Energy Misdefined in Physics

reply to post by Arbitrageur


Does an instance of energy exist without it being momentum? Can momentum have zero energy associated with it?

I think the squaring has to do with the exponential nature, the thing that is often said about requiring an infinite amount of energy to accelerate a mass approaching light speed, its like a true zenos paradox of physics. The cases we are discussing that start as a rest mass and then accelerating to a velocity, is squared (if I now remotely grasp this at all) because the amount of energy needed to consistently accelerate is a curve and not an angled line?

But now I dont know about this, because in free space if you accelerate a rocket to 30 mph, an object in motion stays in motion and that momentum is conserved right? So you can turn off the propulsion/energy you are burning, and your velocity can remain the same, so now from a steady 30 mph, to get to 60 mph would its be the same as 0 to 30? But you are suggesting in a rocket in free space, that the energy used to bring a rocket to a perfect rest would be more then double the energy from 30 to 0, when bringing 60 to 0 to perfect rest? Equaling the notion of your car example and the subsequent need to square the velocity in the equation.

ImaFungi
reply to post by Arbitrageur

 

Does an instance of energy exist without it being momentum?

Gravitational potential energy is mgh. You could stop the roller coaster at its highest point with brakes, just before it makes the big drop. When it's stopped of course it has no momentum.

Can momentum have zero energy associated with it?

Only if you change reference frames to make the apparent motion go away in the new reference frame. The energy didn't go away, just your perception of it did. Example, energy from the train kills the person standing on tracks in front of it. Get on board the train, and in your new reference frame, you don't see the momentum and energy of the train the same way, as they now seem to be zero. Aside from changing reference frames, no.

But you are suggesting in a rocket in free space, that the energy used to bring a rocket to a perfect rest would be more then double the energy from 30 to 0, when bringing 60 to 0 to perfect rest? Equaling the notion of your car example and the subsequent need to square the velocity in the equation.

Have you tried doing the math? Here is a link to some rocket equations to get you started:

Basics of Space Flight: Rocket Propulsion
That link explains the math, and it also refers to sample problems here:

www.braeunig.us...

reply to post by Arbitrageur


Arbitrageur that basics of rocket propulsion page is really awesome! Thanks for that link

ErosA433
reply to post by Arbitrageur

 

Arbitrageur that basics of rocket propulsion page is really awesome! Thanks for that link

Yes thanks to the author, who made the site because he was researching the topic and everything he could find was too simple to be useful, or too complex for a non-expert to understand, so he feels the site fills a gap in the middle of providing information which is useful but not overly advanced, and it does.