Speed of Force or 'Transfer of Momentum'

You are right.

The time element is the instant of time at which contact happens between two masses.

 

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Which brings me back to my point, how does a mass apply a force to another mass in zero elapsed time?

 

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It is a single event, not two separate ones.

 

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So there is no "net force?"

 

In this context, no: a force pair is two equal and opposite forces arising from one event.

Don't get confused: the part of post 13 you quoted that started you down this path was completely wrong.

 

I agree.

 

In the early stages of aerospace design they said it would be impossible to travel at mach one because the transfer of momentum through a medium was limited to the speed of sound. Then any craft would disintegrate from traveling at the speed of sound, so then breaking the sound barrier would be impossible. The sound barrier was then broken. What changes, if any, where then made to allow a jet craft to travel at speeds greater than mach one was made to the science of the transfer of momentum through a medium because of this?

 

Do you have a source for that? As far as I know, that's a myth started by a bad reporter.
http://history.nasa.gov/SP-4219/Chapter3.html

And the way you describe it doesn't make sense: bullets were already traveling faster than the speed of sound and explosions destroy things faster than the speed of sound, so scientists knew you could make things go faster than the speed of sound.

 

I wish I did, I had a science teacher that was a "buff" or a "guru" in early aviation. He claimed it was one of the engineers, but then they pushed it forward and made them do it anyways just to find out for sure. I find it kind of funny, because if like people claim nothing can be pushed faster than the speed of sound then we couldn't have supersonic jets.

I think it is an urban myth created by people on the internet that try to protect the validity of the speed of light barrier. People say that you could push a rod and then the other end would react instantly, and then they say, no no no, the other end only reacts at the speed of sound. But, I think they forget that atoms are bounded together by the electromagnetic force. It is the electrons creating the correct numbers of themselves in the orbitals of the atoms that holds them together. The force on one end of a rod could then be transmitted much faster, if not the speed of light at least close to the speed of light.

I don't think bullets break the sound barrier, and I don't think explosions do either because if they did it would create a sonic boom.

http://en.wikipedia.org/wiki/Sonic_boom

But this says it can be made by a bullet, but that would have to be a really big gun... Doesn't say what kind of gun would have to be used for that. I think most of the sound of gun fire comes from the gun powder in the bullet exploding. If you have ever heard a sonic boom it is very loud, you would be able to tell the difference. It is just like a big loud BOOOOM! If a bullet did this over your head you would go deaf. Guns make more of a BANG! Never heard of one being able to break the sound barrier. In the movies they always, wizz over them.

 

Um, you do realize you have the shoe on the wrong foot, right? The myth is used by people who are against accepted physics, not for it (after all, you are the one who invoked it!). It is a way of saying that "see, scientists were wrong about the sound barrier, so they are probably wrong about the "light barrier"...which wouldn't follow anyway even if the premise was true.

1. This sort of thing has been tested. It is true. Seismology depends on utilizing the speed of sound in solid objects. So does music. And sonar uses the speed of sound in liquids. The speed of sound is so low that it isn't difficult to test. Heck, how do you think they would test the speed of sound in materials in the first place! Come to think of it, you could probably do such a test yourself with a 10' copper pipe, a hammer and two microphones!
2. Electromagnetic energy and physical force are not the same thing: electrical signals do travel near the speed of light. (Electrical signals don't make the whole atom vibrate.)

 

Basically 'applying a force' means 'applying some energy and momentum'.

When two masses are in contact, there is a transfer of energy and momentum from one mass to another mass.

Thus one mass can apply a force to another mass in zero elapsed time.

 

It doesn't really matter what you 'think'. This is not the type of a thing where you can't have an opinion - bullets either break the sound barrier or they don't. The answer is they do break the sound barrier.

The speed of sound at sea level is about 1100 fps.

The smallest common rifle is a .22 caliber the speed of these bullets based on the size of the charge range from the subsonic 850 fps to the supersonic 1400 fps. One of the very common types of deer rifles is the 30.06, these have a muzzel velocity of 2,800 fps or mach 2.5. Another common high powered deer rifle is a .300 mag with a muzzel of 3200 fps or mach 2.9.

 

What is that suppose to mean? Nothing happens instantly. The fastest that any information can be trasfered is the speed of light which mean there will always be some elapsed time in the transfer of any information. As has been pointed out the transfer of the moment and force through a solid occurs at the speed of sound. Do you dissagree with this?

 

Do you mean 'action' and 'reaction' are single event? or, they are separate events but occur simultaneously?

Do you both mean action and reaction are nullifying each other so that there is no "net force"?

Are these two forces acting on the same mass?

Can you explain, where exactly it is wrong? Why and How it is wrong?

If a ball is thrown in a wall, the ball will reflect back immediately. This reflection of the ball is due reaction from the wall.
In the case of Newton's Cradle, the first ball also reflects back after incidence on the second ball. But in this case there is sufficient time-delay for reflection of the first ball after its incidence on the second ball.

Can you explain this time-delay for reflection in the Newton's Cradle?

 

I was replying to MD's question about

I thought he was implying 'contact between two masses' like a 'contact of a ball with a wall' when a ball is thrown to the wall. Duration of this 'contact' can be considered as 'zero elapsed time'.

I dont disagree with the 'speed of sound' conclusion.

 

Supersonic aircraft does not travel at a speed faster than sound. Its speed is only higher than the speed of sound through air. Speed of sound through solid is much higher than the speed of sound through air. Supersonic jet does not exceed the 'speed of sound through solid'.

I think electrons respond to Electro-Motive-Force or EMF, which is quite different from the mechanical force of action-reaction principle.

 

OK, my bad.

You have to be careful with this. If you say the duration of contact is zero elapsed time that ends up meaning an infinite acceleration \(\frac{dv}{dt}\), so an infinite force is applied. If a ball is thrown against a wall the force is F = ma, if the velocity goes to zero instantly then the the acceleration 'blows up' because you have a zero in the denominator of the equation. Now in reality this is an much 'uglier' problem because the forces will involve a deformable body, the deceleration will be different as the body deforms. The point is changes in velocity never happen instantly, this unrealistic assumption is sometimes made to make problems easier to solve, like neglecting friction.

OK. I am glad your question was answered and you incorporated this knowledge.

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You are right, duration of contact can not be exactly zero but it is some 'time duration' tending towards zero.

Thanks.

 

A force pair is a single event/interaction, viewed from two sides. Consider the following statements:

1. A ball impacts a wall.
2. A wall impacts a ball.
3. A wall and ball impact each other.

All three of these statements are describing the same event/interaction in different ways. It works the same with force:

1. A ball applies a force to a wall.
2. A wall applies a force to a ball.
3. A ball and wall share a force.

Again, that's a single event/interaction viewed from different directions. This is the entire point of Newton's 3rd law. If you're having trouble grasping this, please read the wikipedia entry on it. A snippet:

http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_third_law

What I think is confusing people is that while every event is symmetrical, the event has a non-zero time duration. So while at every instant in time during the collision you can say the ball and wall share an equal and opposite force pair, they are in contact for a finite amount of time and the exchange (or reversal) of momentum takes a finite amount of time.

The way a collision between a ball and wall actually works is that at the instant they first touch, the force (again: there is only one force) is infinitessimally small. Over time, the force increases. The increase in force is caused by the inertia and compressibility of the ball converting kinetic energy of motion into potential energy of compression.

And not even the act of compression is uniform: the ball has to transfer deformation through it, so the rates of deceleration and forces are not the same everywhere in the ball throughout the collission. For example, for the next few miliseconds after the ball first touches the wall, the leading edge of the ball is stationary against the wall while the trailing edge is still traveling at its pre-impact speed. It hasn't felt the impact yet.

 

Another problem here is people analyzing events using human perception. You have to realize that in terms of human perception, "instantly" is about a tenth of a second. You have no hope of understanding physics if you make that assumption. A tenth of a second can be a long time at the speed of sound and is an absolute eternity at the speed of light. Equating all of these because your eyes can't tell the difference is a HUGE mistake.