Questions about the irrelevance of energy (ft-lbs)

[eric1115]

It's all good.

This is why these types of discussions are usually such crap-shows. It is also why KE is such a shit unit to use in the context of what we are trying to accomplish. KE doesn't actually do anything. It has to be converted to work in order to affect anything.

In response to your post, I would say that I think where we are getting crossed up is that acceleration is inversely propotional to mass. I think that is what you were trying to say in that last sentence that I highlighted, but I want to make sure.

Eh, sort of. I was saying that the rate of acceleration of the rifle compared to the bullet is directly proportional to the ratio of their masses. If the mass of the rifle is, say 500 times that of the bullet, it will accelerate at 1/500th the rate of the acceleration of the bullet. Ratio of masses and ratio of acceleration (and therefore velocity since the force acts on each for the same amount of time) being equal but inverse explains how/why conservation of momentum happens.

When you said that the bullet ends up with more momentum than the rifle, how did you arrive at that? Were you taking the mass of the shooter and/or friction with whatever the rifle is resting on into account? Free body diagram of rifle and bullet should give them equal momentum if we hand wave away the powder/gases and assume we're talking about a free recoil condition, and more momentum for the rifle if we assume powder/gases are moving in the same direction as the bullet.

As for interia, even if a force is applied, if the object doesn't move, no energy was transferred. It was still converted to force (i.e. "used up") but if the force applied doesn't impart motion it wasn't transfered. In the context of this discussion there are other forces that need to be overcome besides just inertia, but for the sake of making it easier for most to understand, interia is a good "catch all" term.

I think that's where some folks are getting hung up (and part of where I'm quibbling with you). Energy can be transferred in the form of heat/sound/light/etc even if the object doesn't move. Force that doesn't move an object is not too weak to overcome its inertia, it's just not enough to overcome the force of static friction. It's been a minute since I last did a FBD for anything other than making my kids' eyes glaze over, but a very small force on a very large mass just has small acceleration (not none). I realize I'm being a bit pedantic and hopefully not too assholeish about it, and partly just want to make sure I am not being one of those confidently wrong people. I never took anything above 200 level physics in college, and it's been a long damn time since I used it seriously so I could be due for a refresher.

As an aside, I'm with you (if I understand your position correctly) on KE being a completely useless metric for anything happening forward of the muzzle. I think it has value describing recoil characteristics though, since it gets transferred fairly consistently to the shooter's shoulder. So in some ways this really doesn't matter all that much other than to satisfy my own curiosity.

If that doesn't clear it up, I am happy to share my number via PM and am willing to get on a call to discuss this more dynamically.

I appreciate that, if we're still not connecting after this exchange I may take you up on it.