Wikipedia defines mass thusly: “In the SI system of units, mass is measured in kilograms.” Fine, but in space, objects are weightless, so isn’t that definition rather circular?
I intuitively understand the rudiments of gravity, inertia, acceleration and angular momentum. I can play with equations all day. But my mind balks at the real-world applications of weightless bodies having mass. That said, I fully understand that the Space Shuttle has tremendous inertia in space–as does a blimp or zeppelin. Something just isn’t connecting.
Mass and weight are different things. On Earth they’re conviniently equal. In space, the weightlessness means nothing is pulling an object anywhere, but the mass will keep it from moving without a force being applied. It would take the same effort on your part to push a big rock around on Earth or in space (except for surface friction).
Well, part of the confusion comes from the way we measure mass. grams is a measurement of mass, NOT of weight, but most laypeople tend to think of mass and weight as the same thing. Mass is a measurement unit describing how much, um, mass something has.
Let me put it this way…on earth, you might have a 1kg sack of potatoes and a 2kg sack of potatoes. You think of this as weight, since you put it on a scale and it tells you it is 1 or 2kg, respectively. In space, both have no real weight, but you can obviously tell that one is larger than the other, and contains approximately the same density, and so you would say that one is more massive than another. The 2kg bag has more mass than the 1kg bag, even though both have the same weight (0).
Someone much closer to an actual scientific degree will probably be along to explain this better than I can…I remember this from HS Chemistry, but that was … a long time ago.
There is a distinction between mass and weight. The earth has a huge mass, right? But it sits there in space. What’s the weight of the earth?
Mass is simply the resistance of objects to being moved. Objects with a lot of mass take more energy to move than objects with low mass. In a constant gravity field we can measure the mass of an object by putting it on a scale and seeing how hard gravity pulls against the mass, and we call that weight.
Kilograms are a unit of mass. Pounds are a unit of force/weight. In metric, newtons are a measure of force/weight, in english “slugs” are the unit of mass.
Well, a neutron has a peculiar property called “mass”, protons have a different value for that property, and electrons have a still different value. So if you add up the number of protons, neutrons, electrons, and whatever other particles an object has, you’ll know the object’s mass. Various other exotic particles have mass and some like photons have no mass. Now, why do these teeny little particles have a resistance to being moved? No one really knows, they just DO.
Avogadro’s number is a consequence of this. If you have 6.02 * 10^23 atoms (a mole) of an element, the mass in grams of that element will exactly equal the atomic weight of the element, depending on the isotope ratio. So if you have 12.0107 grams of carbon with a typical isotope ratio, you know you’ve got a mole of carbon. This property is extremely important in chemistry if you want to control the reactions you get. So you if you want water, you react hydrogen to oxygen in a two mole to one mole proportion to get one mole of water. If you have more hydrogen you’ll have hydrogen left over after the reaction, more oxygen you’ll have oxygen left over.
And on earth, F=mg, where g is the acceleration due to gravity, which is 9.8 m/s^2, or 32 ft/s^2. In other words, gravity is the acceleration that gives mass a force, which we call weight.
This is true, though engineers will often use lbm. (pounds mass) these being equal to 1/32.2 slugs. This is convenient because in English units, weight is frequently specified, but mass seldom is.
In metric units:
I you push on a 1 Kg mass, with one Newton of force for 1 second, it will accelerate by 1 meter per second.
In English units:
If you push on a 1 slug mass, with one pound of force, for one second, it will accelerate by 1 foot per second.
Another interesting thought for you: A “bathroom scale” uses a spring to measure force. The fancy “scale” with the movable weights your doctor uses is actually a “balance” and measures mass.* The latter will read the same if used on the earth or the moon, the former won’t.
*OK, OK, it compares the force of gravity on two masses, so the force terms cancel out…happy now?
Picture Johnny Spaceman, wearing his little rocket backpack and asbestos-assed space trousers, zooming up to an empty oil drum in space (I can’t tell you why it’s there, it’s a NASA secret) and pushing on it, using the momentum the rocekt pack has given him. He’ll have the thing up to speed in no time. Full of oil, it will be tougher to get moving. If he tries to push the Space Shuttle the same way, he might get it to move (no friction to hold it where it is) but the effort and time will be greater to get it to the same speed as the empty oil drum.
Inertia is the definition of the property that brings about these differences in effort, and mass is the measure of inertia, with kilograms being the unit.
Because Earth’s gravity is more or less uniform, an object with a certain number of kilograms will have the same weight everywhere, so kg’s are used to indicate weight (the amount of force exerted by a body’s mass under acceleration), even though the actual metric unit of weight is the dyne, a unit you don’t here about as often, except in company names (Teradyne, Yoyodyne, etc.).
BAck in the English system, we use pounds for weight, and the more abstract unit of mass is the slug (a think a five-slug object weighs one pound in Earth’s gravity, but my memory is hazy).
This is absolutely true, but someone who misunderstands the difference between “intertia” and “weight” might say this is because the bricks are “pushing down harder” and so providing more friction to the roll of the wheels.
While friction due to weight is part of it, it is not (according to Newton’s theory) the primary reason why it’s harder to move a pile of bricks than an empty skateboard. One of Newton’s great physical insights was to separate the ideas of weight (the pull of an object toward the center of the earth) and inertia (the tendency of an object to resist changes in its motion), even though both items are proportional to the object’s mass. After 400 years of Newtonian physics being commonplace, the genius of this insight–one that is not at all obvious from casual observation–is often forgotten.
Mass then is a fundamental property belonging to an object, one we can determine by measuring an object’s weight or inertia. Being stuck on Earth, weight is the dominant manifestation of an object’s mass, but in a weightless environment mass still shows up as inertia. In the 20th century, the discovery of E=mc^2 gave us yet another way to measure an object’s mass (although this measurement would have to be done by destroying the mass).
Think of it like this…
On earth, 1kg of potatoes weighs 2.2 pounds
On Jupiter, 1kg of potatoes weighs 5.6 pounds
In space, 1kg of potatoes weights 0 pounds
No matter where it is, it’s still 1kg of potatoes. That may help a little, but you still might be puzzling over "But in space, what gives it its 1kg-ness? The answer is, if you wanted to change its velocity you’d have to apply a certain amount of force to it. This also happens to be the same amount of force that would be required to change the velocity of a certain platinum-iridium cylinder that has been kept at Sèvres, France since the 1880’s (the international prototype kilogram). This also happens to be half the force you’d need to change the velocity 2kg of potatoes.
No, it is not a circular definition, because kilograms are not a unit of weight. The unit of weight (force) in the SI system is the newton.
Scotandrsn, the “dyne” is not really used much these days. It is a relic of the old centimeter-gram-second (cgs) system. Nowadays, the meter-kilogram-second (mks) system is preferred, which has evolved into the International System of Units (SI).
Neither of these statements is strictly true. Even on Earth, weight and mass are different properties with different units; although in the Imperial or U.S. Customary foot-pound-second system there are units of pound-force and pound-mass that have the same value (assuming gravity to be a unity factor) for the sake of common convenience, the appropriate mass unit for performing engineering calculations is the slug (or in a poundal-force system, the pound). The obvious difference (aside from the difference of units) is that weight has an orientation with relation to the gravitational field, while mass is directionless and the resultant inertia acts in direct opposition to movement in any direction.
If that explanation is confusing, consider this: you can push something on a cart that is much, much too heavy to lift. This is because weight is a force (in the down direction) that has to be overcome before movement can occur, whereas mass is simply a resistance to a change in momentum (inertia), and the magnitude of that change per unit time is proportional to the effert exerted. In an ideal frictionless world, you’d still have to overcome weight to lift something, but any amount of force to the side would cause it to move, if slowly.
Regarding your second statement (“In space, the weightlessness means nothing is pulling an object anywhere, but the mass will keep it from moving without a force being applied,”) weightlessness doe not mean that there are no applied forces–an object in a freefall orbit most certainly is influenced by gravity, which keeps it swinging around the planet rather than flying off in a straight line–but rather that the applied forces are balanced by interia, so that no net force is felt within the orbiting body (neglecting tidal and rotational forces). And mass/inertia doesn’t just keep an object from moving, but also keeps it from stopping if it’s already in motion.
Sorry to be pedantic, but this is actually at the core of the OP’s question, and it’s important to frame the concepts of mass, inertia, momentum, and weight correctly in order to speak clearly on the topic.
It’s important to understand that weightlessness has nothing directly to do with being in space. Instead, it’s due to being in freefall (which most objects in space are, most of the time). You would be just as weightless if you were in a free-falling elevator as you would on board an orbiting Space Shuttle.