# Any physics people here?

Discussion in 'THREAD ARCHIVES' started by Ruko, Feb 11, 2015.

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1. I have no idea how to physics so I decided to ask some people.

I want to know what the effects of a very high speed pebble would be.
Can someone explain what would happen if a sphere of density 2.5 g/cm3 and radius 1.5cm moving at 700 m/s were to collide with
A: a stationary wall of insect carapace 3 cm thick?
B: a stationary wall of mammal flesh 10 cm thick?
C: a stationary wall of steel 2 cm thick?

#1
• It would shatter the insect carapace most likely. Unless its some ridiculous chitin from a space bug, it would still likely shatter it.
• Penetration and likely an exit wound.
• The projectile would probably be flattened upon impact.
What it seemed like you described is an aluminum musket ball fired just below normal speed for a modern day bullet.

The carapace would be easily shattered in most all circumstances. If we had more information about the size of the carapace target we could make better guesses about it, but in all likelihood it wouldn't provide much defense.

Google bullet wounds and you'll understand the second one better.

Same thing, google shooting a steel plate. Even high powered rifles aren't very good at penetrating steel plating. Unless said projectile is designed for it.

This better not be some anime physics shit.

#2
• Thank x 1
2. If it were anime physics I wouldn't need to ask would I? If it were anime physics it'd probably explode on impact or something ridiculous like that. How fast would it need to be going to penetrate steel then?

#3
3. That's a whole other can of worms.

It all comes down to speed, velocity, angle, and hardness of the steel.

Steel comes in several strengths - soft, mild, medium, and hard depending on the Rockwell Hardness of each different plate's alloy. Sometimes the bullet will not pierce the steel plate - but - the plate is so hard it shatters like glass when fired straight on - but - has no effect at a slight angle.

Too many variables to narrow it down. Essentially, a pebble would have to have such an extreme speed to penetrate that friction simply won't allow it.

#4
4. Considering the RP I need the information for has magic, air resistance isn't an issue.

#5
5. That all depends on how realistic you wanted to keep your RP, which seems you do since you were very specific with your density, size, and speed of your chosen projectile. Not to mention the thicknesses of the targets.

Just bullshit it then if physics don't matter.

#6
6. Well even though I don't like physics, I like to make any magic I use have some kind of scientific basis, so magic is only being used to cause the effect in the first place (IE, in this case it's gravity manipulation and density in relation to another object manipulation, and while I don't know much about physics I figured that making the pebble near infinitely dense in comparison to the target should increase it's weight enough to generate a very strong gravitational force between the two, and making the air have 0 density in relation to the pebble would mean air resistance didn't affect it.)

#7
7. So... You asked for help with regards to physics and then bullshit it away. Right.

• Making anything infinitely dense would be catastrophic. The pebble would implode in on itself and then likely explode, which would be rather comical as a fantasy hand grenade. But dropping it could also possibly make it fall straight through anything it touches.
• That pebble is likely already more dense than the target.
• If the air had zero density that would cause a vacuum, which could cause an extremely loud bang akin to a sonic boom and would generate (possibly) gale force winds as the surrounding air tries to fill in the vacuum you created.
If you don't understand what you're doing and can't write your way creatively around it, it's probably a better idea not to bother with it.

Or..

Just shoot a fireball at the bastard and call it a day.

#8
Last edited: Feb 11, 2015
8. I asked for help in regards to physics BECAUSE I wasn't sure what I was talking about. If I thought I was right I wouldn't be asking. I already knew making something infinitely dense would cause problems, which is why I was thinking that making it extremely dense IN RELATION to another object might be different. In relation to the rest of the world and itself it's still density 2.5, but to the creature, it feels a massive pulling force generated by the pebble and flies towards it. (I just realised in this case i probably mean make the creature very dense in relation so that it was the creature's massive gravity field rather than the pebble's). I also already knew that a pebble is likely more dense than a living object because living objects are primarily water which I'm pretty sure is density 1 g/cm3 . And the pebble itself accelerating to 700 m/s would cause a sonic boom in itself, wouldn't it?

#9
9. Uh, well. In the cases you mentioned, gravitational force exerted on one body by another isn't really anything to do with their relative densities/masses. Like, let's say a cubic metre of air is way less dense and has a way lower mass than a cubic metre of steel, but the gravitational force exerted by that block of steel on that block of air is still going to be almost negligible even in a vacuum (never mind on a planet or something where the gravitational effect of the planet would render the effect completely unnoticeable.) The thing is, gravity is really, really weak. You need ridiculously large masses for it to have any notable effect.
The other thing is that I'm not sure what you're intending to do with this considering you're referring to a target... If you increased the mass of the pebble to the point at which its gravitational effect was noticeable, it would effect everything around it, not just a target.
As for the air resistance stuff, there are similar sorts of issues. By the time you increased the mass enough for it to make that much of a difference, you'd probably be facing other problems.

My advice is: rather than try to numerically calculate what would happen in each situation, just give a nod to the concepts. This can just be as basic as acknowledging that to punch through steel you'd need it to have a much higher mass and speed than to punch through tissue paper. Just by that basic bit of logic, you're backing it with science. Don't worry about making it too realistic in terms of number-crunching, nobody cares. What's more important is to factor in scientific ideas, and for that the best thing to do is look into it yourself (e.g. looking up the equation for gravitational force and thinking about what you'd need to change to produce the effect you want.)

#10
• Thank x 2
10. THANK YOU, @Halo

You're making it way more complicated than it needs to be. If it's for an RP I doubt anyone's going to bother checking your numbers and doing the crunching rather than going "Heeeey... That pebble doesn't have nearly enough massity and denseness to cause harm.."

Besides, if you can control gravity in any sense of the word. Just create a tiny column of it being super dense on the person and watch them die instantaneously.

#11
11. Having just read the post made while I was writing my own - if you're planning to make it so that you're changing the pebble's gravitational influence only compared to one target, well, you're already bullshitting through so much physics that it doesn't matter anyway. That isn't a criticism, I literally said previously it doesn't matter for shit xD.

But yeah, if you're gonna toss entire scientific concepts (like air resistance being a thing, or gravity being a field that effects all objects) out of the window I don't know why you're worried about whether the numerical aspects work out in each specific scenario.

#12
• Like x 1
12. Yeah, I know no one cares about numbers. Neither do I really, but I thought the physicists might find them helpful. What I was really wanting to know was whether it'd create a small hole and penetrate straight through or whether it'd create some kind of impact crater.

#13
13. It would just create a small hole through the person. Only likely fatal if it hits something major, otherwise just really painful.

Bullets are specifically designed to break apart and cause massive internal damage.

Musket balls and other projectiles like that don't.

#14
14. This is the point I'm making - you don't know what it will do in each specific scenario until you plug the numbers into the relevant model of equations. One pebble might go straight through, one might impact but not penetrate, one may burst apart on impact - it all depends on the specifics of the scenario.

What's more important is to consider the scientific concepts as a whole. But as it is, I mean... you're ignoring the nature of gravity and air resistance, but still want to know if it's reasonable for a pebble to go through some steel. Like, if you're tossing out those fundamental concepts from the start, you may as well also toss out the "realism" of what the pebble will do. It seems very odd to be so selective in which concepts you want to uphold - it's already ridiculously unscientific, so why worry about the science of object penetration?

You're writing. You can have it do whatever you want it to, regardless of whether that's what'd actually happen if you fired a real pebble at that speed at a real steel plate. ^^

#15
15.
#16
• You Get a Cookie x 2

#17
17. What? Must admit, I'm somewhat confused.

I meant "you are writing". As in, the activity they are doing is writing, not physics. When you are writing, you can make whatever you want happen in the story, regardless of whether it's scientific, particularly if magic is involved.

#18
• Thank x 1
18. The spelling of 'your' and 'you're.

Was confused as well since I normally see it written as "It's your writing." or "You are writing this."

All cleared up~!

#19
19. #20
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