# Power Over An Object's Density



## Addison (Feb 1, 2017)

Science wasn't my best class in school, nor was it the worst. It was in that annoying sweet spot where it wouldn't go above or below a "B". So I'm not so sure that I'm describing a character's power, and the science behind it, correctly. 

The character, as currently described, has the power to manipulate an object's density. The first hint to this is when he discovers his powers. The hydraulic lift holding up his truck fails and the truck nearly falls on his friend's head. But by touching it the car doesn't drop like a steel ton. It falls slowly like a balloon. 

Later, when he and his friend are at a shooting range to test the theory. (The story's set in Montana), they mess with the targets and bullets by increasing and decreasing the density of both. Higher density to a bullet and it hits the ground before it reaches the target. Lighter density and it hits the target but barely breaks the surface. Higher density to the target and a hunting slug ricochets off. Lighter density and the same slug blows it up like C4. Finally the character gives himself a lighter density and jumps, reaching the top of a cliff sixty feet above. He does it again and can basically glide through the air. 

So, would that be changing the density? Or mass? Volume? What? Where's Sheldon Cooper when you need him?


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## DragonOfTheAerie (Feb 1, 2017)

Addison said:


> Science wasn't my best class in school, nor was it the worst. It was in that annoying sweet spot where it wouldn't go above or below a "B". So I'm not so sure that I'm describing a character's power, and the science behind it, correctly.
> 
> The character, as currently described, has the power to manipulate an object's density. The first hint to this is when he discovers his powers. The hydraulic lift holding up his truck fails and the truck nearly falls on his friend's head. But by touching it the car doesn't drop like a steel ton. It falls slowly like a balloon.
> 
> ...



M/V=density (I think), so by affecting an objects density, you are affecting its mass and volume. I think. 

Honestly, it's probably best to just work out the effects of powers rather than their definition in physics. I've done things like this a lot for one of my WIP's (centering around superpowers.) Things stop making sense quickly when you try to explain things scientifically. I gave up on it. 

Your best bet is to say the character can control density and don't even go into all the implications. You are breaking very fundamental laws of reality here. That has consequences.

Though you might want to consider how overpowered this could make your character. What happens if he drastically decreased the density of someone's skull cavity or internal organs? This character could fly, walk on water, kill anyone he wants, become invulnerable to bullets by decreasing their density drastically...you've got to give him huge handicaps to prevent him from becoming overpowered. Basically all powers that manipulate basic physics and properties of substances become super overpowered, fast.


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## TheCrystallineEntity (Feb 1, 2017)

^It's like trying to work out how many stomachs a centaur has, or how a dragon's fire works.


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## Penpilot (Feb 1, 2017)

As DOTA said the formula for Density is D = M/V so if you change density you either change Mass or Volume or both. But from your description, nothing changes size, so it seems that you're changing mass, which in turn changes density. 

I'm not an expert, but there are a few things I see that can be problems if you try to science this out too much. If you alter density, you're making things more fragile or more sturdy. And this can have significant consequences if it's done to a living thing. A heart can tear itself apart by simply beating. Bones will crush under the weight of air. Muscles can't contract because their composition is to inflexible. Lungs can't function because its tissue is too tightly packed to absorb oxygen. 

So yeah, don't go too far down the rabbit hole. Lots of poop you can step in.


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## Russ (Feb 1, 2017)

I think PP is dead on, don't overthink the science.

Also, if you do, why does the low density target explode like C4?  I have shot lots of low density things and don't remember them exploding.

The physics (if you care) behind bullets are pretty well known.  I think if you changed the density of a bullet a great deal it would be very hard to hit the target.  Of course I have never actually tried it...

By the by does the character have to touch the object to change its density?


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## Addison (Feb 1, 2017)

Russ said:


> I think PP is dead on, don't overthink the science.....
> 
> By the by does the character have to touch the object to change its density?



No overthinking the science, got it. And at first, yes the character does need to touch the object. But as he grows stronger and more powerful, he won't have to if they're within a certain range.


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## psychotick (Feb 2, 2017)

Hi,

There's a few issues with your bullet idea. First changing the density won't affect the bullets flight path in the way you imagine. In our gravitational field a bullet falls at an acceleration of 9.8 m/s/s regardless of it's density. What does change is the momentum. This means that the bullet will still take much the same time to hit the ground say 1/10th of a second if your firing it only a metre or two above the ground, but because it has less mass and hence momentum, friction will overcome it's velocity much more quickly. Your bullet might not even reach it's target. (Note if you're firing the bullet at a much greater height, then yes the less dense bullet will fall more slowly because friction interfers with its falling velocity.)

Next, when the bullet reaches its target, assuming it does, it's impact will be far less, and it might just bounce off.

After that assuming you've got the bullet etc maintaining the same size, then its mass must have decreased. That's essentially the stuff it's made of. So where did it go? Would a less dense bullet retain the same tensile strength as the original? No because there's less mass - ie less atoms and molecules etc, providing the strong and weak nuclear forces that hold it together. So it might well turn to powder in the gun barrel when it gets hit by the gun powder explosion. Think of your metal bullet suddenly being replaced by one made of pumice and you'll get the idea.

As others have said, good idea to leave the science out of it. Alternatively you could instead make the power not to decrease density but rather the effect that gravity has on the mass. So the bullet maintains the same size and mass, same momentum to when fired, and hits just as hard, but isn't pulled down to the ground as quickly.

Cheers, Greg.


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## CupofJoe (Feb 2, 2017)

If I had to explain it, I'd be tempted to make a [deliberately] vague comment about warping space-time/reality and how the usual laws of physics don't apply... And then wander away whistling hoping know one notices too much.


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## Vaporo (Feb 2, 2017)

psychotick said:


> After that assuming you've got the bullet etc maintaining the same size, then its mass must have decreased. That's essentially the stuff it's made of. So where did it go? Would a less dense bullet retain the same tensile strength as the original? No because there's less mass - ie less atoms and molecules etc, providing the strong and weak nuclear forces that hold it together. So it might well turn to powder in the gun barrel when it gets hit by the gun powder explosion. Think of your metal bullet suddenly being replaced by one made of pumice and you'll get the idea.



Not necessarily. If the character's power changed the mass of each individual atom without affecting the number of atoms present, the material would be just as strong as normal. And it's not strong and weak nuclear forces that hold physical objects together. Chemical bonds do that.

Changing the mass of the bullets actually would affect their trajectory, just not for reasons that you might think. If your character were to change a bullet to have zero density, it would get shot out of the barrel much faster than normal, then stop a few inches away and begin floating upward like a very aerodynamic helium balloon.

If you're character made their bullets heavier, they'd leave the gun moving slower, and so may fall short of their target if they don't have enough speed. However, they would also hit a lot harder.

If he made them even heavier, the bullets might not make it out of the barrel, which could be very bad for you character.

What are the limits of this power? Can he keep increasing density until the object begins generating its own gravity field and turns into a black hole? Or will he eventually hit a limit?

Is the mass change permanent? If it is, the guy could probably make quite a lot of money creating massless steel for skyscrapers and... pretty much anything else that would work better if it were made of a lighter material. If he played his cards right, he could easily become the most politically and economically powerful man in the world, simply by picking who he gives his services to.


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## psychotick (Feb 2, 2017)

Hi Vaporo,

Forgot that chemical bonds are based on the electromagnetic field - so good catch. And I was also assuming that the bullet left the gun at the same speed, but yes in the actual world increasing the mass will decrease the velocity of the bullet imparted by the energy of the propellant explosion.

However the problem remains. Hadn't considered the possibility of getting the bullet to complete masslessness. Not even sure a massless bullet would do anything once it left the gun barrel. It's momentum after all is zero so the only impetus it would have would be the force provided by the expellation of the combustion gasses. But more puzzling still is the question what exactly is a massless object? What does it consist of? Or is this simply an oxymoron?

As for making the bullet heavier yes - make it too dense and you risk the gun exploding in the shooter's face.

Cheers, Greg.


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## Vaporo (Feb 4, 2017)

psychotick said:


> However the problem remains. Hadn't considered the possibility of getting the bullet to complete masslessness. Not even sure a massless bullet would do anything once it left the gun barrel. It's momentum after all is zero so the only impetus it would have would be the force provided by the expellation of the combustion gasses. But more puzzling still is the question what exactly is a massless object? What does it consist of? Or is this simply an oxymoron?



Well, it is fantasy. I'd have no problem accepting an object with size and structural integrity, but no noticeable mass. I'd just assume it was made of a magical force field that looked and behaved exactly like the object it was made from from, minus the mass.

A massless bullet would be exactly as useless as you think. It would be lighter than air, so it would float upward. More importantly, for bullet physics it would be completely unable to penetrate the atmosphere with its own momentum as it would have no momentum of its own. If it collided with even one atom in empty space, it would stop and... Actually, I'm not sure what it would do. With a truly massless object, it kind of becomes a divide by zero problem. I suppose you could treat the mass as existing, but being so tiny as to be insignificant for any situation.

Even doing that, I guess there are still problems with a truly massless object. Radiation pressure is a tiny thrust effect produced any time light is reflected off of an object. It would act as a constant force pushing against the object regardless of its mass, and a finite force pushing against an object with zero mass means the object's acceleration is infinity (technically a divide by zero error, but whatever). So, it would instantly accelerate to the speed of light.

But, if it collided with any object when at the speed of light, it would stop. Although because of relativity, it would be impossible for the whole object to feel the collision instantly, so it would fragment, and... This is getting into physics I'm not entirely familiar with, and I could talk all day about it.

The way I would fix the issue just be to put a limitation on the character's power. Say, he can only reduce the mass of an object to 1/1000000000 of the density of air. That way you never quite get a divide by zero error, but for most situations your character is likely to encounter, you can treat things like they have zero mass.

Yeah, trying to figure out physics in a fantasy world can be problematic. Sooner or later, you find you're breaking some law or another. The best you can do is try to make things behave as realistically as you can in the situations your characters encounter, but try to avoid the situations where your assumptions break down. The problem people like me have with this kind of thing isn't usually the implications it would have for theoretical physics, it's when the author puts their magic in a situation where it should have a specific interaction with real physics (and they haven't defined anything about their magic that could nullify those physics), but they don't think it through or don't do their research and they end up with bad physics in their book.


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## K.S. Crooks (Feb 5, 2017)

I would echo the idea to not over-think the science of how it happens and focus on the effects. Make the effects realistic and within acceptable bounds, consistent for your story. For example if you alter the density of a bullet when this happens matters. If you increase the density as it is fired then it will fall short of the target, but is density increase a fraction of a second before hitting, then it will do more damage. Same with decreasing the density- early and the bullet may never reach the target (like trying to throw tissue paper), late and less damage is done.

Is there a limit to the character's power? Can he alter an object or clothing to allow him to fly? How much of a change can he accomplish?


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## Vaporo (Feb 5, 2017)

The other half of the question regarding the targets wasn't talked about much, so I guess I'll talk about that as well.

If the target is fixed to the ground, changing the density of the target would have very little effect on what happens when a bullet hits it, since you're not actually changing the structural integrity of the target. Technically, yes. The bullet would have a harder time penetrating denser material, as it has to move more mass out of the way. However, if you're using a regular wooden or cardboard target, increasing the density enough so that the bullet would bounce off would probably cause the target to collapse to the ground under its own weight. The idea that making something denser always makes it stronger is a common misconception because, in real life, it's often true. However, that's not always the case. Try thinking about the difference between lead and aluminum. Lead is extremely dense, but bends easily. Aluminum is fairly light, but is very strong.

In fact, increasing density would be what your character would do if he wanted to destroy things. Get put in jail? Make the walls so dense that they can't support themselves. Angry assassin coming at you with a knife? Make his body liquefy under its own weight (or make him so light that he floats upward like a helium balloon, if you want to be a bit less deadly). This ability really is very, very powerful, when you think about it.

Theoretically, your main character could briefly (as in, less than a few milliseconds) increase the density of his body so that he would be bulletproof (or at least partially bullet-resistant). However, if he did this for any meaningful length of time, his body wouldn't be able to hold up its own weight, so he would be crushed if he didn't immediately revert to normal. It would be far and easier safer to simply make all of the incoming bullets weightless before they ever left the gun. 

As a little aside, unless your character has superhuman reflexes, it would be impossible for him to stop bullets in midair unless he knew exactly when it was going to be fired. Most people know this, but somehow people in movies and books are still always able react to a bullet after it's fired. _However_, your character could believably train himself to affect his own bullets in midair. He knows exactly when the bullet is fired, and has a few moments to plan out the action before he actually does it, so he could train himself to fire a bullet and immediately change it's mass after it leaves the barrel. It would probably take a lot of practice, but I could believe it happening. 

Your character changing the density of his own body too much would probably be bad. If he became too dense, he wouldn't be able to stand, and his heart would no longer be able to pump blood. I'm also pretty sure that there are chemical reactions in the body that rely on some elements having a particular density, so becoming too light could probably make him extremely sick if he did it for too long. This is a bit of a fuzzy area though, so you could probably get away with a few things here.


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