Ask any question you want about Physics

originally posted by: eManym
What force is being exerted on the vehicle body to make it move?

Yes you did sort of answer your own question. That's actually in interesting and relevant question which has been brought up recently in the context of EM drives, where the thrust is provided by photons, which have no rest mass therefore there is no "reaction mass". There is apparently some question about whether existing theory can adequately explain some recent experimental results in that regard, so it should be interesting to watch the developments on that topic to see the ultimate resolution.

But in a more conventional rocket design, you have a barely controlled explosion. In an explosion without any constraints the particles may tend to go flying in all directions, but with the rocket nozzle, the "exploding" particles moving toward the rocket push against the rocket moving it forward, and the rocket pushes back, and ultimately all the particles are forced out the back of the nozzle as there is no other direction in which they can expand, and this reaction mass ejected at high velocity allows the rocket to move forward.

This is the best site on the internet regarding rocket technology, for non-rocket scientists (I saved it for offline-browsing in case it ever goes offline, as there is really no good alternative that I've found):

Rocket and Space Technology

Isaac Newton stated in his third law of motion that "for every action there is an equal and opposite reaction." It is upon this principle that a rocket operates. Propellants are combined in a combustion chamber where they chemically react to form hot gases which are then accelerated and ejected at high velocity through a nozzle, thereby imparting momentum to the engine. The thrust force of a rocket motor is the reaction experienced by the motor structure due to ejection of the high velocity matter. This is the same phenomenon which pushes a garden hose backward as water flows from the nozzle, or makes a gun recoil when fired.

a combustion chamber with an opening, the nozzle, through which gas can escape. The pressure distribution within the chamber is asymmetric; that is, inside the chamber the pressure varies little, but near the nozzle it decreases somewhat. The force due to gas pressure on the bottom of the chamber is not compensated for from the outside. The resultant force F due to the internal and external pressure difference, the thrust, is opposite to the direction of the gas jet. It pushes the chamber upwards.

To create high speed exhaust gases, the necessary high temperatures and pressures of combustion are obtained by using a very energetic fuel and by having the molecular weight of the exhaust gases as low as possible. It is also necessary to reduce the pressure of the gas as much as possible inside the nozzle by creating a large section ratio. The section ratio, or expansion ratio, is defined as the area of the exit Ae divided by the area of the throat At.

The thrust F is the resultant of the forces due to the pressures exerted on the inner and outer walls by the combustion gases and the surrounding atmosphere, taking the boundary between the inner and outer surfaces as the cross section of the exit of the nozzle. As we shall see in the next section, applying the principle of the conservation of momentum gives

where q is the rate of the ejected mass flow, Pa the pressure of the ambient atmosphere, Pe the pressure of the exhaust gases and Ve their ejection speed. Thrust is specified either at sea level or in a vacuum.

originally posted by: eManym
They hit the Moon's surface at the same time. I say that objects don't fall at the same speed.

Not sure how it helps you to cite an experiment which contradicts your model?

If the force of gravity is greater than the force that is holding a mass together than the mass will have a slower fall rate. In a very high force of gravity environment masses will fall at a slower rate than masses in a lower gravity environment.

I don't follow that at all. Is there any real world example of what you're talking about? You already mentioned the moon, and compared to the Earth, it's lower gravity and we can see things fall slower on the moon which again contradicts your hypothesis.

The Earth doesn't have a gravity field that exceeds the force that holds a mass together i.e. the binding force. I am talking about a force of gravity that is greater than the binding force holding the mass together.

a reply to: eManym
I repeat: "Is there any real world example of what you're talking about?"
There is "spaghettification" of objects falling into a black hole, but they would only appear to fall slowly to an external observer due to time dilation. If you were falling into a black hole, you would see yourself falling very fast, until you were killed by spaghettification from the immense gravity.

near a black hole (assuming that there is no nearby matter), objects would actually be destroyed and people killed by the tidal forces, because there is no radiation. Moreover, a black hole has no surface to stop a fall. As an object falls into a black hole, the tidal forces increase to infinity, so nothing can resist them. Thus, the infalling object is stretched into a thin strip of matter. Close to the singularity, the tidal forces even tear apart molecules.

Nothing of that magnitude but cool, thanks for the input. I was checking into the math, but its late. Thanks.

A celestial body with the diameter of the earth with about the mass of the sun would be enough to produce the effect.

originally posted by: Arbitrageur
a reply to: eManym
I repeat: "Is there any real world example of what you're talking about?"
There is "spaghettification" of objects falling into a black hole, but they would only appear to fall slowly to an external observer due to time dilation. If you were falling into a black hole, you would see yourself falling very fast, until you were killed by spaghettification from the immense gravity.

Not necessarily true. You will actually see yourself falling extremely slowly as time ticks very very fast there, whereas to an external observer would appear to fall very fast.

originally posted by: eManym
The Earth doesn't have a gravity field that exceeds the force that holds a mass together i.e. the binding force. I am talking about a force of gravity that is greater than the binding force holding the mass together.

I wouldn't use the term "binding force" here, that's generally reserved for the interatomic forces holding an atom together.

Maybe "tensile strength" is more like what you want.

In a simple situation where you have two masses not rotating about each other, the gravitic force of each body on the other tends to accelerate the thing in one piece. If you get out into weird cases, you can have a lot more force on the inner edges than the outer ones, and the gradient can break up one or both bodies if their tensile strength is insufficient.

In a more practical problem, you will get this faster due to tidal forces if the two bodies are in some sort of orbit around each other, or one is moving past the other at a reasonable speed.

Either way, the point at which the material a body is made of failing and the object coming apart due to gravitic gradient or tidal forces is called Roche's limit.

So, what is gravity interacting with? If its not the binding force between nucleons. I was under the impression that the gravity interaction to a mass was down to its smallest constituent.

I may be delving into unknown territory. If so, then that it is.

a reply to: eManym
All mass interacts with all other mass via gravitational interaction.
The reason it usually doesn't rip molecules apart, and may not rip subatomic bonds apart is because it is such a weak force compared to those other forces.

Electromagnetism (that holds molecules together) is 1000000000000000000000000000000000000 times stronger than gravity.

The strong nuclear force is 100000000000000000000000000000000000000 times stronger than gravity.

Source: Fundamental interaction

Those are huge numbers with lots of zeroes (36 and 38 respectively if I didn't make a typo). Gravity just can't compete with the other forces on small scales. It pulls on every component of atoms and molecules but it can't pull them apart, with the possible exception of right next to the black hole singularity if there is such a thing.

The reason gravity can pull objects apart inside the Roche limit, is that gravity is not competing with other forces like electromagnetism or strong nuclear in that case, it's gravity between objects competing with gravity that holds each object together, or in other words, gravity versus gravity, so you don't have those huge numbers of force strength difference.

a reply to: Arbitrageur

You confused me with the gravity vs gravity thing at the end. When something crosses the roche limit it isnt gravity vs gravity its gravity vs centrifugal force. Its a difference in tidal forces that rips things apart. But if their tensile strength is strong enough it will stay together just not in the shape one might expect. If mass is unevenly distributed it will be torn apart but im unclear if you believe gravity can separate individual atoms at the roche limit??

originally posted by: dragonridr
You confused me with the gravity vs gravity thing at the end. When something crosses the roche limit it isnt gravity vs gravity its gravity vs centrifugal force. Its a difference in tidal forces that rips things apart.

Did you notice I said "gravity can pull objects apart inside the Roche limit" and not "gravity *always* pull objects apart inside the Roche limit". Your question seems to infer the latter which isn't what I said.

Tidal forces come from what?

Gravity.

And what is trying to hold the pieces of the satellite together?

Gravity. (In some cases)

Thus, gravity (tidal) versus gravity (satellite cohesion), as explained here:

Since tidal forces overwhelm the gravity that might hold the satellite together within the Roche limit, no large satellite can gravitationally coalesce out of smaller particles within that limit. Indeed, almost all known planetary rings are located within their Roche limit

But yes if the satellite is held together by tensile strength instead of gravity, then it's gravity versus tensile strength, and if the tensile strength is stronger, it will win.

Some real satellites, both natural and artificial, can orbit within their Roche limits because they are held together by forces other than gravitation.

Jupiter's moon Metis and Saturn's moon Pan are examples of such satellites, which hold together because of their tensile strength

im unclear if you believe gravity can separate individual atoms at the roche limit

Note the aforementioned rings. That's what you get inside the Roche limit.

Also "centrifugal force" is fictitious, it's really just inertia.

a reply to: Arbitrageur

OK ill assume i misread that tidal forces come from gravity and inertia that causes the sheering effect. Basically the roche limit it the area where inertia is trying to throw an object out of orbit and gravity tries to drag it in. Its these two opposing forces that cause things to tear apart. I thought i read where you said its gravity vs gravity its not at all no inertia no roche limit you just plummet towards the massive object.

Building on my post earlier in the thread, what do you guys make of this apparently controversial op-ed op-ed by John Horgan:
Gr owing time lag for nobels portend the end of fundamental discoveries in physics.

From the looks of it, he's mainly talking about particle physics here. In condensed matter, plasma, and AMO (Atomic, molecular, optical), obviously this would not apply. But it seems to me experimental HEP has a very dire future, and that the next generation accelerators (assuming they are built) will be the last once they turn up nothing.

a reply to: Diablos
When I read your post, I was thinking I should cite Feynman's interesting thoughts on the topic, but then I read your link and saw the link already cites them. I'll pull them out here for emphasis (from your link):

The age in which we live is the age in which we are discovering the fundamental laws of nature, and that day will never come again. It is very exciting, it is marvelous, but this excitement will have to go. Of course in the future there will be other interests. There will be the interest of the connection of one level of phenomena to another—phenomena in biology and so on, or, if you are talking about exploration, exploring other planets, but there will not be the same things we are doing now.

I think he's right to an extent, but I don't think we are near the end of that fundamental discovery. Your point that we will reach limitations on the size of particle accelerators we are economically willing to build is valid. There is a possibility that cosmic rays with much higher energies than any particle accelerator produces might be utilized even more than we are already doing. There is a limited, expensive experiment for cosmic ray observation on the ISS, but much larger and even more expensive space observation apparatus is conceivable. We do have observation arrays for cosmic rays on the ground, but those are less likely to lead to fundamental new discoveries than observations in space before all the atmospheric particle showers are created.

I'd be happy if we could just figure out dark matter and dark energy/cosmological constant versus the vacuum catastrophe during my lifetime. In other words, if we only know what about 5% of the universe is made of, it sure would be nice to figure out the other 95%.

a reply to: PraiseTheHighestOne

It's called heat transfer. It's used in conjunction with soundless technology. The burnt smell is ions from plasma shielding. It's probably against the law to post this but what the hell.

Inside the sphere on top is an alluminum rod that goes almost to the bottom. At the end is a small alluminum ball. A hole is cut at the bottom and a smooth copper ring is placed and insulated from the alluminum. The alluminum ball sits in the center of the copper ring. Inside the alluminum sphere is 4 rotating electrically charged coils of copper that has it's own power source.
Now my theory is that when a positive charge is placed on the alluminum ball and a negative charge on the copper ring sparks will occurr causing the air to be pulled inside the sphere and ionizing the air within. More and more of the air will get sucked into the sphere until the air moves no more and only the electrons will gather inside the sphere. Once the inside of the sphere air is ionized You charge the coils inside and spin them. The charged coils create a magnet that the ionized air within will follow. This will cause a buildup of electrons in the inside and a buildup of protons on the outside of the sphere. A coander effect will occurr and air from top will flow to bottom. This will cause a static charge to build on the outside of the sphere. It will then rise no matter how much it weighs and will power itself after a jump start. Sort of like a magnet. The same poles will repell each other. Oh, and it is placed on a sheet of cardboard. DO NOT TOUCH.

I'm just guessing here, i barely made it thru geometry in school , so don't bash me to hard.

a reply to: cloaked4u

originally posted by: cloaked4u
I'm just guessing here, i barely made it thru geometry in school , so don't bash me to hard.

Since the topic of this thread is "Ask any question you want about Physics", are you asking a question? I couldn't find one easily and I don't really understand the topic of your post. Did you mean to post that in a different thread?

It sounds to me like a topic for the skunk works forum, which is sort of a forum for wild guesses with no evidence, among other things.

originally posted by: Diablos
Building on my post earlier in the thread, what do you guys make of this apparently controversial op-ed op-ed by John Horgan:
Gr owing time lag for nobels portend the end of fundamental discoveries in physics.

From the looks of it, he's mainly talking about particle physics here. In condensed matter, plasma, and AMO (Atomic, molecular, optical), obviously this would not apply. But it seems to me experimental HEP has a very dire future, and that the next generation accelerators (assuming they are built) will be the last once they turn up nothing.

Though it may be harder to branch out in to unexplored territory i think discoveries are far from over at this point anyway.

you guys admit to your self yet that light is emitted '360' degrees surrounding an accelerated charged particle? What physics stuff yall been thinking about lately?