OK, in cecil’s thing about whether the earth was getting heavier or lighter, he gave some estimates of the earth’s weight, but how can the earth have a weight. We know that “weight” is really just the gravitational force between two objects, so if that’s the weight of the earth, reletive to what? If you talk about weight, its usually assumed that you’re talking about weight on earth, so you can’t just give a weight of the earth itself without specifying what you’re talking about.
You gotta read carefully, Cecil got the weight of the earth by putting it on a scale.
Actually, I assume that he got the “weight” of the earth by determining how much an object with the mass of the earth would weigh if it were in normal earth gravity. (But I really shouldn’t presume to speak for His Cecilness).
To review, boys and girls, mass is a measure of how much “stuff” an object has, while weight is a measure of how much force of gravity is acting on an object. The English unit pounds is a measure of weight, while the metric kilogram is a unit of mass.
Imagine a one kilogram hammer (takes a lot of imagination, doesn’t it.) Here on earth it weighs about 2.2 pounds, because that’s the force of the earth’s gravity acting on it. Now let’s take it up to the moon. On the moon, it would still have a mass of one kilo. However, since the moon’s gravity (more properly, the force of the moon’s gravity on a given mass on the surface of the moon) is about 1/6 of earth’s gravity (on a given mass on the surface of the earth), it would have a weight of about 0.37 pounds (1/6 of 2.2).
Anyway, Chrisooya, I think that you’ve caught the Master in an inclarity, to say the least. Good job.
The metric unit of weight (for a physicist) would, of course, be a Newton, though in common parlance people usually describe weight with kilograms, assuming earth gravity.
presumably, since every mass associated with the Earth has a weight, it would be possible to calculate a total for the Earth’s mass, even though the net effect of this weight would be more or less negated by the distribution around the center of gravity.
Then again, defining a discrete set of matter as being associated with Earth gets fuzzy. The weight of any one object being the net force acting upon it by all the rest of that mass, including, to some infinitessimally small degree, the gravitational forces acting throughout the entire cosmos.
The slug is still in use. As a matter of fact I used it often in a few fluids courses as recently as 1998. The sea level density of air will forever be engraved in my mind as 0.002377 slugs per cubic foot.
Hey, measurements and units are funny things.
I remember the double take the kid gave me when I told her that temperature is not a measure of heat. WELL WHAT IS IT THEN? I had to get an old college textbook and look it up before I could tell her.
(Basically, it’s a measure of whether heat will flow between two bodies. You can put 80 calories of heat into a gram of ice at 0 degrees C and the temp will not change; the ice will only melt.)
Unless they insist on measuring mass in those god-awful pound-mass things. A pound-mass is the mass of an object with a weight of one pound at sea-level. But if you want a real stumper for them, try a degree Celsius is to a Kelvin (not a degree Kelvin) as a degree Fahrenheit is to a ???. Answer: degree Rankine.
