# Is gravity really a force?

## Recommended Posts

Now wait. Hear me out first before jumping to conclusions.

We often take for granted the thought that gravity is a force, but if it were a force, it wouldn't be constant because if it were constant, an object of less mass would accelerate more rapidly than an object of greater mass due to inverse proportionality. Force is mass times accelertion, so if the force were constant, and the mass decreased, then the acceleration must increase. But that's not the case. All free-falling objects in a vacuum affected by the same source of gravity accelerate uniformly.

The only way these two problems could be solved is if you were to say that gravity is not constant, but that wouldn't make sense in Newtonian physics because absolutely nothing about the earth changes, not its mass, not its density, nothing, and according to everything I've been raised to know, you cannot change just one thing in the universe without changing at least one more thing.

And to top it all off, gravity, supposedly a force, is not even measured in Newtons, the standard unit of force. It's measured in the unit of acceleration, 9.8m/s/s, which is what remains constant under gravity, not the force being applied.

So wouldn't it be more accurate if gravity was defined as an acceleration rather than a force? It certainly makes sense to me. Even my physics professor agrees that my logic is water tight.

• Replies 66
• Created

#### Popular Days

We often take for granted the thought that gravity is a force, but if it were a force, it wouldn't be constant because if it were constant, an object of less mass would accelerate more rapidly than an object of greater mass due to inverse proportionality. Force is mass times accelertion, so if the force were constant, and the mass decreased, then the acceleration must increase. But that's not the case. All free-falling objects in a vacuum affected by the same source of gravity accelerate uniformly.

Can you elaborate on what you mean by "constant" here?

##### Share on other sites

You´re pretty much right except with your premise that gravity was the gravitational force acting on an object. You´d like to look up the term "gravitational field". Sadly, the english wikipedia entry on the subject sucks (the german and the spanish version seem ok, in case you speak either language).

EDIT: http://en.wikipedia.org/wiki/Law_of_universal_gravitation under the heading "gravitational field" might be a start.

##### Share on other sites

Gravity IS measured in Newtons. The 9.8m/s^2 thing is just a shortcut approximation, when the object's mass is negligible compared with the Earth.

Remember, gravity is not a case of a lone force acting on successive objects. It's a force of attraction (in Newtonian physics, that is, which is what you're dealing with) BETWEEN objects. The rock pulls on the Earth exactly as much as the Earth pulls on the rock, and that mutual force is proportional to the PRODUCT of their masses divided by the square of the distance between them. In other words, yes, it does exert twice as much force on a 20kg rock as a 10 kg rock, but that's fine, because it's the mass of the rocks that is the "source" of the force in the first place.

##### Share on other sites

Dstebbins: No, gravity is not a force. But I'm afraid your logic is incorrect.

##### Share on other sites

Gravity is a Force. Acceleration due to gravity is not a force - it is an acceleration.

As Cap'n asked, it would be good if you defined 'constant'. Constant with respect to what? I presume you mean mass? If so, then you are correct, the gravitational force on a mass is not a constant with respect to the mass - objects with higher masses get a larger force under gravity. It is the acceleration which is constant.

In other words you seem to be confusing the term 'gravitational force' and 'gravitational acceleration' (the latter of which is the 9.81m/s^2 on the surface of the Earth).

##### Share on other sites

hai,

ur understanding of gravity & gravitational pull has to be changed.

As per sir Issac newton ,Gravitational pull exerted by earth to a body of mass m is F= GMm *t 2/ l 2,.

where G = gravitational constant

M= mass of earth.

t= time

& l = distance from earth.

so if we substitute this to equation F = m x a = GMm t 2/ l 2,.

Here U can see m on both sides will cancell to give an acceleration(g=9.8m/s2) which is not depended on the mass of the object. Any doubts?

##### Share on other sites

As per sir Issac newton ,Gravitational pull exerted by earth to a body of mass m is F= GMm *t 2/ l 2,.

where G = gravitational constant

M= mass of earth.

t= time

& l = distance from earth.

There is no "t" term in the equation

##### Share on other sites

spunnery:

That's what you call an axiomatic explanation. If you start with the G you will end up with acceleration that is independent of mass. Interestingly, it isn't quite correct.

We can see this we conduct an experiment with masses of 1x, 10x, and 100x in free space. The 100x mass stands in for the earth. If we "drop" the 1x and 10x masses together they will "fall" towards the 100x "earth" mass at the same rate, and will "hit the ground" at exactly the same time. Note that the 100x mass is alse attracted towards the smaller masses which act like a single smaller mass of 11x.

If however you repeat the experiment with the 1x mass, then later with the 10x mass, they will collide with the 100x mass after different elapsed times in accordance with the attraction of the larger mass to the smaller.

Obviously with the earth and normal masses this effect is miniscule, but it does demonstrate that G isn't a perfect constant.

Severian:

My "gravity is not a force" answer is not a casual answer. However you might view my explanation as speculative, and this thread is perhaps not the best place for me to give it.

##### Share on other sites

Dstebbins: No, gravity is not a force. But I'm afraid your logic is incorrect.

Refrain from Spouting off this Pseudoscience and claiming it as Fact!

##### Share on other sites

No matter what we call it, gravity is everpresent. In my opinion what it is called means nothing as long as the effects are observed, and recorded using the same wordage.

We often take for granted the thought that gravity is a force, but if it were a force, it wouldn't be constant because if it were constant.

Gravity is a constant in the means of its effects are observed and can consistantly be predicted, given the right information. erg. It can be found as a constant force because the effects are completely dependent on the consistincy of the objects mass and acceleration. On the same note it is NOT seen as a constant like the speed of light. The truth of the matter is that the velocity of light... C, isnt even a constant speed. Light travels slower through any medium including the earth's atmosphere. in the same scence would you not consider light to be called a constant force?

The only way these two problems could be solved is if you were to say that gravity is not constant, but that wouldn't make sense in Newtonian physics because absolutely nothing about the earth changes, not its mass, not its density, nothing, and according to everything I've been raised to know, you cannot change just one thing in the universe without changing at least one more thing.

And to top it all off, gravity, supposedly a force, is not even measured in Newtons, the standard unit of force.

Im not sure here, but, im pretty sure you could solve for the force of gravity between two objects. erg. The force measured in newtons, between two objects in the vaccuum of space.

id post more but the bell rang and im out of time to type.

##### Share on other sites

spunnery:

That's what you call an axiomatic explanation. If you start with the G you will end up with acceleration that is independent of mass. Interestingly, it isn't quite correct.

We can see this we conduct an experiment with masses of 1x, 10x, and 100x in free space. The 100x mass stands in for the earth. If we "drop" the 1x and 10x masses together they will "fall" towards the 100x "earth" mass at the same rate, and will "hit the ground" at exactly the same time. Note that the 100x mass is alse attracted towards the smaller masses which act like a single smaller mass of 11x.

If however you repeat the experiment with the 1x mass, then later with the 10x mass, they will collide with the 100x mass after different elapsed times in accordance with the attraction of the larger mass to the smaller.

Obviously with the earth and normal masses this effect is miniscule, but it does demonstrate that G isn't a perfect constant.

The fact that both the large (100x) and small masses will accelerate does not support the conclusion that G isn't constant. It just means you have to apply the physics correctly.

##### Share on other sites

Swanson: tell me about the 1x mass and 100x mass attracting. Then tell me about the 10x mass and the 100x mass attracting. If you apply the physics correctly I'll be interested to see how you use the G. And if you manage to demonstrate that spunnery was correct, please accept my apologies in advance.

##### Share on other sites

Swanson: tell me about the 1x mass and 100x mass attracting. Then tell me about the 10x mass and the 100x mass attracting. If you apply the physics correctly I'll be interested to see how you use the G. And if you manage to demonstrate that spunnery was correct, please accept my apologies in advance.

Each object feels a force GMm/r^2

From F = ma, they will respectively have accelerations of GM/r^2 (for mass m) and Gm/r^2 (for mass M). From there on it's just a little calculus, integrating and applying the boundary conditions (once for velocity and twice for position). If they are point masses they should meet up at their center of mass, e.g. if m = M then they will meet at r0/2

##### Share on other sites

Hey hey hey hey HEY! One at a time, folks. I can't talk to everyone at once. What do you think I am, a computer?

Anyway, when I say "constant force," I mean suppose the earth's gravity was a force of 2.00 x 10^20 N (I came up with that number out of the blue. It's so large because I imagine the earth would have a large force). That means that an object with a mass of 1.00kg would accelerate to the earth at a rate of 2.00 x 10^20 m/s/s. Likewise, if an object had a mass of 2.00kg, it would accelerate in free fall at a rate of 1.00 x 10^20 m/s/s. It's called inverse proportionality. If c is a constant, then c=xy, or in this case, f=ma. The force formula is just an example of inverse proportionality with different variables. F is constant, so if mass goes up, acceleration must go down.

But that's not the case. The acceleration is constant, so gravity is more like a=fm. That's not the equation for force, so can gravity really be considered a force at all?

##### Share on other sites

Gravity is a force by definition...

http://en.wikipedia.org/wiki/Force

##### Share on other sites

Hey hey hey hey HEY! One at a time, folks. I can't talk to everyone at once. What do you think I am, a computer?

Anyway, when I say "constant force," I mean suppose the earth's gravity was a force of 2.00 x 10^20 N (I came up with that number out of the blue. It's so large because I imagine the earth would have a large force). That means that an object with a mass of 1.00kg would accelerate to the earth at a rate of 2.00 x 10^20 m/s/s. Likewise, if an object had a mass of 2.00kg, it would accelerate in free fall at a rate of 1.00 x 10^20 m/s/s. It's called inverse proportionality. If c is a constant, then c=xy, or in this case, f=ma. The force formula is just an example of inverse proportionality with different variables. F is constant, so if mass goes up, acceleration must go down.

But that's not the case. The acceleration is constant, so gravity is more like a=fm. That's not the equation for force, so can gravity really be considered a force at all?

"The Earth's force" is a phrase that doesn't mean anything. Gravity is a force BETWEEN objects, and is derived from (and proportional to) BOTH of them. Specifically, both their masses. Specifically, the product between them. The force acting on (and exerted by) a 20kg mass is twice that acting on (and exerted by) a 10kg mass because 20 is twice 10, and the Earth hasn't changed.

##### Share on other sites

The acceleration is constant, so gravity is more like a=fm. That's not the equation for force, so can gravity really be considered a force at all?

Have you looked up the term "gravitational field"? Since I seem to be a bit lonely with my view that the term "gravity" does not mean "gravitational force", let me briefly present what I consider gravity:

Gravity is the concept that explains the observed attraction between massive bodies (the gravitational interaction).

Clasically, theories of interactions consist of two parts:

- The field equation that defines the field (the gravitational field, here) as a function of its sources (mass, here) - and describes its dynamics, but that´s not important, here.

- The part that defines the influence of the field on other particles. This part comes as an additional term to the equation of motion of those particles. In Newtonian Gravity, this additional leads to the equation of motion of a particle to alter from $a=0$ to $a = \frac{m}{m} g$, where g is the gravitational field (I´ve explicitely written m/m as a hint on the problem with massless particles, which cannot be described with Newtonian physics, anyways).

So now back to your original post: Whether the term gravity really is defined as the gravitational force or not seems pure semantics to me. What you seem to be bothered about is why the gravity of earth (whatever that is) is not constant but depends on the object it acts on (if it is the gravitational force). My point is quite simple: There is an object (a concept) associated to gravitational interaction, the gravitational field, that has exactly the property you are missing in the gravitational force: It only depends on the source, not on the object it acts on.

##### Share on other sites

Have you looked up the term "gravitational field"? Since I seem to be a bit lonely with my view that the term "gravity" does not mean "gravitational force", let me briefly present what I consider gravity:

Gravity is the concept that explains the observed attraction between massive bodies (the gravitational interaction).

Clasically, theories of interactions consist of two parts:

- The field equation that defines the field (the gravitational field, here) as a function of its sources (mass, here) - and describes its dynamics, but that´s not important, here.

- The part that defines the influence of the field on other particles. This part comes as an additional term to the equation of motion of those particles. In Newtonian Gravity, this additional leads to the equation of motion of a particle to alter from $a=0$ to $a = \frac{m}{m} g$, where g is the gravitational field (I´ve explicitely written m/m as a hint on the problem with massless particles, which cannot be described with Newtonian physics, anyways).

So now back to your original post: Whether the term gravity really is defined as the gravitational force or not seems pure semantics to me. What you seem to be bothered about is why the gravity of earth (whatever that is) is not constant but depends on the object it acts on (if it is the gravitational force). My point is quite simple: There is an object (a concept) associated to gravitational interaction, the gravitational field, that has exactly the property you are missing in the gravitational force: It only depends on the source, not on the object it acts on.

I don’t have a formal education in physics so what I say may be wildly off the mark.

From what I understand one could look at matter really as a composite of subatomic particles really in various arrangements held together or operation via various forces, is that would gives us waves or mechanics, as such either changes or energy or information moves through it, rearranges, overall don’t know what to say.

If so, is it possible to suggest that gravity may not act as a constant because it might be a product of various interactions? I mean looking at the residual strong interaction alone or the fact that some forces if you will cant be fully isolated I come to think that maybe gravity is not some isolatable force or constant or whatever the correct term is for it.

I think understanding subatomic behavior and dark matter would help a lot in understanding gravity, of course I know so little that its really easy for me to be wrong, still does not quench my desire to talk about it though:D

I mean the standard model works without fully explaining gravity, so the math can work and in turn be usable in real life, but it still simply does not account in full detail for gravity, but the model can still work is what I find interesting.

##### Share on other sites

Thanks Swansont. That's perfectly fine. I hang my head in shame. Apologies. All: I was wrong in post 9. Sorry. I mixed up my Ggs and my relativity.

##### Share on other sites

thanks both of u .by mistake 't' came in the equation(this is b'cause i have mixed up something-i am trying to calculate the distance travelled to fix the orbital paths of planets).

##### Share on other sites

Have you looked up the term "gravitational field"? Since I seem to be a bit lonely with my view that the term "gravity" does not mean "gravitational force", let me briefly present what I consider gravity:

Gravity is the concept that explains the observed attraction between massive bodies (the gravitational interaction).

Clasically, theories of interactions consist of two parts:

- The field equation that defines the field (the gravitational field, here) as a function of its sources (mass, here) - and describes its dynamics, but that´s not important, here.

- The part that defines the influence of the field on other particles. This part comes as an additional term to the equation of motion of those particles. In Newtonian Gravity, this additional leads to the equation of motion of a particle to alter from $a=0$ to $a = \frac{m}{m} g$, where g is the gravitational field (I´ve explicitely written m/m as a hint on the problem with massless particles, which cannot be described with Newtonian physics, anyways).

So now back to your original post: Whether the term gravity really is defined as the gravitational force or not seems pure semantics to me. What you seem to be bothered about is why the gravity of earth (whatever that is) is not constant but depends on the object it acts on (if it is the gravitational force). My point is quite simple: There is an object (a concept) associated to gravitational interaction, the gravitational field, that has exactly the property you are missing in the gravitational force: It only depends on the source, not on the object it acts on.

quick question:

Wtf?

##### Share on other sites

You're going to have to be more specific about that. Atheist didn't say anything that was incorrect. "Gravity" is really just the particular relationships between observed behaviors of objects. Whether you describe it as forces or fields or anything else is just a matter of preference. There is certainly no PROBLEM with treating it as a force, however, as I and others have explained.

##### Share on other sites

quick question:

Wtf?

Ok, quick answer then: You´re trying to heat cold coffee to sell it as an energy drink.

##### Share on other sites

Ok, quick answer then: You´re trying to heat cold coffee to sell it as an energy drink.

what does coffee have to do with anything?

## Create an account

Register a new account