Conservation of Energy: Superstrings, Einstein & Birds

[Mattofix]

[SOLVED] Conservation of Energy

From 'Superstrings and Other Things' by Carlos Calle

'Einstein’s extension of the principle of conservation of energy is a profound generalization. In everyday life, however, the limited principle of conservation of energy is sufficient. A flying bird has more energy than a bird standing on the branch of a tree, but the increase in the bird's mass due to its greater energy when flying is so small that it cannot be measured by any experiment.'

I understand and agree with everything up to where he says

but the increase in the bird's mass due to its greater energy when flying is so small that it cannot be measured by any experiment

, shouldn't increase be decrease. I can't see how the bird can gain mass, only loose it by converting it into it kinetic energy? which would make sense. I might be missing the point completely or its simply just a printing error?

This then lead me to thinking about us, when we move about, does the energy come from particles in our body undergoing reactions giving off energy, resulting in an overall decrease in our mass and increase in kinetic energy? and using E=MC^2 calculates the energy given off?

 

[ZapperZ]

I think he is invoking the increase in "mass" as in the "relativistic mass".

Zz.

 

[Mattofix]

I sort of understand relativistic mass, but it has not been mentioned prior in the book, E = MC^2 has only been mentioned so far, but i do appreciate that as the
birds speed increases so does its mass due to the relativistic mass idea.



So say the bird at rest has a mass M1, therefore using E=MC^2 gives the energy equivalent of its mass which we shall call E1.

The flying bird has gained kinetic energy, say e2. So e2 = MC^2 , so e2 has an equivalent mass of say m2.


The bird must have lost a mass of m2 in order for it to have gained the kinetic energy (e2)?

Is this true? if not where does e2 come from?

and is this idea totally independent from relativistic mass?

So this idea decreases its mass but relativistic mass increases its mass? Ahhh! I am totally making a mess of this,

 

[Proggle]

I think you're mixing two concepts.

If you count the bird's metabolism in the equation, then yes, there would be a minuscule mass loss as respiration takes place and the compounds the bird uses for energy are chemically changed. This energy allows it to fly.

However, I think in the problem you're describing, you don't care how the bird got that energy, you're just using its movement as an example for relativistic mass, which purely from that assumption, increases.

 

[Mattofix]

Great, so i have confirmed there are 2 processes going on here,

1. being the conversion of the birds mass to allow it to fly (decrease in mass)

2. being the increase in the birds mass because of its increase in speed.

As to which one he was referring to, i guess i will never know for certain but if you read what came before the original quote...

Einstein deduced that mass and energy were equivalent. The famous E=MC^2 gives the energy equivalent of a mass m.
According to Einstein, the mass of an object is a form of energy. Conversely, energy is a form of mass. For example, the combination of one pound of hydrogen with four pounds of oxygen to form water releases enough energy to run a hair dryer for about 10 hours. If we had an extremely precise balance, we would discover that the mass of the water formed is less than the total mass of the oxygen and hydrogen used by about one part per billion. The mass loss is exactly equivalent to the energy released in the process. A more dramatic example - as we shall see in chapter 23 - is the release of energy in a nuclear reaction, we could obtain about ten million times more energy. As in the chemical process, the end product would weigh less than the original material. The difference in mass is converted into energy.

...followed by...

'Einstein’s extension of the principle of conservation of energy is a profound generalization. In everyday life, however, the limited principle of conservation of energy is sufficient. A flying bird has more energy than a bird standing on the branch of a tree, but the increase in the bird's mass due to its greater energy when flying is so small that it cannot be measured by any experiment.'

...because there is no mention of relativistic mass and that he has only just introduced the concept of E=MC^2 i think he must be referring to the conversion of the birds mass into energy. So he has made a mistake by using increase instead of decrease in the birds mass?

 

[Doc Al]

...because there is no mention of relativistic mass and that he has only just introduced the concept of E=MC^2 i think he must be referring to the conversion of the birds mass into energy. So he has made a mistake by using increase instead of decrease in the birds mass?

No, looks to me he's talking about relativistic mass. Note that he says "Conversely, energy is a form of mass" in the first passage you quote, from which I deduce that he means that the kinetic energy of the bird gives it more "mass". I think he's doing you a disservice by not being clear.

Lot's of luck trying to learn physics from a popular book!

 

[Mattofix]

Thanks for that, your most probably right. i thought that this book would give me a nice refresher guide to most physics topics, instead it seems to creating lots of problems and waisting lots of time! On the plus side, its making me question things more and probably in the long run understanding things better.