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# Why is mass not conserved unlike energy? watch

1. I thought by E=mc2 that both mass and energy are interchangeable ?
http://www.edexcel.com/migrationdocu...e_20110127.pdf
such as in Q7 why is the answer is C NOT B
2. Because mass and energy are the same thing: they are referred to as mass-energy.

This is why photons of a certain frequency (and thus a certain energy according to E=hf) can sometimes produce a pair of particles.

In essence, if you're conserving mass, you're conserving energy. But since mass can convert to energy matter itself isn't really conserved is it?

I hope this helped
3. (Original post by Kuki)
Because mass and energy are the same thing: they are referred to as mass-energy.

This is why photons of a certain frequency (and thus a certain energy according to E=hf) can sometimes produce a pair of particles.

In essence, if you're conserving mass, you're conserving energy. But since mass can convert to energy matter itself isn't really conserved is it?

I hope this helped
but energy isn't also really conserved alone. I understand the idea of mass/energy is conserved but situations like pair production energy completely disappears (to matter) & in annihilation where matter completely disappears (to energy) so why do we choose energy conservation over matter? im really confused
4. (Original post by >>MMM<<)
but energy isn't also really conserved alone. I understand the idea of mass/energy is conserved but situations like pair production energy completely disappears (to matter) & in annihilation where matter completely disappears (to energy) so why do we choose energy conservation over matter? im really confused
Energy is the intrinsic property of which mass is a subset. Mass is a form of energy, but not all energy is a form of matter. Energy can also be kinetic energy, or mechanical energy, or gravitational potential energy etc.
5. basically, matter IS energy

so you can just need to remember that energy is conserved at all times
6. (Original post by Jack93o)
basically, matter IS energy

so you can just need to remember that energy is conserved at all times
so your basically saying that matter is energy and it cant be the other way round?
7. (Original post by F1 fanatic)
Energy is the intrinsic property of which mass is a subset. Mass is a form of energy, but not all energy is a form of matter. Energy can also be kinetic energy, or mechanical energy, or gravitational potential energy etc.
so your basically saying mass/matter is not conserved unlike energy?
8. (Original post by >>MMM<<)
so your basically saying mass/matter is not conserved unlike energy?
correct, mass is not conserved as evidenced by nuclear fission, where the combined mass of the child nuclei is less than the mass of the parent nuclei because of the release of binding energy, which we use to power steam turbines and generate electicity (or to cause mass destruction in a nuclear weapon).
9. (Original post by >>MMM<<)
so your basically saying that matter is energy and it cant be the other way round?
It can be the other way round, energy can convert to mass, and mass can convert to energy.

I think the best way is to just think of them energy and mass as the same thing when you consider pair production and annihilation. It really isn't important whether you only get matter or only energy out of a collision, because mass and energy are basically the same thing through the 'e = mc^2' equation
10. (Original post by F1 fanatic)
Energy is the intrinsic property of which mass is a subset. Mass is a form of energy, but not all energy is a form of matter. Energy can also be kinetic energy, or mechanical energy, or gravitational potential energy etc.
This.

The full equation is:

Where p is momentum. If p = 0, i.e. the particle has no kinetic energy, we get:

Which you may have heard about.

EDIT: The confusion probably comes from use of the terms "rest mass" and "relativistic mass". Relativistic mass is basically the same thing as energy so people don't tend to use it. "Rest mass" is what I'm talking about in my post - the symbol m.

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