Explain, Mass?

Question

E=m/c2 The difference of mass in a nuclear and a coal burning power plant? Obviouly mass has nothing to do with size? In layman terms please.

Answer 1

Mass has two meanings. The inertial mass is the property which resists acceleration. When Newton came up with his F=ma law, he observed the inertial mass of an object subjected to a force is what determines how fast it accelerates. An object with a lot of mass didn't accelerate as much as an object without much mass.

The other meaning is the gravitational mass. This is the property which results in gravitational attraction. Put an object with a lot of mass in a gravitational field and a large gravitational force acts on the object. If the object doesn't have a lot of mass, the gravitational force is much less. The fact that gravitational and inertial mass are equivalent is a subtle result of the theory of general relativity.

When coal burns, the reaction is chemical. It occurs at the atomic level. The nuclei of the carbon and oxygen atoms are unchanged; they just swap some electrons. The heat is generated by atoms bouncing around at higher speeds due to the chemical changes.

In a nuclear reactor, the nuclei of the fuel split. The fragments of the split atom have less mass than the original nuclei. The missing mass is converted into energy, found by Einstein's E=mc2. This is the energy generated by nuclear reactors.

I wouldn't say mass has nothing to do with size. Obviously, the smaller of two objects can have more mass. But just as obviously, size and mass are at least partially correlated.

Answer 2

E = mc^2 is not important except in nuclear reactions. Mass is several things (I think there is still a Nobel prize for someone why the different meanings of mass are really the same).

Mass has to do with the resistance to change in velocity (inertial mass) F=ma and all that.

Mass also has to do with the gravitational attraction. Weight is the force gravitiy has on a object it is proportional to mass (why that should be the same as interial mass?)

Back to E=mc^2. In this equation mass is the rest mass.

By the way, It is called rest mass because if something is moving it weighs more! The difference between mass and rest mass is only important when the mass is moving nearly the speed of light.

OK so E=mc^2 is only part of the equation but it is the part that tells you how mass is related to energy. In a nuclear reaction (like a bumb) there may be a large net conversion of mass to energy. E=mc^2 tells you the absolute maximum energy you can get from a mass at rest should it be completely converted to energy. This does not happen in everyday life. One exampel would be a electron and positron hitting each other. They will be completely converted to energy in the form of photons (light). Since photons do not have mass, the most have e=mc^2 energy, where in this case m would be the rest mass of the electron and positron.

In chemical reaction (like burning coal) you would never be able to measure the change in mass, but there would be some.

Answer 3

So far as extracting energy goes (or so I assume from your question), there are two ways you can get energy from mass, from nuclear or chemical energy. Nuclear is far, far more powerful.

First of all. Mass is a concept so "primitive" (that is, it can't be broken down any further) that it is hard to explain, but I'll say it this way: it is a measurable property of an object's resistance to acceleration.

What do I mean by this? Obviously, a truck weighs more than you do - but mass is only indirectly related to weight, because weight is a property of what happens to mass in a gravity field.

We can still say that the truck *masses* more than you do, though - because it takes more energy to start moving the truck, or to change its motion, than it does to move you or change your motion. See what I mean?

And you are right: mass has nothing to do with size. An inflatable, family-sized tent may be much larger than you, but it'd take less energy to push it than to push you (assuming of course it wasn't staked to the ground)

(revision: Jim_H below puts the mass-size question better than I do here.)

So far as how mass relates to energy, that's two whopper subjects, but think of it this way:

A coal-fired plant uses chemical energy to make heat. In a chemical reaction, you convert a substance from one chemical compound that has some energy bound up in it, to another form that has less energy in it.

Put it this way. You've enjoyed fires in a fireplace, or a camp outing, right? Now, of course, wood isn't a chemical element, its a lot more complicated, but just for illustration's sake - those fires were continuous chemical reactions between the cellulose in the wood and the oxygen in the air around you.

Note two things:

1. Once the wood is completely burned, good luck trying to "light it on fire" again. It's been converted to a form that has less energy in it than wood.

2. If you were to completely weigh the material products of the fire - all the ash, all the smoke particles - you would find that it massed the same as the wood did before the fire.

Nuclear energy is not a chemical reaction. It gets heat from converting mass, typically enriched uranium, into energy.

A fellow by the name of Einstien pointed out that mass and energy are two different forms of the same thing - that is, E=MC^2. And there is a *LOT* of energy in mass.

If you weighed the products of a nuclear reaction, you would find that they weighed less than from before the reaction. Mass was turned directly into energy.

Whew. You asked a big question there! I think it's time for me to raid the fridge and turn some pasta into energy... :>

Hope this helps.

Answer 4

E=mc^2, not E=m/c2

In relativistic theory, this is the rest energy of an object with mass m.

A coal burning plant has little to do with this formula. A nuclear does; by definition it releases nuclear bond energy. For simplicity, if to assume that speeds in the nuclear reaction are close to zero, approximate energy released in a nuclear reaction is the difference of such energies before and after the reaction. Adjustment for non-zero speed is trivial.

m=rV, where m is mass, r is density, and V is volume. r is different for different states of different matter. For ideal gases,

p=rRT/u

where p is pressure, r is density, R is the universal gas constant, and u is the atomic mass.

Answer 5

I have to clear up something the previous responses seem to have wrong.

In fact, there is a slight difference in the mass of chemical reactants before and after burning. This is of course expressed as heat. However the mass difference is undetectable in practice, because (change in mass)=(released energy)/c^2, and c^2 is a very large number.

In contrast the energy released in a nuclear reaction is much larger, so the change in mass is on the order of grams per tens of kilogram. The energy released is electrostatic potential enerergy (fission) or strong-force potential energy (fusion).

Edit: On closer reading, at least one of the responses I thought to have this incorrect got it right.

Answer 6

Mass is how much matter is in an object any where in space.

Weight is the force of gravity on an object .
E=MC2
E=Energy m=Mass C2=the speed of light squared

Mass times the speed of light squared = Energy

Answer 7

It is also important to realize that this equation is only useful if you are studying the CHANGE in mass as it relates to it's energy. The key to the formula is that Einstein (and maybe some others) discovered that Energy = mass when the mass is divided by the speed of light squared. So as an atom mass is changed, it releases a tremendous amount of energy because of the speed of light. The key is to realize that energy, mass and the speed of light all are so inter-related as to simplify into a small equation. Hope that makes sense (And I hope that's all correct)

Answer 8

According to my understandings, mass is the amount of matter or "stuff "within or confined in a certain volume.

Hence, mass and weight being different.

Answer 9

Hi say your weight is 322 #. this is your weight in earth. Your weight in moon, or mars will be different. But your mass will be 10 (m=w/g) This mass will remain same wherever you go. constant. That is mass.

Answer 10

mass is defined as the amount of substance / matter found in an object. it SI unit is kg. weight on the other hand is the amount of force that acts on an object. its SI unit is N (NEWTONS). its SI unit is newtons because weight is a force. you can calculate weight by multiplying mass and gravitational force.