# Conservation of Energy

In my calculus bases physics 1 class our teacher assigned us a 4-5 page paper on conservation of energy. He said to talk about a specific topic of our choosing that had to do with conservation of energy. I'm an astronomy/mathematics major, and I was hoping somebody could help me decide on a topic. I really want to do something on astronomy, but I can't seem to find anything that I could write that much about.

Thank you,
Pcrouse

D H
Staff Emeritus
Newton's vis viva equation.

A physics class assigned a paper? One of the reasons I am doing physics is because I dont have to write papers!

You could talk about how our universe supposedly has a fixed amount of energy but is increasing its expansion. Or maybe how we are able to only see some really small percentage of the amount of "stuff" in the universe.

Bobbywhy
Gold Member
My suggestion is for the subject of Astrophysics, not Astronomy. Examine how the energy of radiation is NOT always conserved. Light emitted by a distant quasar is redshifted while travelling for billions of years so that longer wavelengths (lower energy) arrive here at our detectors. So what happened to that difference in energy?

For example, see lots of threads right here in PF on this.

Ken G
Gold Member
Or do a paper on supernova blast waves, and describe what are called the "energy conserving" phase (the "Sedov" phase) and the "momentum conserving" phase (the "snowplow" phase). This may sound technical, but the physics is very simple, and it is interesting that we use language like that even though energy and momentum are always conserved quantities (outside of general relativity, where we don't have a concept of a global inertial reference frame in which to conserve energy and the situation gets pretty technical to say the least). In a nutshell, whenever we say energy is not conserved, we really mean it is going into some form we are not tracking (often heat, or in the supernova case, light from the shocked gas). That might be an interesting tack to take for a paper in any event-- consider some astronomical context where it appears that energy is not being conserved by the behavior witnessed, and then dig a little deeper to figure out where the energy really went to recover the concept of conservation of energy (again avoiding GR!).

D H
Staff Emeritus
Ken, the OP is a freshman.

Ken G
Gold Member
It's OK, the energy conserving phase of a supernova blast is nothing but the energy in an ideal gas, given the rate that new gas is piled up into the expanding bubble. I think they can handle that! They are in calculus-based physics, no need to baby them! They don't even need to consider how fast the bubble expands, they can just ask what the temperature inside is, relative to the amount of mass in there. In the "energy-conserving phase", the temperature is inversely proportional to the amount of gas piled up in the bubble, but after that phase is over (the "momentum-conserving phase"), the temperature is much lower than that (because of the light that has escaped from the bubble). That's all there is to it.

Last edited:
russ_watters
Mentor
Analyze a perpetual motion machine.

G01
Homework Helper
Gold Member
A physics class assigned a paper? One of the reasons I am doing physics is because I dont have to write papers!

Yes, because physicists never have to write papers in their jobs.

D H
Staff Emeritus