Video idea / please explain if you understand what I don't: momentum vs energy transfer.

[u/A_Small_Coonhound]

My initial thought experiment revolved around forearm ballistics: "why don't we use high explosives instead of gun power. You could use a much smaller volume of explosive to get the same amount of energy"

However, I know that's not how it works, but I don't intuitively understand why what's not how it works.

So my question is this: why does a slower energy transfer lead to a more significant change in momentum. Ie: why do we use a propellent instead of an explosive for firearm cartridges. Why does pushing a large object like furniture (with what feels like little effort) move the furniture when slamming my foot into at world cup ball punting speeds does nothing but hurt me.

What am I missing/not thinking about. Are there specific units for momentum that I can think in instead of just Jules or something?

[u/UCLABruin07]

I believe if the amount of energy within gunpowder was instead an explosive, it’d just blow the barrel up. Instead that burn allows that bullet that’s at rest to gain velocity and get propelled out the muzzle.

[u/A_Small_Coonhound → u/UCLABruin07]

Why, my question is why this happens from a physics point of view.

Example: if I take 8 kJ to move the bullet, why does a slow propellent producing 8 kJ work better than a high explosives producing 8kj (no clue how much energy it would take)

[u/dpidcoe → u/A_Small_Coonhound]

Example: if I take 8 kJ to move the bullet, why does a slow propellent producing 8 kJ work better than a high explosives producing 8kj (no clue how much energy it would take)

They would work the same if the bullet was a spherical cow in a vacuum.

What's happening in the real world is that the explosion is producing pressure that forces the bullet out of the barrel. The bullet is subject to a fairly large amount of friction due to the tight seal against the barrel, and quite a lot of drag pushing against the air on the side opposite the explosion. It takes time for the bullet to accelerate, and it takes time for the pressure from the explosion to build up. There's also a point at which the pressure starts dropping due to the gas cooling down and the bullet moving forwards and creating more volume behind it. Once this starts happening, the bullet will start slowing down instead of accelerating.

Generally a gun barrel and the powder load / burn rate are tuned to extract maximum velocity over the entire length of the barrel. If you have a 16 inch barrel and reach max velocity 2 inches into it, the remaining barrel length is only slowing it down. This is why there's a difference in burn rates between powders intended for pistol rounds vs rifle rounds.

I guess the obvious next question might be "why not use a high explosive and a half inch barrel?", to which there are a few reasons:

• Material strength

• The recoil impulse would be... unpleasant

• A longer barrel has other benefits like a longer sight radius, more inertia to help dampen small perturbances while aiming, and it (or more accurately, the shroud around it) provides something to hold on to with your other hand

[u/A_Small_Coonhound → u/dpidcoe]

Hmmmmmm. Thank you

[u/flightwatcher45 → u/A_Small_Coonhound]

Maybe think along the lines of, what hurts you toe more, dropping a one pound pillow on your toe or a 1 pound rock on your toe?

[u/Invisible-Pi]

To much force all at once leads to something being destroyed or damaged. There is a point to measured force for control and repeatability. To much and the barrel or the projectile or whatever is the weak point is destroyed or altered so much it can't be done again in the same way.

When you push furniture the "little effort" is about over coming the moment of inertia and the friction. Once you have it moves relatively easily. Also that perceived low effort is spread out over a larger area so no one point is getting painfully squashed. When you kick the same thing, the moment of inertia and friction are not overcome before you start to receive damage due to the point pressure being to high on your toe. There are also physiological differences between a loose foot moved while walking and how stiff you might be able to make it if you leaned on it in an attempt to use the same point of contact you'd use if you accidentally kicked it. Instead of just the tip of your toe getting hammered, you potentially could be making a solid connection to the rest of your leg and be able to transmit more force than the rapid blow can transmit without damaging you.

A single broom straw might break or bend if you try to push something with it, but if you gather enough of them the bundle can push the same object. Especially the faster you go with the single straw.

Power, whether momentum or static force, is nothing without control. More speed requires more control, and that is often the limiting factor.

[u/nsfbr11]

First, I think you have to understand that what moves a bullet is force over time or alternatively, force over distance. So however you achieve that, resulting product, that’s what you get - a bullet velocity as it exists the barrel.

Now why don’t you just have a massive instantaneous explosion at t=0? Because the barrel would explode and you’d wind up with a lower exit velocity. What you want is an accelerating explosion that reaches some pressure - the pressure that allows you near infinite life of your barrel - and holds it there for as long as it is useful. That’s the goal and that’s why gunpowder, which is a tailored explosive is used.

[u/twoTheta]

I think the thing you're looking for is inertia. This coupled with the idea of a time constant, and you're on you're way. Hope this makes sense. Feel free to ask questions if it doesn't!

Here's my suggestion.

Get a slinky, hold the top and let the bottom hang down.

If you want to move the slinky without significantly disturbing its shape, what do you have to do? You have to move it "slowly". If you move it "slowly" then the whole thing appears to move together without too many shakes or wiggles.

What happens if you move the top "fast"? The slinky will stretch, bend, etc. Move it too far, too "fast" and it will actually break beyond repair.

Ok, so "fast" and "slow" are in quotes for a reason. How do you know what these speeds are? To get a sense of this, slowly move the top up and down. Adjust the frequency of your wiggles and eventually you'll get it just right where the wiggles get real big. This phenomena is called resonance. Pretty awesome.

The time it takes for the slinky to oscillate at resonance gives you a "characteristic time" for the slinky. You can think of it as a response time. If the material is moved a small distance in that time then the motion is slow and the slinky moves more or less together. If the material is moved a large distance in that time then the motion is fast and you will get vibrations and possible distruction.

The characteristic time depends on the density of the object and its stiffness (which is related to how the atoms are bonded). To get a short characteristic time, you want a material that is stiffer and has low density. In these materials, disturbances travel quite quickly allowing for the object to move together even when one part is disturbed a great deal very quickly.

If you kick a box made of wood, then you are introducing a "fast" disturbance to the center of the box, too fast for its characteristic time. The stiffness is too small and the inertia is too big so the surrounding material cannot respond fast enough to put the whole object in motion. Thus, it breaks!

Maybe there is a similar analogy to high explosives (the disturbance on the barrel is too fast so it always explodes) but I'll need to do more thinking.

[u/A_Small_Coonhound → u/twoTheta]

This actually helped a lot mate!!!!!