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Author Topic: mass has the tendency to reduce its own gravity as its velocity increases  (Read 10222 times)

Offline Bored chemist

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I'm not denying the evidence. (and implications that I was doing might be construed as an ad hom)
The evidence is that I'm still sat on this chair.
I have not moved.
My velocity remains zero.
The change in velocity is zero.
The rate of change of velocity is zero
the acceleration is zero.

If you took the chair away then  I would experience an acceleration downwards which I would be able to observe, not least when I hit the ground.

"You can assume that your acceleration is caused by a force"
And I do. As you say, if I didn't do that I would be ignoring F=ma.
However I believe in that formula.
And, since I know my mass, I can measure my acceleration (from how fast the world and I get together) and I can calculate the force.
It remains about 700N
That's not an acceleration, its a force; it's called weight.
The fact that it is proportional to mass isn't the issue.

Perhaps you would like to answer the question I posed earlier
"With respect to  what am I currently accelerating that explains the dent in the springs in the chair I'm sitting on?"


 

Offline imatfaal

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If it is merely an acceleration that is completely independent of mass (the Geezer position) and not (what I think BC is contending) the result of a calculation of opposing forces - how does one explain a helium filled balloon.  It should accelerate to the floor just as BC's bum does without the chair if all there is a globally applicable acceleration of 9.8ms-2 (and we have moved on from those mad imperial units that only the americans now cling to); of course we know that other forces (not necessarily an acceleration) also play a role; and it is the vector sum of all these forces that determines the motion
 

Offline Geezer

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If gravity is a "force", isn't it a bit strange that it is measured in units of acceleration rather than force per mass? Could this possibly be a clue?  ::)

What y'all obviously fail to realize is that "weight" is simply the force necessary to prevent an acceleration. A mass is still being "accelerated" whether it is moving or not, and that is why the springs in your chair are stressed while it is preventing your acceleration.

As I proved earlier, if F=ma, gravity can only be an acceleration. I'm only applying the rules. If you have a problem with this you should complain to your science teacher for giving you a bum steer.

 
 

Offline Bored chemist

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"If gravity is a "force", isn't it a bit strange that it is measured in units of acceleration rather than  "

It isn't
http://en.wikipedia.org/wiki/Gravitational_constant
You seem to be basing your point of view on a false premise.
However it's a moot point. The two sets of units you propose are the same anyway.
Acceleration
m/s/s

Force? Well we both agree that f= ma
so
"force per mass"
has units of force ( which is ma)
kg m/s/s
divided by the units of mass
(that's kg)
 so kg m /s /s  divided by kg is
m/s/s
which is  the same as acceleration.

And, as I have said, I remain a firm believer that F =ma

"What y'all obviously fail to realize is that "weight" is simply the force necessary to prevent an acceleration. "
That's true of any force.
If I push on a rock, the force that stops it accelerating is friction.

Is friction an acceleration?
« Last Edit: 03/09/2011 02:16:50 by Bored chemist »
 

Offline Geezer

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Well, if you want to come up with an alternative to General Relativity, knock yourself out.

"From the viewpoint of GR, there is no gravitational force. Rather, in
the absence of electromagnetic and other forces, particles follow the straightest possible
paths (geodesics) through a spacetime curved by mass. Freely falling particles define
locally inertial reference frames. Time and space are not absolute but are combined into
the four-dimensional manifold called spacetime."

http://web.mit.edu/edbert/GR/gr1.pdf
 

Offline Bored chemist

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I'm not trying to come up with an alternative to GR.
I'm trying too get you to explain yourself, but you seem unable or unwilling to do so.

So, for the third time of asking

"With respect to  what am I currently accelerating that explains the dent in the springs in the chair I'm sitting on?"

And, while you are at it, why do you think you can add together quantities with different units? And do you consider friction to be an acceleration?
 

Offline Geezer

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"With respect to  what am I currently accelerating that explains the dent in the springs in the chair I'm sitting on?"

You seem to have missed this.

http://www.thenakedscientists.com/forum/index.php?topic=40713.msg365800#msg365800
 

Offline MikeS

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I'm not trying to come up with an alternative to GR.
I'm trying too get you to explain yourself, but you seem unable or unwilling to do so.

So, for the third time of asking

"With respect to  what am I currently accelerating that explains the dent in the springs in the chair I'm sitting on?"

And, while you are at it, why do you think you can add together quantities with different units? And do you consider friction to be an acceleration?


You're accelerating in time.
« Last Edit: 04/09/2011 08:41:32 by MikeS »
 

Offline Bored chemist

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Geezer,
that purports to be an explanation based on redefining the word "acceleration" to include standing still or alternatively failing to explain that what you meant was this odd thing called "proper acceleration"
Is it worth asking you to reply to the other questions about dimensional analysis?
Mike,
Thanks for actually answering the question.
 

Offline MikeS

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BC
You're welcome.

You might think that saying "You're accelerating in time" is obvious but it goes much deeper.

If gravity and acceleration are equivalent then gravity is no more than an acceleration but an acceleration in what?  It has to be non-directional in a spatial sense and that only leaves an acceleration in time.  Acceleration contains a time factor, therefore any change in the passage of time will affect acceleration.  The passage of time is slower on the surface of the Earth becoming progressively faster the further away you get.  If the passage of time is slowest on the Earth and the Earth is traveling through time then the Earth must be accelerating in time.

Therefore, mass accelerating through time causes the effect of gravity.

See http://www.thenakedscientists.com/forum/index.php?topic=40746.msg365767#msg365767
« Last Edit: 04/09/2011 14:07:16 by MikeS »
 

Offline JP

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"With respect to  what am I currently accelerating that explains the dent in the springs in the chair I'm sitting on?"

And, while you are at it, why do you think you can add together quantities with different units? And do you consider friction to be an acceleration?

Ok, I'll steal Geezer's thunder a bit.  The acceleration is with respect to an inertial reference frame.  If you were free-falling, you wouldn't dent the springs.  Since you're accelerating with respect to a free-falling reference frame, you dent the springs. 

For the second bit, of course force isn't acceleration.  But calculations within accelerating reference frames require the use of fictitious forces if you want F=ma to hold (centrifugal force is the classic example).  Fictitious forces aren't real forces and are due entirely to acceleration: if you properly write out Newton's laws, accounting for the accelerating coordinate system, you can eliminate the fictitious forces and explain their effects in terms of accelerating reference frames.  Friction wouldn't be an acceleration since it can appear in an inertial reference frame.

If you assume that gravity is a fictitious force, you notice right away that the force of gravity disappears when you're in free fall.  If you then figure out the proper way to change variables to non-free-falling coordinate systems, you can eliminate the force of gravity and describe it in terms of accelerations of coordinate systems with respect to free-fall.  This is what GR does.

All that Geezer seems to be pointing out here is that if you use GR, you can explain the force of gravity as a fictitious force that comes from not properly accounting for accelerating coordinate systems.  Of course, you can also describe gravity as a force and use Newtonian mechanics quite well in most cases that appear in daily life. 
« Last Edit: 04/09/2011 17:41:47 by JP »
 

Offline Bored chemist

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Now we have that sorted out, perhaps we can get back to the OP's point.
They contend that "person inside the air craft reduces its own gravity force due to speed."

And I disagree, because I don't seem to weigh less when I'm in a plane.
In a plane I am travelling fast so, WRT me, the earth is travelling fast. But it still seems to hold the same attraction for me.
As I said, we are not talking about the effect of altitude on gravity here, nor the effect of the plane rising or falling.
Just to keep it simple we are on a plane in level flight (i.e. at a constant height WRT the Earth's C of G).
Given that starting point, it seems to me that the OP is mistaken.
 

Offline JP

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Now we have that sorted out, perhaps we can get back to the OP's point.
They contend that "person inside the air craft reduces its own gravity force due to speed."
I agree.  Speed shouldn't effect gravitational force.

Quote
And I disagree, because I don't seem to weigh less when I'm in a plane.
In a plane I am travelling fast so, WRT me, the earth is travelling fast. But it still seems to hold the same attraction for me.
As I said, we are not talking about the effect of altitude on gravity here, nor the effect of the plane rising or falling.
Just to keep it simple we are on a plane in level flight (i.e. at a constant height WRT the Earth's C of G).
Given that starting point, it seems to me that the OP is mistaken.

I think what the poster might be describing (though the question was worded in a confusing manner) is that you appear weightless when in orbit.  As your plane goes faster and faster maintaining a constant distance from the earth's center of mass, your weight as measured on a scale on board the plane will decrease.  We can argue about whether that's a proper definition of weight, but so far as you and the scale are concerned, it's reading a lower number.  If your plane is actually a spaceship, and you reach a stable orbit, the scale reads zero.  But it's not your speed causing this.  It's the fact that you have to be falling to maintain a constant distance from the earth's center of mass, and falling makes you appear to weigh less on that scale. 
 

Offline Geezer

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I think what the poster might be describing (though the question was worded in a confusing manner) is that you appear weightless when in orbit. 


But you don't need to be in orbit. To become weightless, all you need to do is jump so that your feet are no longer in contact with anything.
 

Offline MikeS

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I think what the poster might be describing (though the question was worded in a confusing manner) is that you appear weightless when in orbit. 


But you don't need to be in orbit. To become weightless, all you need to do is jump so that your feet are no longer in contact with anything.

Whilst you are accelerating upwards you weigh more.  At the top of the jump you weigh the same and only on the way down do you start to weigh less.  It's got nothing to do with your feet touching the floor.  It's all to do with acceleration in space-time.
 

Offline Geezer

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I think what the poster might be describing (though the question was worded in a confusing manner) is that you appear weightless when in orbit. 


But you don't need to be in orbit. To become weightless, all you need to do is jump so that your feet are no longer in contact with anything.

Whilst you are accelerating upwards you weigh more.  At the top of the jump you weigh the same and only on the way down do you start to weigh less.  It's got nothing to do with your feet touching the floor.  It's all to do with acceleration in space-time.

Not really. You are literally "weightless". It's only possible to "weigh" something when it is in contact with something else. You do weigh more while your legs are accelerating your mass and your feet are still in contact with the ground, but as soon as they leave the ground, you are weightless.
 

Offline JP

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I think what the poster might be describing (though the question was worded in a confusing manner) is that you appear weightless when in orbit. 


But you don't need to be in orbit. To become weightless, all you need to do is jump so that your feet are no longer in contact with anything.

True.  I was trying to figure out what the OP meant.  He mentioned flying in a plane and microgravity, so I thought about orbits.  If you free fall and weigh yourself on a scale, you get zero weight.

The force of gravity isn't reduced in any case (according to Newton). 
 

Offline MikeS

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I think what the poster might be describing (though the question was worded in a confusing manner) is that you appear weightless when in orbit. 


But you don't need to be in orbit. To become weightless, all you need to do is jump so that your feet are no longer in contact with anything.

Whilst you are accelerating upwards you weigh more.  At the top of the jump you weigh the same and only on the way down do you start to weigh less.  It's got nothing to do with your feet touching the floor.  It's all to do with acceleration in space-time.

Not really. You are literally "weightless". It's only possible to "weigh" something when it is in contact with something else. You do weigh more while your legs are accelerating your mass and your feet are still in contact with the ground, but as soon as they leave the ground, you are weightless.

Geezer
You are quite right if using the purely gravitation explanation "defining the weight of an object as the force measured by the operation of weighing it (using a force-sensitive scale, such as a spring scale), in vacuum."
but there is another definition defining it as "When used to mean force, its magnitude (a scalar quantity), often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g;[3] thus: W = mg."
http://en.wikipedia.org/wiki/Weight
 

Offline Geezer

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Mike,

Wikipedia isn't always correct. The g in W=mg is constant which means that W must be proportional to mass, but it clearly is not. If you stand on a scale and bend your knees, your weight varies substantially.
 

Offline CPT ArkAngel

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Gm1m2/r^2 = m1a

a = Gm2/r^2, not really a constant, only for a specific geodesic orbit it is constant.

 

Offline MikeS

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Mike,

Wikipedia isn't always correct. The g in W=mg is constant which means that W must be proportional to mass, but it clearly is not. If you stand on a scale and bend your knees, your weight varies substantially.

Surely W=mg is just a way of knowing the weight of an object without actually 'weighing' it.  When you bend your knees your rest mass remains the same but you have added an inertial mass component.  Likewise, when you jump off the ground the action of jumping gives you inertial mass which is 'heavier' than your rest mass.  At the top of the jump you have lost all of the inertial mass and your weight returns to normal.
 

Offline Geezer

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Surely W=mg is just a way of knowing the weight of an object without actually 'weighing' it. 


W=mg is an approximation for weight assuming that is only valid in particular cases. It's an unreliable method of determining weight in general. It's not easy to alter your mass, but it's very easy to alter your weight.

It's a definition thing. Weight is a measure of net force on an object. If the forces change, the weight has to change.
 

Offline Bored chemist

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Weight is a measure of the force that local gravity exerts on something.
So, while W=mg, g isn't a constant.

Currently I weigh about 700 N, but if I moved to the moon, I would weigh about a sixth of that.
If I moved upstairs I would weigh slightly less than I do here.
 

Offline Geezer

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Weight is a measure of the force that local gravity exerts on something.
So, while W=mg, g isn't a constant.

Currently I weigh about 700 N, but if I moved to the moon, I would weigh about a sixth of that.
If I moved upstairs I would weigh slightly less than I do here.

True. Not only that, but g does not take into account the force produced by the atmosphere that's trying to lift you off the scales.

A mass of 1 kg can have a negative weight on Earth if it happens to be hydrogen gas.
 

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