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frb
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is it, cause velocity isn't.
frb said:is it, cause velocity isn't.
frb said:is it, cause velocity isn't.
It depends on what movie you are watching?dextercioby said:Since coordinates,velocity and acceleration are ALL OF THEM CINEMATIC QUANTITIES,they need to be defined wrt a reference system.
frb said:is acceleration absolute?
is it, cause velocity isn't.
marcus said:what about rotation?
can an observer decide whether his or her frame of reference is rotating or not? can different observers agree?
jdavel said:But unlike Newtonian physics, according to SR the magnitude of acceleration is different in different inertial frames.
Actually it is not acceleration that is absolute. It is the rate of change of momentum. In SR, dp/dt is invariant and absolute.jdavel said:marlon,
You said,
"Acceleration is absolute in every theory before General Relativity. So, Newtonian physics (here, also speed is absolute), Special relativity (no absolute speeds, only absolute acceleration)"
But unlike Newtonian physics, according to SR the magnitude of acceleration is different in different inertial frames.
Then perhaps you would care to enlighten us. Why is/is not dp/dt at least absolute and invariant?salazar18 said:it seems that this dialogue is lacking in depth of knowledge of momentum, angular momentum, time-space curvature, general relativity, and cyclical energy fluctuations
I don't think that can be right. Although position is dependent upon a co-ordinate system, change in position and rate of change of position is not (at least to inertial observers where relative v<<c). Mass is not dependent upon the frame of reference either (if v<<c). To any inertial observer, (even those moving at speeds close to c) dp/dt is invariant and absolute under Newtonian/Galilean relativity and under relativity.reilly said:There's no way acceleration is anything but relative. It's the second deriv. of a distance, or coord, which necessarily refers to two points, and that's the source of the relative nature of spatial coordinates, and functions thereof.
I don't follow you there. Force is defined as [itex]\vec F = d\vec p/dt[/itex]. So it has to be in the direction of the momentum change, which means it has to be in the direction of the acceleration.jdavel said:That's not true! In general, a Lorentz transformation will change both the direction and magnitude of force. In fact force and the acceleration it causes can end up in mutually different directions after applying an LT!
If dp/dt is not invariant under relativity then momentum cannot be conserved. Neither can energy because energy is related to momentum by:Nature paid a huge price for insisting on there being one speed that would be the same in all reference frames! The price was that almost nothing else could be.
I thought we were talking about inertial reference frames. Acceleration is relative under Newtonian mechanics if you use non-inertial frames of reference.reilly said:Fred's watching me go down in a free falling elevator -- Albert Einstein Model # 32g.
Unless Fred is deranged, he will say that I'm moving down with acceleration g, and he's not.
Of course, I say something different. Fred's moving up with g, and I'm not accelerating.
That's about as relative as it gets.
regards,
Reilly Atkinson
reilly said:Fred's watching me go down in a free falling elevator -- Albert Einstein Model # 32g.
Unless Fred is deranged, he will say that I'm moving down with acceleration g, and he's not.
Of course, I say something different. Fred's moving up with g, and I'm not accelerating.
That's about as relative as it gets.
regards,
Reilly Atkinson
Andrew Mason said:I don't follow you there. Force is defined as [itex]\vec F = d\vec p/dt[/itex]. So it has to be in the direction of the momentum change, which means it has to be in the direction of the acceleration.
The path of motion changes, but does the acceleration?jdavel said:Andrew,
It seems that way, but it doesn't because the mass changes as well as the velocity.
Suppose an object is moving in the x direction at .99c. Now apply a force to it with components Fx = 10 and Fy = 1. That's almost along the x direction. But look at the resulting acceleration. The Fx = 10 can barely cause any acceleration because the object is already almost going at c. But the Fy = 1 can increase the y component of velocity to 0.1c without the resultant velocity exceeding c. The force was almost all in the x direction, but the acceleration is almost all in the y direction!
Good point. The direction is not absolute - I retract my earlier statement about direction! But I think |dp/dt| is still the same for all observers. Try working it out and see.jdavel said:AM
"Your example applies equally to Newtonian mechanics as well."
No it doesn't; you misunderstood my example.Try this.
Suppose you're in a frame of reference where a particle is moving slowly (v<<c). If you apply a force to the particle its momentum will change at the rate given by dp/dt = d(mv)/dt = m(dv/dt). So dp/dt = ma. That's a vector equation and since dp/dt and a are the only two vectors, they have to be pointing in the same direction.
But now if you look at the same thing in a frame of reference where the particle is moving at .99c, the force will cause the momentum to change at the rate given by dp/dt = d(mv)/dt = m(dv/dt) + v(dm/dt). So dp/dt = ma + v(dm/dt). Again, that's a vector equation, but this time there are three vectors, dp/dt, a and v. So unless v is in the same direction as a, dp/dt will point somewhere in between v and a, not in the same direction as either one.
frb said:is acceleration absolute..is it, cause velocity isn't.
But according to you there is always a gravitational field and therefore it is always absolute. Stop posting nonsense that even you don't believe has any physical meaning.Mueiz said:In gravitational field it is absolute and equal zero in free-falling frames.
In the absence of gravitational field it is relative.
Hi prasanth.s, welcome to PF. Usually rather than responding to a post which is 6 years old it is better to start a new thread. That said, there are two kinds of acceleration that we can discuss, one is called "proper acceleration" and it is absolute, the other is called "coordinate acceleration" and it is relative. If you would like to discuss the difference I recommend a new thread.prasanth.s said:Am new here ... ACCELERATION IS RELATIVE
You can set up a gravitational field that will make any local experiment confirm rotation without there actually being any rotation. Frame dragging can even simulate Coriolis Effect.marcus said:what about rotation?
can an observer decide whether his or her frame of reference is rotating or not? can different observers agree?