Hovertanks

[Lancefighter]

Its also worth noting that most of the time, the bullet penetrates the target, and keeps going and keeps most of its KE to itself.

thats what i said . . . . . . . .

The problem is this is completely irrelevant. Where the prop gets its air is of no consequence to where the turbine gets its air, and thats what you guys were arguing about.

lol I did say i was just arguing because im bored >.>

[3drts]

You weren't wrong on that point (bullet punches through, takes most of its KE along with it), you were wrong with the first word of your response to his question:

Yes

Your subsequent explanation was irrelevant, as if the gun recoiled with 7 joules of energy, that doesn't mean it only takes 7 joules of energy at the target to stop the bullet.
While specifics *parts* of your argument were correct, the position of your argument was untenable after your first word.

Isn't pointless arguing fun?

[Lancefighter]

RABBLE RABBLE!

the Yes was mostly because i didnt understand much how joules convert into kinetic energy >.>
Now that I wikipedia it... wait i was under the impression that the force exerted on the shooter was the exact same as the addition of all the kinetic force applied to what the bullet touches, adjusted for all the fancy words a bullet goes through..
Eh >.< nvm all this, bullets arent my line of work..

[3drts]

I'm surprised no one questioned me about how lasers produce recoil (even if it is ridiculously low).

Light is weird......
Relativity is weird.....

To a photon, it gets anywhere, instantly, due to time dilation. I've also heard it said that from the frame of reference of a photon, due to dilation as you approach the speed of light, to the photon the universe is a dimensionless point.

Going less abstract here:
momentum = mass* velocity.
Einstein showed up energy= mass *c^2, and that mass can be converted to energy, and vice versa.
Its what nukes do, and particle accelerators can take energy and turn it into mass (though given the small amount of mass converted to energy in a nuke, and the much lower power ratings of particle accelerators compared to nukes, the mass gained is only measurable at the atomic/subatomic level).
So then would energy traveling at high speed have momentum? based on the photon, it seems yes.
Its also been shown that photons can accelerate other objects, and exert a force upon striking.

Its been shown they behave like waves, and particles, but the same has been shown for electrons, which unequivocally have mass.
So what is the difference between a particle with only energy, and only mass?
Wtf is mass anyway? Is mass like water, and energy like steam or something? composed of the same building blocks?

Does a photon have mass like some version of the product of x*1/x, taking lim x->0 of x*1/x gives a value of one, when x=0, you get zero*infinity, and if you take the limit as the value of zero*infinity, you get one.
You could also get 2 if you take the limit of x* 1/2x - its still 0 * infinity at the limit, except now based on the limit, you sya the product is 2 - which is why you don't say the limit is equal to the product of the value at the limit - though this works for every case when you don't divide by zero, limits are the only way to "investigate" behavior of things at zero.
So does infinite speed* zero mass = a number greater than zero for momentum?

From the point of view of an observer going at light speed, they go arbitrarily fast.
Suppose you had a ship that could accelerate to 0.999999.... % the speed of light, and send it to a star millions of light years away, the journey takes a very short time for them, but millions of years to an observer on earth.
If you could accelerate arbitrarily close to light speed, in an arbitrarily short period of time, you could be anywhere in the universe in an arbitrarily short time, from your point of view.

One second you are on earth, observing an alien species that just discovered the radio, the next second, your ship is there, but they have evolved and advanced by millions of years, and laugh at your pathetic technology when you get there (it took you a second by your time to get here?! lol, we'd get there in 0.001 seconds, haha noob!)

Light speed is basically infinite speed(to someone on a space ship going at a high percent of the speed of light, they may perceive their acceleration rate as constant), and yet, due to relativity, not infinite at all, at the same time.

If you actually reach the speed of light, time stops for you.
But yet, you arrive at your destination (likely plowing right into the planet, as you'd have no way of decelerating if you actually reached the speed of light, and didnt just get very close - and the planet would be blown to smithereens).
How can you cover all that distance, without any time passing for you?
simple you have infinite velocity, where distance travelling is v*t, t = zero, v = infinity?
But there is also spatial dilation... if you reach the speed of light, from your point of view, all distances have reached zero, you got to your destination with no time for you passing, because to you, there was no distance to travel...
But if there is no distance to anywhere, are you everywhere at once from your frame of reference?
If so, why do you only impact into that planet/star, at a single place?

Relativity, light having momentum, etc, all make my head hurt.

But the simple, testable conclusions, are that light has momentum, and can exert a force.

Even at this level, it starts destroying classical things like F=MA
And it only gets worse from there...

[TVR]

... Lets be clear, 7 joules is not a valid recoil value.
Recoil may be measured in units of momentum, mass * velocity, such as kg*m/s ...

It is, I just realized the true purpose of v = (e/0.5m)^(0.5), and why the Wikipedia article on recoil gave that as the value.

... i was under the impression that the force exerted on the shooter was the exact same as the addition of all the kinetic force applied to what the bullet touches, adjusted for all the fancy words a bullet goes through.. ...

It is, since the launcher, projectile, and target are closed system as far as Newtonian mechanics are considered, the second law of thermodynamics means that it takes more work to fire that projectile, which means that launcher will have equal or more waste heat to deal with than the target.

Some of that energy can be converted to momentum, the upper limit being the recoil experienced by the shooter.

... To a photon, it gets anywhere, instantly, due to time dilation. I've also heard it said that from the frame of reference of a photon, due to dilation as you approach the speed of light, to the photon the universe is a dimensionless point. ...

Interestingly, a photon will never reach the speed of light in a vacuum, as a true vacuum would have to completely devoid of mass/energy, BUT that would mean that the speed of light in a vacuum would equal 0, as per c = (e/m) ^ (0.5).

[3drts]

... Lets be clear, 7 joules is not a valid recoil value.
Recoil may be measured in units of momentum, mass * velocity, such as kg*m/s ...

It is, I just realized the true purpose of v = (e/0.5m)^(0.5), and why the Wikipedia article on recoil gave that as the value.

Well.... if we want to quote wikipedia:

Recoil is explained by the law of conservation of momentum, and so it is easier to discuss it separately from energy.

If you want to say recoil is constant, ie a 105mm howitzer firing the same shell type, always has the same recoil, then we cannot talk in terms of energy, only momentum.

A howitzer mounted in bedrock, with no spring/recoil dampner, could then be said to have nearly zero recoil, because its velocity term is near zero (like 10^-20 or something), and thus the energy is nearly on the order of 10^-40 or something like that.
We would have to say the same gun has different recoil depending on if it is mounted on a tank, a ship, or an aircraft (such as an AC-130).
This seems counter intuitive, but if we speak in terms of momentum, the recoil is constant.

Recoil will result in the recoiling object having some KE, but that KE is dependent on the mass of the firing system, not the recoil of launching the projectile.

[Cyp]

...

Interestingly, a photon will never reach the speed of light in a vacuum, as a true vacuum would have to completely devoid of mass/energy, BUT that would mean that the speed of light in a vacuum would equal 0, as per c = (e/m) ^ (0.5).

E=mc² is only valid for things that aren't moving. If something is moving, then E²-p²c²=m²c⁴, where p is the momentum in the space directions, and E is the momentum in the time direction, and the factors of c are just to fix the units.

[TVR]

c = ((E²-p²c²)/m²) ^ (0.125) would still result in a c = (0/0), but it's not very important because there would be nothing in that universe to travel at the speed of light in a vacuum.

... If you want to say recoil is constant, ie a 105mm howitzer firing the same shell type ...

Remember, the mass of the launching platform is not a variable in the case of an M16, that 7 joule value can be converted back into Newtons granted the gas expansion rate and barrel size.

[3drts]

Remember, the mass of the launching platform is not a variable in the case of an M16

Yes it is.
Does the M-16 have a full magazine or is it firing the last round in the chamber.
Is there an M203 attached?
Is it currently loaded?
How heavy is the guy holding it
Has the stock been removed?
If the barrel is attached to something else (ie mounted, or held against a shoulder), then the masses are coupled, and the energy as a result of the recoil changes

that 7 joule value can be converted back into Newtons granted the gas expansion rate and barrel size.

Huh?

KE is just a bad way to measure the recoil.
Momentum is a good way to do so.

[KukY]

Let me contribute to this topic:

• Nerds

[JDW]

There is evidence that Einstein's way smarter than you imagined him to be...

[EvilGuru]

Going less abstract here:
momentum = mass* velocity.
Einstein showed up energy= mass *c^2, and that mass can be converted to energy, and vice versa.

Wrong. p = m*v is only valid when v << c; special relativity states p = gamma*m*v with gamma = 1/sqrt(1 - v^2/c^2). In the case of v << c we have gamma -> 1/sqrt(1 - 0) = 1 => p = m*v;

Energy in special relativity is E = sqrt(p^2*c^2 + m^2*c^4); if you're at rest then p = 0 and hence E = sqrt(m^2*c^4) = m*c^2. If you have no rest-mess then E = sqrt(p^2*c^2) = p*c; using E = h*f (Planck's equation) and c = f*l where f = frequency and l = wavelength we get: h*f = p*f*l => l = h / p which is the de Broglie relation.

Finally, binomial expand the above expression for E, ignore the constant term m*c^2 and you'll recover E = p^2/(2*m) = 1/2*m*v^2.

Its what nukes do, and particle accelerators can take energy and turn it into mass (though given the small amount of mass converted to energy in a nuke, and the much lower power ratings of particle accelerators compared to nukes, the mass gained is only measurable at the atomic/subatomic level).
So then would energy traveling at high speed have momentum? based on the photon, it seems yes.

Its also been shown that photons can accelerate other objects, and exert a force upon striking.

They have momentum. Indeed this is something of a problem for those designing satellites; those solar panels have a large enough area that the force impacting on them from the photons being emitted by the Sun is a real issue.

Its been shown they behave like waves, and particles, but the same has been shown for electrons, which unequivocally have mass.
So what is the difference between a particle with only energy, and only mass?

Actually, it has been shown that everything in the known universe can exhibit wave-like properties. This is known as wave-particle duality.

Wtf is mass anyway? Is mass like water, and energy like steam or something? composed of the same building blocks?

Mass is annoying. There are several reasons for this. Firstly, it is not one quantity but two: inertial mass which is responsible for inertia of objects (F = m*a; resistance to acceleration) and gravitational mass (F = G*m1*m2/r^2) which is responsible for the force experienced between two objects due to gravitational attraction. There is no reason that this should be the same. But they are; at least down to 10^-14. Secondly, it does not fit well with the idea of gauge invariance (broadly speaking: this is the existence of free parameters in Maxwell's equations) which states that things should have 0 mass. (The solution to reconcile these two ideas in the standard model is everyone favourite undiscovered particle: the Higgs boson which is responsible for giving objects mass.)

Does a photon have mass like some version of the product of x*1/x, taking lim x->0 of x*1/x gives a value of one, when x=0, you get zero*infinity, and if you take the limit as the value of zero*infinity, you get one.
You could also get 2 if you take the limit of x* 1/2x - its still 0 * infinity at the limit, except now based on the limit, you sya the product is 2 - which is why you don't say the limit is equal to the product of the value at the limit - though this works for every case when you don't divide by zero, limits are the only way to "investigate" behavior of things at zero.
So does infinite speed* zero mass = a number greater than zero for momentum?

Photons have zero rest mass and hence are able to travel at the speed of light, c. However they are affected by massive bodies, such as planets, in an effect known as gravitational lensing.

From the point of view of an observer going at light speed, they go arbitrarily fast.
Suppose you had a ship that could accelerate to 0.999999.... % the speed of light, and send it to a star millions of light years away, the journey takes a very short time for them, but millions of years to an observer on earth.
If you could accelerate arbitrarily close to light speed, in an arbitrarily short period of time, you could be anywhere in the universe in an arbitrarily short time, from your point of view.

You'd be dead. Anything more than 2-3g of acceleration would quickly take its toll. But yeah, if you could get to those speeds, that is what you'd observe.

One second you are on earth, observing an alien species that just discovered the radio, the next second, your ship is there, but they have evolved and advanced by millions of years, and laugh at your pathetic technology when you get there (it took you a second by your time to get here?! lol, we'd get there in 0.001 seconds, haha noob!)

Not unless they do not have to abide by the same laws as physics as us. If we were close to c then you wouldn't be able to get there much faster. It is a fundamental limit.

Relativity, light having momentum, etc, all make my head hurt.

[/quote]
Try doing general relativity (which is when you add acceleration into the mix); that makes your head hurt.

Polemically yours, Freddie.

[3drts]

Well, I study molecular biology, not physics....
So while I was good at calculus and can do derivatives and integrals and such, basic classical physics, when it comes to relativity and quantum mechanics/physics, My understanding is far far worse.

Actually, it has been shown that everything in the known universe can exhibit wave-like properties. This is known as wave-particle duality.

I know, but the wave properties of electrons are the most easily observed phenomenon.
It was the most unequivocal example of something with mass and momentum acting like a wave.

If both mass and energy(in this case, photons) act as particles and waves, and can be converted back and forth (seems going from mass to energy is easier than vice versa)
Then it seems to me that mass is just a specific form/organization of energy, and that energy itself is what has momentum.

So if light is affected by gravity, do photons generate their own gravity?

It seems they must for conservation of momentum to hold.
Its been established photons have momentum, if a photon is moved by gravity towards a gravity well, then its momentum has changed, and the gravity well must have a corresponding momentum change, ie a (very very very) small amount of movement towards the photon, as if pulled by the gravity of the photon.

So then it would seem gravity is a property of energy, not just energy in the form of mass.

* edit:
Did anyone account for this in trying to explain "dark matter", where based on orbits and such, they conclude there is a lot of "missing" matter, or matter we can't see that makes things orbit the way they do?

[TVR]

... If the barrel is attached to something else (ie mounted, or held against a shoulder), then the masses are coupled, and the energy as a result of the recoil changes ...

This is only the case when the objects are rigid, and in that case, we would also need to apply elastic collision between the gas, rifle, and bullet, instead of 'waste heat'.

... So then it would seem gravity is a property of energy, not just energy in the form of mass. ...

The entire point of mass-energy equivalence is that the only difference between them is radius as a fraction of c

... Did anyone account for this in trying to explain "dark matter" ...

There's not enough computational power to calculate the gravitational (space-time) distortion of all the mass and energy in this galaxy, and from other galaxies.

[EvilGuru]

So if light is affected by gravity, do photons generate their own gravity?

It seems they must for conservation of momentum to hold.
Its been established photons have momentum, if a photon is moved by gravity towards a gravity well, then its momentum has changed, and the gravity well must have a corresponding momentum change, ie a (very very very) small amount of movement towards the photon, as if pulled by the gravity of the photon.

So then it would seem gravity is a property of energy, not just energy in the form of mass.

Ever see the Matrix? There is a scene where a small boy, who is bending a spoon, looks up at Neo and says "there is no spoon".

The effect is known as gravitational lensing (and is actually a prediction of general relativity) that occurs because massive objects warp space time. It is not the photons which bend, but space.

Polemically yours, Freddie.