Can you always tell who is accelerating?

[coconut]

Hi,

 

I'm getting a bit giddy thinking about how to determine whether something has experienced an acceleration or not.

 

eg. General Relativity has some examples.. how free falling in elevator is the same as floating in space, and standing in elevator same as rockets moving spaceship in space.

 

But in all these examples couldn't you just say that the person never experienced any acceleration, that they were always perfectly stationary, and everything else just moved all funny.

 

In fact couldn't you even stretch, twist and tie it to a yoyo, and yet it could still stay "hey, I never moved" everything else is just moving a bit crazy.

 

If you treated an object as a bunch of particles each with an initial position relative to the others, you might say that acceleration would alter the relative positions, but if that is the definition of acceleration, then wouldn't it be possible to invent a clever device which accelerated each particle uniformly, so that it didn't deform at all. So then how would you know that it accelerated?

 

Any thoughts welcome!

[swansont]

Basically, if Newton's laws work, you are not being accelerated.

[Tesseract]

acceleration is the rate of change of velocity with respect to time, so to see if something is accelerating you have to measure its rate of change of velocity compared to time.If the particles in an object are moving (motion from one point to another) at the same speed the entire distance it is not acceleration.

You know something is accelerating if the velocity changes as time passes.The definition of acceleration is not the particles distance compared to the others.

[[Tycho?]]

Uh, I think you can always tell who is accelerating I think. Namely that when you accelerate, a force is applied on you, which you can feel and measure. If it was another object that was acclerating, you would not feel a force, so you would know you were not acclerating. I think.

[Tesseract]

I think acceleration is being confused with velocity.Acceleration is a=v/t so you can tell is something is accelerating by measuring the speed at which velocity is increasing.

[swansont]

I think acceleration is being confused with velocity.Acceleration is a=v/t so you can tell is something is accelerating by measuring the speed at which velocity is increasing.

 

The velocity with respect to what?

[Tesseract]

The velocity with respect to what?

 

What do you mean.

[jordan]

'']Uh, I think you can always tell who is accelerating I think. Namely that when you accelerate, a force is applied on you, which you can feel and measure. If it was another object that was acclerating, you would not feel a force, so you would know you were not acclerating. I think.

 

The motion of the other object could cause the force you feel. If two planets were in space, planet A is stationary and planet B passes by, wouldn't planet A have the appearence of feeling a force?

 

 

What do you mean.

 

The point is that acceleration can not absolutely determined for one object. We only say everything is in motion because it is the most convenient. It would be too difficult to calculate everything if we assumed you, Tesserect, we stationary and the universe revolved around you (though you may like that idea ). Therefore, you need to determine how you are establishing something has a definite velocity before talking about how velocity effects acceleration.

 

*Disclaimer: everything written above is from memory. It is taken from a book read years ago and may not be eniterly correct.

[Tesseract]

Everything in motion is determined by a starting point.The starting point is relatve in all math.All the atom in an object move at the same speed when in motion.

We measure distance by definite intervals away from a starting point.If an object moving from point (A) has to move to a point (B) (10 metres away)then we measure the velocity by observing the time spent for the object to move through an interval of measurement (say a metre).If the object takes 5 seconds to move from starting point (A) to the one meter interval, we say it moves 1m/second.If there is nothing to slow it down it will move 10 metres in 50 seconds or v=10m/s.

To measure its acceleration we have to see if the interval of time from two definite intervals has increased.Say the object moved from interval 7 to interval 8 in 4.99 seconds.We say it is accelrating 0.01 metres per second or a=0.01m/s.

 

At least I think thats right.

[jordan]

Everything in motion is determined by a starting point.The starting point is relatve in all math.All the atom in an object move at the same speed when in motion.

We measure distance by definite intervals away from a starting point.If an object moving from point (A) has to move to a point (B) (10 metres away)then we measure the velocity by observing the time spent for the object to move through an interval of measurement (say a metre).If the object takes 5 seconds to move from starting point (A) to the one meter interval' date=' we say it moves 1m/second.If there is nothing to slow it down it will move 10 metres in 50 seconds or 10m/s.

To measure its acceleration we have to see if the interval of time from two definite intervals has increased.Say the object moved from interval 7 to interval 8 in 4.99 seconds.We say it is accelrating 0.01 metres per second or 0.01m/s.

 

At least I think thats right.[/quote']

 

Yes, but how can you be sure it's the object moving and not everything else around it moving? Say we have two planets in space again without any refrence frame behind them. They pass by each other and continue to distance themselves. Which one was moving. This is much more difficult to imagine on a "everyday life" scale, but it still aplies.

[Tesseract]

Yes, but how can you be sure it's the object moving and not everything else around it moving? Say we have two planets in space again without any refrence frame behind them. They pass by each other and continue to distance themselves. Which one was moving. This is much more difficult to imagine on a "everyday life" scale, but it still aplies.

 

When the two planets pass each other and are at the closests possible point together we can call that point A.The planet that continues to move from point A is the one that is moving.

[jordan]

And which planet would that be? Planet A or Planet B?

[Tesseract]

Ebverything in this universe is relative you pick a planet.Id do it this way the planet that is still at point A is planet A.The one moving is planet B.

[jordan]

Ebverything in this universe is relative you pick a planet.

 

Not sure what this says, but I think it might be on the right track.

 

 

Id do it this way the planet that is still at point A is planet A.The one moving is planet B.

 

Precisely the point. Where is point A at when you don't have any refrence. You're in black space without anything but two planets, which one is moving? Let's add three. The converge towards a point, then move away at 120^o to each other. At least two would have to be moving, but could you prove which ones are if you don't have "point A" to refrence everything to? I could continue adding objects until we get to a more realistic situation.

[Tesseract]

You cant prove anything if you dont have a point of reference, because you cant prove that anything is moving at all.Whats the question in this thread anyway?

[Tesseract]

Not sure what this says' date=' but I think it might be on the right track.

[/quote']

 

I meant that everything is relative to each other, they all move from a point of reference.

[jordan]

You cant prove anything if you dont have a point of reference, because you cant prove that anything is moving at all.

 

You can't conclusively prove which object isn't moving.

 

 

Whats the question in this thread anyway?

 

If you treated an object as a bunch of particles each with an initial position relative to the others, you might say that acceleration would alter the relative positions, but if that is the definition of acceleration, then wouldn't it be possible to invent a clever device which accelerated each particle uniformly, so that it didn't deform at all. So then how would you know that it accelerated?

 

I wouldn't think it would deform, but rather expand and that is how it would show acceleration.

 

 

In fact couldn't you even stretch, twist and tie it to a yoyo, and yet it could still stay "hey, I never moved" everything else is just moving a bit crazy.

 

The world would spin really fast if everything was measured with respect to a yo-yo.

[Tesseract]

In a plane you cant prove which object is moving or which object isnt moving.Now Im going to leave and go watch a movie, Ill be back tommorow.

[jordan]

When the two planets pass each other and are at the closests possible point together we can call that point A.The planet that continues to move from point A is the one that is moving.

 

 

In a plane you cant prove which object is moving or which object isnt moving.

 

You never cease to confuse me Tesseract. Oh well, I too am leaving for the night. My head now hurts.

[swansont]

The velocity with respect to what?

 

 

What do you mean.

 

You said that you can measure if the velocity changes. Velocity is measured with respect to something else - my point was: how do you know if you are accelerating or if that "something else" is accelerating?

 

That measurement is insufficient to tell who is accelerating.