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geordief

Q's about acceleration (Hijack from What fundamentally is acceleration?)

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I always thought that a system was in acceleration  because  it was expending energy and that its acceleration could be maintained only so long as the energy lasted.

The longer the acceleration was maintained the more the system dissipated and eventually it would vanish.

On the other hand a system that was in motion wrt another system could maintain that state for ever.

 

 

 

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1 hour ago, geordief said:

I always thought that a system was in acceleration  because  it was expending energy and that its acceleration could be maintained only so long as the energy lasted.

The longer the acceleration was maintained the more the system dissipated and eventually it would vanish.

On the other hand a system that was in motion wrt another system could maintain that state for ever.

You can get a constantly changing Vector in circular motion too. Something in orbit can keep on going forever if left to its own devices.

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1 hour ago, geordief said:

I always thought that a system was in acceleration  because  it was expending energy and that its acceleration could be maintained only so long as the energy lasted.

The longer the acceleration was maintained the more the system dissipated and eventually it would vanish.

On the other hand a system that was in motion wrt another system could maintain that state for ever.

 

 

 

geordief,  on reading your post, it brings to mind a point that I never quite understood -  when I was doing a Physics course, my tutor insisted that the Moon is constantly "accelerating" as it orbits the Earth.  This didn't seem convincing to me.   Because as you say, "acceleration" -  ie, an  increase in speed, surely requires an increase in energy input.

Whereas the Moon, when it orbits the Earth, isn't getting any energy input. it just travels round in a a constant orbit,  So  how can it be "accelerating"?

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6 minutes ago, Endy0816 said:

You can get a constantly changing Vector in circular motion too. Something in orbit can keep on going forever if left to its own devices.

Doesn't the orbit decay?

I  feel out of my depth as usual but if it doesn't decay doesn't that mean it is  accelerating?

And if it does decay ,then it is in freefall and so not accelerating.

 

Maybe there is a difference in what acceleration  means in Relativity and in Newtonian mechanics??

 

 

11 minutes ago, Charles 3781 said:

geordief,  on reading your post, it brings to mind a point that I never quite understood -  when I was doing a Physics course, my tutor insisted that the Moon is constantly "accelerating" as it orbits the Earth.  This didn't seem convincing to me.   Because as you say, "acceleration" -  ie, an  increase in speed, surely requires an increase in energy input.

Whereas the Moon, when it orbits the Earth, isn't getting any energy input. it just travels round in a a constant orbit,  So  how can it be "accelerating"?

I hope I just answered your question too.

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13 minutes ago, Charles 3781 said:

geordief,  on reading your post, it brings to mind a point that I never quite understood -  when I was doing a Physics course, my tutor insisted that the Moon is constantly "accelerating" as it orbits the Earth.  This didn't seem convincing to me.   Because as you say, "acceleration" -  ie, an  increase in speed, surely requires an increase in energy input.

Whereas the Moon, when it orbits the Earth, isn't getting any energy input. it just travels round in a a constant orbit,  So  how can it be "accelerating"?

Acceleration is not an increase in speed. It's a change in velocity. Our Moon is gaining energy due to the tidal lock with the Earth, and Luna is also changing velocity due to it's orbital motion.

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6 minutes ago, moth said:

Acceleration is not an increase in speed. It's a change in velocity. Our Moon is gaining energy due to the tidal lock with the Earth, and Luna is also changing velocity due to it's orbital motion.

I think acceleration can just be an increase in speed if the direction does not change

With the Moon ,isn't it in freefall? (so not accelerating)

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Just now, geordief said:

I think acceleration can just be an increase in speed if the direction does not change

With the Moon ,isn't it in freefall? (so not accelerating)

You are correct, I should have said: not only an increase in speed. But velocity is a vector quantity, so a change in direction is also an acceleration

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17 minutes ago, geordief said:

Doesn't the orbit decay?

Double checked myself. You're correct on orbital decay. I was only a considering the more conventional methods of a body losing velocity(drag, tidal forces), rather than gravitational radiation. Would thankfully still take forever and a day lol.

 

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Just now, moth said:

You are correct, I should have said: not only an increase in speed. But velocity is a vector quantity, so a change in direction is also an acceleration

But what about the Moon being in freefall around the Earth ?It does not accelerate

 

Should we say that because it is following a geodesic in spacetime that it is moving in a straight line?

 

Is it just in Newtonian mechanic that it appears to be changing its velocity wrt the Earth and so apparently accelerating?

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9 minutes ago, geordief said:

But what about the Moon being in freefall around the Earth ?It does not accelerate

I think the tidal lock with earth means the moon is not in freefall

 

11 minutes ago, geordief said:

Should we say that because it is following a geodesic in spacetime that it is moving in a straight line?

I don't think so, but a Physicist might disagree.

 

11 minutes ago, geordief said:

Is it just in Newtonian mechanic that it appears to be changing its velocity wrt the Earth and so apparently accelerating?

Circular motion is an acceleration even with no change in speed

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3 minutes ago, moth said:

 

 

Circular motion is an acceleration even with no change in speed

Then isn't the word "acceleration" being used differently in science.  It properly means to speed up.  Like when you "accelerate" your car.   Your car increases its speed.

If you claimed you could make your car increase its speed,  by driving it round in a circle with no change in speed,  that would not make sense.

So I think the word "accelerate" is used differently in science, from what it is in the real world.

 

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4 minutes ago, Charles 3781 said:

Then isn't the word "acceleration" being used differently in science.  It properly means to speed up.  Like when you "accelerate" your car.   Your car increases its speed.

If you claimed you could make your car increase its speed,  by driving it round in a circle with no change in speed,  that would not make sense.

So I think the word "accelerate" is used differently in science, from what it is in the real world.

 

I would say it's meaning in science is more precise than in everyday conversation, but you are not wrong.

Do you know what a vector is?

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There are two concepts that need to be distinguished.
Proper acceleration is relative to a free-fall or inertial reference, while co-ordinate acceleration is based on the choice of co-ordinate system.
They are equivalent in an inertial frame in flat ( no gravity ) space-time.

The moon, being in free-fall ( orbit is a 'fall' ), is not considered to be accelerating in GR, while you, standing on the Earth's surface, are being accelerated 'up' at 1g by the surface.
This link may dispel some of the above confusion...

https://en.wikipedia.org/wiki/Proper_acceleration

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1 minute ago, moth said:

I would say it's meaning in science is more precise than in everyday conversation, but you are not wrong.

Do you know what a vector is?

I think it means " going in a certain direction".  Why giving it the name "vector" makes it more scientific, I'm not so sure about.

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1 minute ago, Charles 3781 said:

I think it means " going in a certain direction".  Why giving it the name "vector" makes it more scientific, I'm not so sure about.

vectors have two qualities, magnitude and direction. Velocity is a vector and acceleration is a change in velocity.

 

17 minutes ago, MigL said:

There are two concepts that need to be distinguished.
Proper acceleration is relative to a free-fall or inertial reference, while co-ordinate acceleration is based on the choice of co-ordinate system.
They are equivalent in an inertial frame in flat ( no gravity ) space-time.

The moon, being in free-fall ( orbit is a 'fall' ), is not considered to be accelerating in GR, while you, standing on the Earth's surface, are being accelerated 'up' at 1g by the surface.
This link may dispel some of the above confusion...

https://en.wikipedia.org/wiki/Proper_acceleration

I should have noticed this is the relativity section, not the physics section.


Is the moon still considered to be in freefall even though the distance between Earth and moon is increasing due to tidal forces?

 

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!

Moderator Note

geordief

You simply have to STOP HIJACKING THREADS

If you have a question that is not directed at the OP, open a new thread to ask it. Link to the original thread, if needed. But it's against the rules to interrupt someone else's thread to talk about whatever is puzzling you

 
8 hours ago, Charles 3781 said:

Then isn't the word "acceleration" being used differently in science.  It properly means to speed up.  Like when you "accelerate" your car.   Your car increases its speed.

If you claimed you could make your car increase its speed,  by driving it round in a circle with no change in speed,  that would not make sense.

So I think the word "accelerate" is used differently in science, from what it is in the real world.

The situation is exactly the opposite. Velocity is a vector, so any change in this vector is an acceleration. That's how it's used in physics, so circular motion is the result of an acceleration. It's the non-physicist who not only limits it to mean a change in speed, but distinguishes between speeding up and slowing down. Slowing down is an acceleration, too.

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+1 was for the explanation, not the mod note.

Learn something new everyday on this site. 

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