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Feb 28th 2018, 07:26 PM   #1
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Active mass

 Originally Posted by topsquark Let's get a fresh start here. -Dan
Good idea Dan .....

Deep breath ....

Dear friends .... I have seen in literature the terms "active gravitational mass" and "passive gravitational mass" , here's a quote ...

"We might distinguish two kinds of gravitational mass, active and passive. The active gravitational mass is the source of the object's gravitational field, while the passive gravitational mass responds to it. "

I wonder if someone could explain these terms , why it was necessary to introduce them ... any experimental evidence to justify their use ...

Please keep it simple , with minimal technical jargon , imagine you are explaining things to a 16 year old with good physics knowledge.

Many thanks ....oz

 Mar 1st 2018, 06:34 AM #2 Senior Member   Join Date: Aug 2010 Posts: 369 We know that the gravitational force between two objects with masses m and M and distance r apart is $\displaystyle \frac{GmM}{r^2}$. m here, because the force is proportional to it, so it "causes" the force, is the "active mass". We also know that, subject to a force, F, a body of mass m reacts by accelerating with acceleration $\displaystyle \frac{F}{m}$. m here, because it is "reacting" to the force, is the "passive mass". There is no "a-priori" reason that these two masses should be the same but repeated experimentation has shown that they are which is why we don't often use the terms "active" and "passive" mass. topsquark likes this.
 Mar 1st 2018, 10:33 AM #3 Senior Member     Join Date: Jun 2016 Location: England Posts: 642 Was it not Newton's intention, when defining the Gravity equation, that the masses referred to should be the same physical property as the mass in Force = Mass x Acceleration? If there is no theoretical requirement that these quantities be the same, I would suggest that the fact that they do seem to be the same, points toward a gap in the theory where this requirement link should be. __________________ ~\o/~
Mar 1st 2018, 10:41 AM   #4
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 Originally Posted by Woody Was it not Newton's intention, when defining the Gravity equation, that the masses referred to should be the same physical property as the mass in Force = Mass x Acceleration? If there is no theoretical requirement that these quantities be the same, I would suggest that the fact that they do seem to be the same, points toward a gap in the theory where this requirement link should be.
I would love to know why everyone keeps ignoring one simple fact - that there is zero evidence that inertial mass and gravitational mass are equal, that they can only be shown to be proportional.

I've said this before and I was ignored. Why?

Mar 1st 2018, 11:11 AM   #5
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 Originally Posted by Pmb I would love to know why everyone keeps ignoring one simple fact - that there is zero evidence that inertial mass and gravitational mass are equal, that they can only be shown to be proportional. I've said this before and I was ignored. Why?
Perhaps because there is more too it than that.

I have never come across the terms active and passive mass before and frankly I don't see the need for them, in fact they obscure Newton's laws as does the idea of proportionality.

 Mar 1st 2018, 07:30 PM #6 Senior Member   Join Date: Apr 2017 Posts: 403 Again we have slipped into a discussion of the equivalence of gravitational and inertial mass . This is not the question ... for the moment lets forget all about mass's inertial properties .. We are solely discussing gravitational effects ... Someone has put forward the idea of of two components in the gravitational effect . Here's the quote again ... "We might distinguish two kinds of gravitational mass, active and passive. The active gravitational mass is the source of the object's gravitational field, while the passive gravitational mass responds to it. " I can understand confusion over this , because it doesn't make sense !
Mar 1st 2018, 07:52 PM   #7
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 Originally Posted by Pmb I would love to know why everyone keeps ignoring one simple fact - that there is zero evidence that inertial mass and gravitational mass are equal, that they can only be shown to be proportional. I've said this before and I was ignored. Why?
Well you have to spell out what you mean , again in more detail ...

Think of the many readers who never post... explain everything as if talking to a 16 year old ...

Let me have a go at explaining why they're proportional

F=Ma .... we have defined the meter ,and defined the second , and defined the Killo and by these definitions we can say 1N causes a mass of 1 Kg to accelerate by 1 m sec 2...

Now to gravity .... F= G mM/r2 ...... Look at that G !!

We have a G in there ... so gravitational and inertial mass are proportional .

If G was 1 then they would be equal !!

Mar 1st 2018, 08:06 PM   #8

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 Originally Posted by oz93666 Well you have to spell out what you mean , again in more detail ... Think of the many readers who never post... explain everything as if talking to a 16 year old ... Let me have a go at explaining why they're proportional F=Ma .... we have defined the meter ,and defined the second , and defined the Killo and by these definitions we can say 1N causes a mass of 1 Kg to accelerate by 1 m sec 2... Now to gravity .... F= G mM/r2 ...... Look at that G !! We have a G in there ... so gravitational and inertial mass are proportional . If G was 1 then they would be equal !!
Where is the r in F = ma?

-Dan
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 Mar 10th 2018, 04:57 AM #9 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,570 He'll never get it. Not to mention that there's no way that a 16 year old can understand everything in physics. But I'm certain most will understand what active, passive and inertial mass is. What he'll never understand is why the entire physics community doesn't think like he does. topsquark likes this.
 Mar 10th 2018, 11:11 AM #10 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,570 This reminds me of electrodynamics. The terms active charge and passive charge can be used there too. Active charge would be described by the 4-current 4-vector while passive charge by the scalar charge of a particle.

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