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Offline Ricardo Pedri Lopes

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Basic question about gravity
« on: 19/12/2010 19:10:53 »
I've searched for this on Google but I didn't found it, so here I go:

If every body with a mass applies gravitational force on other objects, does it mean that my body applies gravity on a pen, for example?

If so, isn't the pen supposed to be attracted by my body, and vice-versa?


 

Offline QuantumClue

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Basic question about gravity
« Reply #1 on: 19/12/2010 19:13:03 »
I've searched for this on Google but I didn't found it, so here I go:

If every body with a mass applies gravitational force on other objects, does it mean that my body applies gravity on a pen, for example?

If so, isn't the pen supposed to be attracted by my body, and vice-versa?

Of course.

Even objects on earth are subject to gravitational laws.
 

Offline QuantumClue

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Basic question about gravity
« Reply #2 on: 19/12/2010 19:14:04 »
It should be noted, that your force on the pen is insignificant next to the strength of the earth, vanishingly small in fact, because gravity itself is so weak.
 

Offline Bill S

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Basic question about gravity
« Reply #3 on: 19/12/2010 19:58:10 »
Enter Mark McCutcheon, who maintains that gravity is a myth.  In his theory, if you pick up a stone, then release it, it does not fall to the ground, the ground comes up to meet it, because all matter in the Universe is constantly expanding.
Because everything expands at the same rate we don't notice it. [8D]
 

Offline imatfaal

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Basic question about gravity
« Reply #4 on: 20/12/2010 11:54:15 »
The guy is a fruitcake who doesn't even belong in New Theories board let alone Physics.
 

Offline lightarrow

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Basic question about gravity
« Reply #5 on: 20/12/2010 17:08:35 »
I've searched for this on Google but I didn't found it, so here I go:

If every body with a mass applies gravitational force on other objects, does it mean that my body applies gravity on a pen, for example?

If so, isn't the pen supposed to be attracted by my body, and vice-versa?
gravitational force is

F = -G*m1*m2/r2

F = force
G = Newton's gravitational constant ~ 6.67*10-11 N*m2*kg-2
m1 = your mass; (kilograms)
m2 = pen's mass; (kilograms)
r = distance between your centre and the pen's centre (I assume here both bodies are spherical, for simplicity); metres.

Give values to the masses and to r, then compute the gravitational force between you and the pen.
 

Offline Ricardo Pedri Lopes

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Basic question about gravity
« Reply #6 on: 21/12/2010 01:59:37 »
The guy is a fruitcake who doesn't even belong in New Theories board let alone Physics.

Seems like someone is so frustrated with life that cares enough to attack random strangers.
 

Offline peppercorn

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Basic question about gravity
« Reply #7 on: 21/12/2010 02:03:23 »
The guy is a fruitcake who doesn't even belong in New Theories board let alone Physics.

Seems like someone is so frustrated with life that cares enough to attack random strangers.

I'm certain Imatfaal is belittling Mark McCutcheon's 'ideas' not you.
 

Offline Geezer

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Basic question about gravity
« Reply #8 on: 21/12/2010 04:09:36 »
Ricardo,

I agree with Peppercorn. Imatfaal is not in the habit of attacking random strangers on the forum. Of course, I can't vouch for his actions when he's not on the forum  :D

BTW - I saw a comment where someone suggested that McCutcheon was giving alternative physics "a bad name"!!
 

Offline imatfaal

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Basic question about gravity
« Reply #9 on: 21/12/2010 12:29:03 »
Bom Dia Ricardo - apologies if you thought you I was insulting you - my ire was directed entirely at Mark McCutcheon and I should have made that clear. 
 

Offline Ricardo Pedri Lopes

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Basic question about gravity
« Reply #10 on: 21/12/2010 13:30:55 »
Bom Dia Ricardo - apologies if you thought you I was insulting you - my ire was directed entirely at Mark McCutcheon and I should have made that clear. 

Hello Matthew,

Thanks for not taking my reply personally. I shouldn't have assumed that your message was directed to me and I apologize for that. [:I]

Have a great day :)
 

Offline yor_on

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Basic question about gravity
« Reply #11 on: 21/12/2010 20:20:38 »
Then the stone would have to expand proportionally right?
So, how do they meet?
 

Offline yor_on

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Basic question about gravity
« Reply #12 on: 21/12/2010 23:39:07 »
Ah, now I see, the air gets thrown out, right?
So in the end we have a giant hand holding a giant stone touching an incredibly giant planet, and no air.

I'm right, I know I am..
 

Offline imatfaal

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Basic question about gravity
« Reply #13 on: 22/12/2010 11:53:08 »
 ;D Yor-on - you have a better grasp of the theory than Mark McCutcheon
 

Offline Foolosophy

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Basic question about gravity
« Reply #14 on: 22/12/2010 12:47:04 »
The guy is a fruitcake who doesn't even belong in New Theories board let alone Physics.

Seems like someone is so frustrated with life that cares enough to attack random strangers.

what is a random stranger?
 

Offline syedbukhari39

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Basic question about gravity
« Reply #15 on: 23/12/2010 08:13:41 »

F = -G*m1*m2/r2


Take a flask and put 1 Kg of High speed diesel.
Now take 1 Kg of water and put it in the flask, you will observe water settle down in the bottom of the flask and HSD will float up.
What causes the difference in behaviors although the mass is constant for both?
 

Offline syhprum

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Basic question about gravity
« Reply #16 on: 23/12/2010 09:18:33 »
Lightarrow

When your quote the familiar formula for the gravitational force between two bodies you use -G does this merely signify that the force is attractive ?.
 

Offline syedbukhari39

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« Reply #17 on: 23/12/2010 10:04:56 »
Yes
only for attractive force
 

Offline imatfaal

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Basic question about gravity
« Reply #18 on: 23/12/2010 11:41:53 »
Lightarrow
When your quote the familiar formula for the gravitational force between two bodies you use -G does this merely signify that the force is attractive ?.

Yes it does Syhprum.  Force is a vector - it has magnitude and direction, the other side of the equation G, masses, and distance have no direction ie they are scalars.  If you wish to be really obsessive you can stick in a unit vector like this:
F12=Gm1m2r-2. r-hat12

in words that is the force exerted by 1 on 2 is equal to Gm1m2r-2 times by the unit vector from from 1 to 2.  The force is thus in the opposite direction to the unit vector - thus we put in a minus sign in to show the reversal of direction.

Reading above I think I have just confused the situation. 

Second try - the direction of the force exerted by the earth on me is the opposite of the direction from the centre of earth towards me. 
 

Offline lightarrow

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Basic question about gravity
« Reply #19 on: 23/12/2010 12:58:46 »
Lightarrow

When your quote the familiar formula for the gravitational force between two bodies you use -G does this merely signify that the force is attractive ?.
Yes.
The formula should be written vectorially:

F = -G (m1*m2/r123)*r12

F = vector force that the point mass m1 makes on the point mass m2.
r12 = vector from the point mass m1 to the point mass m2.

So you see that the force is opposite to the vector r12 and so the force goes from m2 to m1 (m1 attracts m2).

 

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Basic question about gravity
« Reply #19 on: 23/12/2010 12:58:46 »

 

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