid here. We're using Conceptual Chemistry as supplemental reading with the DIVE cd for Integrated Chemistry and Physics.

The author states that he will be measuring mass throughout the book using grams, kilograms, etc. instead of the weight measurements of pounds and tons.

I'm confused beyond measure. Don't grams, kilograms, etc. measure weight? How do they measure mass? How does anyone measure mass without weighing it?

I get that mass and weight are different. I get that your weight would change depending on the location (moon vs. earth, for example) based on the gravitational force. But how does one measure mass without actually counting all the atoms (which I'm guessing isn't how they do it.)

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My understanding is that 1 gram is meant to be equal to 1 cubic centimeter of water, so something with a mass of 1 gram is made up of the same volume of "material" that 1 cubic centimeter of water would have.

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weight is counting gravity. therefore, same mass on earth and moon will have different weight

lbm is 1 lb mass

lbf is 1 lb force with gravity

kg is mass

1 nt (newton) is 1 kg*9.8 m/s2 and that is weight

Edited by jennynd
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I am sure this is totally not helpful, but it is interesting! The standard (American) unit of mass is the slug! Don't have a clue how much mass that is in grams, but the name is awesome, and I hope it brings you a smile as you work through all this!

Blessings,

April

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I'm confused beyond measure. Don't grams, kilograms, etc. measure weight? How do they measure mass? How does anyone measure mass without weighing it?

Mass doesn't change no matter where you go in the universe. Weight is different if there is different (or no) gravity.

If I am 60kg on Earth, I'm still 60kg on the moon, or weightless in space.

If I am 140lbs. on Earth, I weigh something completely different on the moon.

What you weigh on the moon, say, is measured in Newtons - mass (kg) x acceleration of gravity (m/sec^2).

Edited by matroyshka
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OK, all these answers lead me to my next question. What do you do when you are not smart enough to understand the stuff you're supposed to be teaching your kids? Seriously. That's why I got the DIVE cd - I knew I didn't get it before we started. But ds is not a science person either so we have gotten to the point where we are just laughing our heads off - we've gone off the proverbial deep end - both of us. We are sitting here guffawing as we are reading about Newtons. It might as well be Greek.

I want my mommy.:tongue_smilie:

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The author states that he will be measuring mass throughout the book using grams, kilograms, etc. instead of the weight measurements of pounds and tons.

If this is how he said it, it is very confusing. A pound can be a measure of mass or weight(force). It sounds like maybe he is saying the book will be using all metric/SI units instead of English. If that is the case there was no need for him to confuse you about mass vs weight. Anyway......

Weight is the force exerted by an object when its mass is under the influence of gravity.

I found this:

"Use of the gravitational constant, gc, adapts Newton’s second law such that 1 lbf = 1 lbm at the surface of the earth. It is important to note that this relationship is only true at the surface of the earth, where the acceleration due to gravity is 32.17 ft/sec2 ."

http://www.tpub.com/content/doe/h1012v1/css/h1012v1_28.htm

So if you are using the English measures, you will be using pounds-mass (lbm) for mass. If you are using the metric/SI measures, you will be using grams and kilograms for mass. If it was a physics course and you would be doing problems with force, you would use pounds-force for English and Newtons for metric/SI. You are not going to get into forces in chemistry.

http://www.mathsisfun.com/measure/weight-mass.html

Edited by laundrycrisis
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Weight is the force exerted by an object when its mass is under the influence of gravity.

I found this:

"Use of the gravitational constant, gc, adapts Newton’s second law such that 1 lbf = 1 lbm at the surface of the earth. It is important to note that this relationship is only true at the surface of the earth, where the acceleration due to gravity is 32.17 ft/sec2 ."

http://www.tpub.com/content/doe/h1012v1/css/h1012v1_28.htm

So if you are using the English measures, you will be using pounds-mass (lbm) for mass. If you are using the metric/SI measures, you will be using grams and kilograms for mass. If it was a physics course and you would be doing problems with force, you would use pounds-force for English and Newtons for metric/SI. You are not going to get into forces in chemistry.

http://www.mathsisfun.com/measure/weight-mass.html

The first question in lesson 4 of the DIVE lab/workbook says:

Use unit multipliers to convert 6 lbs. to Newtons. If lbs. is weight and Newtons are a force, is this even possible? I don't get it.

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This is the quote from Conceptual Chemistry:

"Because mass is independent of location, it is customary in science to measure matter by its mass rather than its weight. Conceptual Chemistry adheres to this convention by presenting matter in units of mass, such as kilograms, grams, and milligrams. Such weight units as pounds and tons are occasionally provided as a reference because of their familiarity."

My confusion lay in how does one measure mass if not by weighing it on a scale?

GingerPoppy's reply "that 1 gram is meant to be equal to 1 cubic centimeter of water, so something with a mass of 1 gram is made up of the same volume of "material" that 1 cubic centimeter of water would have" is helpful, but honestly, it still confuses me. Doesn't that disregard things that are more dense than water or is that irrelevant?

I really think it's beyond my comprehension.

Edited by Kathleen in VA
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The first question in lesson 4 of the DIVE lab/workbook says:

Use unit multipliers to convert 6 lbs. to Newtons. If lbs. is weight and Newtons are a force, is this even possible? I don't get it.

Weight is a force....it is mass at the acceleration of gravity. If you can make the assumption you are at the surface of the earth, 1 lbm = 1 lbf. So 6 lbm will weigh 6 lbf, and you need to convert 6 lbf to Newtons.

I was always taught to distinguish lbf and lbm - we lost points for just using lb or lbs.

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Weight is a force....it is mass at the acceleration of gravity. If you can make the assumption you are at the surface of the earth, 1 lbm = 1 lbf. So 6 lbm will weigh 6 lbf, and you need to convert 6 lbf to Newtons.

I was always taught to distinguish lbf and lbm - we lost points for just using lb or lbs.

Lightbulb!:) I was forgetting that weight is also a force.

I think I can at least explain the problem to him now. Thanks so much!! (Where's the smiley with tears of gratitude when you need it?)

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T

My confusion lay in how does one measure mass if not by weighing it on a scale?

GingerPoppy's reply "that 1 gram is meant to be equal to 1 cubic centimeter of water, so something with a mass of 1 gram is made up of the same volume of "material" that 1 cubic centimeter of water would have" is helpful, but honestly, it still confuses me. Doesn't that disregard things that are more dense than water or is that irrelevant?

I really think it's beyond my comprehension.

You do measure it on a scale, but you also know the acceleration of gravity where you do it (here on earth) and any other forces pushing on the scale (generally none).

It should be the same amount of material, not volume of material, and the water is at a standard temperature and atmospheric pressure, so the density of the water is not variable. They took a fixed volume of pure water at a standard temperature and pressure and weighed it in the earth's gravitational field - and gave that exact amount of downward force a name.

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Just wanted to say thanks for this link especially - I'm gradually gaining some understanding.:)

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My confusion lay in how does one measure mass if not by weighing it on a scale?

.

That is actually an excellent question. It is not at all trivial that the same quantity that is important for an object's force of gravity (ie weight, which is equal to mg) should be relevant when describing an object's inertia, i.e. its resistance to being accelerated.

You can determine mass by using Newton's 2nd law: F=ma.

If you apply a certain amount of force to an object and and measure the object's resulting acceleration, you can calculate its mass: F/a=m. This will be independent of location, the same on the moon and on Earth.

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OK, all these answers lead me to my next question. What do you do when you are not smart enough to understand the stuff you're supposed to be teaching your kids?

It's not that you're not smart enough!! It's just that you haven't learned it yet yourself. So, you have fun learning it together. It's great for your kids to see you humble yourself and admit you don't know everything. It's even better for them to see you not only willing, but wanting, to learn along with them.

It's also a great opportunity for you to model for your children how you go about understanding something that doesn't make sense initially. Homeschool Moms' boards are a great resource :), but there are lots of good sites online that might help as well.

Have fun, and don't hesitate to come here to ask for help.

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That is actually an excellent question. It is not at all trivial that the same quantity that is important for an object's force of gravity (ie weight, which is equal to mg) should be relevant when describing an object's inertia, i.e. its resistance to being accelerated.

You can determine mass by using Newton's 2nd law: F=ma.

If you apply a certain amount of force to an object and and measure the object's resulting acceleration, you can calculate its mass: F/a=m. This will be independent of location, the same on the moon and on Earth.

Why don't textbook authors tell you this up front? I mean, this is 9th grade science and perhaps they don't feel the student is ready for this, but if you are going to tell me that weight and mass are two different things but that you can measure them both by placing items on a scale you've lost me. That makes no sense to me and I'm left scratching my head and wondering what it is I don't understand. Your formula makes perfect sense to me - I get how you would get the same answer for mass no matter where in the universe you are. Maybe I just over analyze things, by I hit a brick wall if I don't understand why - I need to know why - not just "this is the way it is."

Anyway, all that to say, THANK-YOU for explaining that to me. I understand now.:)

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It's not that you're not smart enough!! It's just that you haven't learned it yet yourself. So, you have fun learning it together. It's great for your kids to see you humble yourself and admit you don't know everything. It's even better for them to see you not only willing, but wanting, to learn along with them.

It's also a great opportunity for you to model for your children how you go about understanding something that doesn't make sense initially. Homeschool Moms' boards are a great resource :), but there are lots of good sites online that might help as well.

Have fun, and don't hesitate to come here to ask for help.

Thank you so much for your encouraging words. It's been a hard day all around - this was just one little thing out of many (I won't bore you.:D) You are right, I don't know this stuff, but I really want to. I read The Making of the Atomic Bomb this summer (you might have seen me mention it in the "intellectually challenging books" thread), and I tried very hard to grasp the physics and chemistry but always felt it was just beyond my grasp (sometimes way beyond my grasp). I understood it in a very limited sort of way - enough for the rest of the book to be interesting. But I desperately wanted to understand the physics and chemistry parts, too, so here is my opportunity to learn it. I'm excited.:)

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Why don't textbook authors tell you this up front? I mean, this is 9th grade science and perhaps they don't feel the student is ready for this, but if you are going to tell me that weight and mass are two different things but that you can measure them both by placing items on a scale you've lost me. That makes no sense to me

You can, of course, use a scale to measure mass as well, because you know there is a one, well-determined factor of g=9.8m/s^2 between weight and mass.

The way a scale works is to have the weight, i.e. force of gravity, balanced by another force, such as the force in a spring, or a normal force ( bathroom scale). The object is then at rest and not accelerating at all (you stand still when you are on your bathroom scale). The scale measures the normal force or spring force and, since the system is balanced, knows it equals the weight.

Now, instead of writing the weight force on the scale numbers (in pounds or Newtons), you can take the number for the force in Newtons and show it divided by 9.8- that would be your mass in kilogram, because W/g=m.

So, the author is entirely correct: you CAN use a scale to find the mass, if you use the fact that on the surface of Earth W=mg, and g is known. Only it does not make them the same quantity.

Hope that makes sense and did not confuse you.

(Gosh, this is sooo much easier in Europe where people's bathroom scales show masses in kilograms and there is no such thing as a pound.)

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