10 ms·

This probably isn't the right place for this question. But the right kind of people will be reading this thread so I will ask it. In college physics my teacher

This probably isn't the right place for this question. But the right kind of people will be reading this thread so I will ask it. In college physics my teacher insisted that, despite gravity being popularly referred to as a force, it is not a force. Weight is indeed a force, but gravity is more like a field. I understood it like this: Say you have a point mass in an isolated system. There is certainly gravity all around that mass, but there are no forces anywhere in the system. Not until another mass is introduced into the system do you have any forces. It is apparent from the formula for force since then you have that new mass times the acceleration of gravity. Or is this just being pedantic and does it even matter?

TchoBeer 5y ago
Isn't the same true of electromagnetism?
cyphar 5y ago
Seems needlessly pedantic to me, since by that definition electromagnetism isn't a force either -- it's not possible to have a force on a charged object without there being another object that creates the field the first object is interacting with (and vice versa). In fact, Newton's third law kind of implies that it's impossible to have any force in isolation. The same formula argument would apply to Coulomb's law. Now, is it fair to point out that you can model these forces as fields, and that such a model is very useful? Yeah. But it's silly to argue that something is or is not a "force" but is instead a "field" -- these are mathematical constructs used in the models we use for physics. I mean, it's not really accurate to say that "forces" and "fields" exist in the first place (in the sense that the mathematical models themselves are a real thing that exist in the world -- they are simply models). And in the end it definitely doesn't matter, neither the maths nor the real world cares whether you personally consider gravity a force or not. (I expect this is also the case for the strong and weak forces, but I haven't done post-grad physics.)
- derefr 5y ago
  EM isn’t a force. EM flux is a force. The pedantry is important because it tells you which formulae/units are compatible with one-another. Weight (~gravitational force), or EM flux (~electromotive force), being forces, can be directly translated into impulse in a dynamic system—reduced to an instantaneous acceleration upon a mass, and thus into the fundamental units of meters, seconds, and grams/mols.
  - cyphar 5y ago
    Dimensional analysis seems to me to be a far more fool-proof way of knowing whether a quantity should be used with a particular formula. Also the other issue is that the word "force" in physics doesn't just refer to F=ma, it also refers to the general concept of an interaction mediator. That's why they're called the four fundamental forces -- meaning that saying "gravity is not a force, it's a field" is not only overly pedantic but is also arguably wrong if you take it literally.
monster_group 5y ago
IANAP but wanted to say this - Same argument could be made for EM force. There is electric field around a charge but only when you place another charge the two charges attract or repel as per electromagnetic force. That way gravity and EM seem to be similar. But gravity does seem to be different as it can be thought of as not a force (in agreement with your physics teacher). It curves space and time and objects just move along the geodesics (straight lines on curved surfaces). So gravity gives rise to space time geometry. Once that happens, you can stop thinking about gravity as a force. Worth emphasizing again - IANAP.
- geomark 5y ago
  Right. Just like the force between opposite charges. That's how I've always thought of it. Small nit - I think it is actually called electrostatic force, at least when we are talking about charges at rest.
  - hexane360 5y ago
    Is there a way to describe electrostatics as objects following inertial trajectories through curved space?
- DangitBobby 5y ago
  See, that makes sense. I've heard it said before that gravity is not a force, and my thoughts were just that it obviously is a force. And of course here we are, cutting through the pedantry to learn that they were in fact not communicating anything actually meaningful with the phrase. You could simply say that potential energy and kinetic energy are different and it would be just as meaningful as what people mean when they say gravity is not a force. It's misleading. A single particle creates a gravitational potential that exerts nothing on its own. Okay. I say this as someone with a BS in Physics. Thank you for your explanation.
  - kjeetgill 5y ago
    If you haven't seen it yet, this thread hes devolved into a refutation of OPs recollection of their teacher's explanation. There are other people rice chimed in about why Gravity isn't a force: Basically because gravity is space-time curvature and it's objects in free fall that are at rest.
deleted 5y ago
[deleted]
jessermeyer 5y ago
In a Newtonian context, gravity is a force. In an Einsteinian context, forces are not even discussed.
- Enginerrrd 5y ago
  They are, in the context of F=ma where m is the inertial mass and an object is deviating from a spacetime geodesic, which explains why we feel a force from gravity as we sit here.
  - lupire 5y ago
    Do you feel a force from gravity? I thought you feel the electromagnetic force (normal force) of your chair pushing you away from the Earth and stopping your free fall.
    - DangitBobby 5y ago
      If not for the gravitational force, the normal force would have nothing to exert. So I guess you feel the magnitude of the force from gravity via the normal force.
  - tsimionescu 5y ago
    Well, if you are currently at rest in relation to the surface of the earth, that means you are moving with speed c towards the future (the ct direction in Minkowsky space time). The shortest path from the past to the future, near a large mass like the Earth, is curved towards the center of the Earth. So, because of inertia, your trajectory in spacetime will be curved towards the center of the Earth. However, the surface you are standing on is preventing this motion via an electromagnetic interaction.
im3w1l 5y ago
You can have a gravitational field with waves in it without any charges at all. Place a single test particle in this field and it be affected by the field
forgotpwd16 5y ago
Mathematically all interactions can be described as fields. Where gravity differs from other interactions/forces is that it lacks, according to standard model but hypothesized to exist, a carrier (boson). Instead gravity is described by general relativity as the result of curved spacetime. Edit: This is what article is about and how double copy helps with this. The previous is described in the article's second section.
- criddell 5y ago
  > Mathematically all interactions can be described as fields. Is there any other way a field can be described? What's the physical reality of a field?
  - forgotpwd16 5y ago
    Particles as shown in most introductory particle physics courses. Because of particle/wave (alternatively particle/field) duality the question of whether particles or fields are the fundamental constituents remains open but the current consensus is the later (fields).
    - criddell 5y ago
      Describing a field in terms of particles seems circular if particles are excitations of a quantum field.
      - forgotpwd16 5y ago
        Indeed. That's why it is a hard question to answer which is fundamental.
- DangitBobby 5y ago
  How does it being a consequence of curved spacetime make it not a force? When we learn the more fundamental origins behind the other forces, will they also cease to be forces? It's not helpful, IMO.
teachingassist 5y ago
I say that 'weight is the force due to gravity' - that weight is the force and gravity is the abstract concept (in college-level physics terms). Same for 'EM force' and 'electromagnetism' Conflating the two is a type of 'metonym' that is common in every-day and journalistic speech as here. If it's obvious what aspect you mean, then it doesn't really matter.
_Microft 5y ago
Gravity is what we call it when objects interact via their masses. Other fundamental interactions are electromagnetism, the weak or the strong interaction (often also called "weak (nuclear) force", "strong (nuclear) force"). In the static case (i.e. time-independent), there is a (let's e.g. focus on electric / gravitational) potential. It's spatial change ("gradient" / "derivative in space") is its electric / gravitational field which is also the ratio of the force on an infinitesimally small charge/mass at the given distance to the charge/mass (it has to be negligibly small so that it does not influence the field). So there are few different concepts that usually all get mixed up and lead to some confusion. It hope the overview helps a bit. If you want to read more, look up the italic terms in Wikipedia.
climech 5y ago
Veritasium made a cool video about this some months ago: https://www.youtube.com/watch?v=XRr1kaXKBsU https://www.youtube.com/watch?v=XRr1kaXKBsU
alberth 5y ago
This video really helped me understand why it’s not a force. Also awesome physics channel as well. https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU
jerf 5y ago
In General Relativity, gravity is not a force. Despite the fact we know that GR is broken and can't be a description of reality, even educated physicists seem to persist in speaking as if it is absolutely true, even though they know better. See also all the talk about "what happens in a black hole", which should almost always be qualified with "in General Relativity". When we have a unified theory a lot of the crazy stuff, like rotating singularities leading to "other universes", will probably disappear. Is gravity going to be a force in the Unified Theory? Probably. I believe it is in our current best candidates. But they aren't right either yet, so I don't know. Edit: Some modders seem to be confused and are probably reading this as some sort of criticism of science itself or something. It is not. It is literally true. It is not a force in GR, but that doesn't make it "not a force", because GR is known to be false. It is an exceedingly good approximation to something, but we know it is not the underlying truth, which is why we are still seeking out a Unified Theory... precisely because GR isn't it. Thus, I find confident proclamations about whether or not gravity is or is not a force to be premature. We don't know. The thing it is definitely not a force in is known to be not the truth. So to the extent you are confused about why it isn't a force... well, stay tuned, because this whole area is ripe for reconsideration in the next few decades. There are also theories that have attempted to rewrite all of physics as "not a force"s like gravity too, turning all forces into geometry. While these are not currently favored, IIRC these are the orginal physics theories that added rolled-up dimensions. String theory built on those.
cygx 5y ago
Nope, that's not what a physicist means when they say gravity isn't a force. What is meant is that apparent gravitational acceleration can be understood as a consequence of Newton's first instead of his second law. According to Newton's first law, a body on its own (ie with no net external forces acting) will continue to move uniformly in a straight line. Now, drag a marker uniformly and in a straight line (from your perspective) across a spinning disc. It will trace out a curved line, meaning an observer sitting on the disc will conclude that a force must have been present as the line would have been straight otherwise. We call this particular apparent force the Coriolis force, one of the pseudo-forces (aka fictitious forces or inertial forces) present in rotating reference frames. Being accelerated by such a pseudo-force won't register on an accelerometer, and the force vanishes if we analyze the motion from an inertial reference frame. According to General Relativity, gravity is like that, a pseudo-force, except that there's generally no frame that can make gravitational (pseudo-)forces vanish in an extended region. Taking a differential-geometric perspective, we note that there's no inherent notion of "continuing on in the same direction" on arbitrary manifolds. We need additional structure such as a covariant derivative, which gives us a notion of velocity change along a trajectory. Gravity hooks into that, with bodies in free-fall moving in 'straight lines' according to the Levi-Civita connection of spacetime.
- dilippkumar 5y ago
  > ...Being accelerated by such a pseudo-force won't register on an accelerometer... Just checked the accelerometer on my phone. It reads around 9.8m/s/s pointing downwards.
  - m4r35n357 5y ago
    _You_ are providing that acceleration. You need to drop it!
  - cygx 5y ago
    Assuming you're standing on the surface of the earth, that's wrong: You're being accelerated upwards, not downwards. Cf the introductory paragraph and "Physical principles" section of https://en.wikipedia.org/wiki/Accelerometer https://en.wikipedia.org/wiki/Accelerometer
grae_QED 5y ago
This sums it up [1]. [1] https://www.youtube.com/embed/XRr1kaXKBsU https://www.youtube.com/embed/XRr1kaXKBsU
prof-dr-ir 5y ago
Others have answered your question rather well. Let me offer a perspective. I believe it was Feynman who said that theoretical physicists have a very simple goal: they just wants to predict the future. Now this might be impossible in full generality, but in controlled circumstances (which we usually call "experiments") they can often do a pretty good job of predicting the outcome. As far as we know, predicting the future requires mathematics which is therefore the physicists' main tool. In doing so it is practical to give a name, like 'force' or 'geodesic motion' or 'particle' or 'wave', to certain mathematical concepts. But in my view one should not get too hung up on the precise meaning of any of those words, simply because it is not productive if the mathematics is already clear enough. In fact, I think this is the most common misconception for people with a passing interest in physics. (See also discussions elsewhere on this page...) So allow me to stress this: these words really mean very little without the equations. Of course, mine is just a physicist's perspective. I imagine your question would be the bread and butter for a philosopher.
- deleted 5y ago
  [deleted]
vincent-toups 5y ago
Whether it is pedantic depends on your goals and perspective, but if you are interested in a history and philosophical development of Force that culminates in the observation that gravitational forces are fictitious, I can heartily recommend "Concepts of Force" by Max Jammer.
- polynomial 5y ago
  What about the opposite? A clear explanation of what the strongest evidence for gravity NOT being fictitious but rather being a real force is?
  - vincent-toups 5y ago
    Such a point of view is pretty much incongruous with contemporary physics. I'm not sure how one would go about even formulating the case. Despite its central presence in physics 101 the notion of force is actually not all that clearly present in fundamental physics. Newton's innovation might be said to be the representation of gravity as a force, but even in the Principia he doesn't commit to the idea because of an intuitive rejection of the non-locality implied by such a conception. So the idea that gravity is a force of the same sort as you pushing on a cart full of apples (whatever _that_ is) was never a particularly strongly held idea. I'm not an expert on the history of physics (I have read the Pricipia in translation, though) and I suspect Newton, if questioned appropriately, might have himself admitted there are issues with the conception that go beyond just non-locality. Even at the level of Lagranian physics in the contemporary style (which despite that description is hundreds of years old) the idea of forces has an ad-hoc quality to it. The description of a completely closed system doesn't really contain forces unless you explicitly include them in the form of lagrange multipliers for the constraints, and then the forces that you calculate are, in a sense, provided "externally" by processes outside of the system itself. For instance, the easiest way to represent the motion of two particles held together by a perfectly rigid rod doesn't mention the force exerted by the rod to maintain that constraint and yet simulates the motion perfectly. If we are interested in how much force the rod must exert to maintain the constraint, we can add in a largrange multiplier, but what that quantity tells us is in a sense a summary of the non-represented physics of the rod itself, which is still excluded from the system. A force then is a kind of interface between that which you are considering in detail and that which is assumed to be part of the external world. If everything in the world were simulated, then nothing would deviate from its expected motion, and thus there would be no forces at all, since all a force is is a description of how something deviates from the path it would ordinarily take if not for the intervention. On this view Newton's first law is much more important and fundamental than his second: objects always move in straight lines but we must take great care in understanding what a straight line actually is for a given physical system. Really, the seed of general relativity and the geometric formulation of field theories is right there in the first law. This explains why the second law is of so much less importance in modern physics. I'm not a professional, just an interested failed physicist, so standard equivocations apply to the above. This video by an actual professional hits on some of these ideas: https://www.youtube.com/watch?v=IBlCu1zgD4Y https://www.youtube.com/watch?v=IBlCu1zgD4Y
agumonkey 5y ago
I thought it was an abstraction trick. Just like they derive the electric field of a charge from abstracting away the second charge from the classic electric force equation.
jhardy54 5y ago
I’d highly recommend this explanation: https://m.youtube.com/watch?v=XRr1kaXKBsU https://m.youtube.com/watch?v=XRr1kaXKBsU
kazinator 5y ago
Yes, gravity is "more like a field". But what kind of field? A force field. The right kind of object in the right kind of force field experiences a force. A mass experiences a force in a gravitational field; a charge experiences a force in an electric field. A point mass is difficult to reason about because a point mass has infinite density, and a gravitational field that gets arbitrarily large the closer that the point is approached. Non-point masses, like uniform spheres, have a gravitational field that increases up to their surface, and then decreases. A particle at the exact centre of a uniformly dense field experiences no net force. Also, a particle floating inside a hollow sphere of uniform thickness experiences no net force; the field sums to zero everywhere inside. This is why the field gets weaker toward the centre of a solid sphere. Every point inside a solid sphere can be regarded as simultaneously being inside a hollow sphere (experiencing no net force from that), and being just on the surface of a smaller interior sphere: that part of the sphere which is deeper than the particle.

TchoBeer 5y ago

Isn't the same true of electromagnetism?

cyphar 5y ago

Seems needlessly pedantic to me, since by that definition electromagnetism isn't a force either -- it's not possible to have a force on a charged object without there being another object that creates the field the first object is interacting with (and vice versa). In fact, Newton's third law kind of implies that it's impossible to have any force in isolation. The same formula argument would apply to Coulomb's law. Now, is it fair to point out that you can model these forces as fields, and that such a model is very useful? Yeah. But it's silly to argue that something is or is not a "force" but is instead a "field" -- these are mathematical constructs used in the models we use for physics. I mean, it's not really accurate to say that "forces" and "fields" exist in the first place (in the sense that the mathematical models themselves are a real thing that exist in the world -- they are simply models). And in the end it definitely doesn't matter, neither the maths nor the real world cares whether you personally consider gravity a force or not. (I expect this is also the case for the strong and weak forces, but I haven't done post-grad physics.)

derefr 5y ago
EM isn’t a force. EM flux is a force. The pedantry is important because it tells you which formulae/units are compatible with one-another. Weight (~gravitational force), or EM flux (~electromotive force), being forces, can be directly translated into impulse in a dynamic system—reduced to an instantaneous acceleration upon a mass, and thus into the fundamental units of meters, seconds, and grams/mols.
- cyphar 5y ago
  Dimensional analysis seems to me to be a far more fool-proof way of knowing whether a quantity should be used with a particular formula. Also the other issue is that the word "force" in physics doesn't just refer to F=ma, it also refers to the general concept of an interaction mediator. That's why they're called the four fundamental forces -- meaning that saying "gravity is not a force, it's a field" is not only overly pedantic but is also arguably wrong if you take it literally.

monster_group 5y ago

IANAP but wanted to say this - Same argument could be made for EM force. There is electric field around a charge but only when you place another charge the two charges attract or repel as per electromagnetic force. That way gravity and EM seem to be similar. But gravity does seem to be different as it can be thought of as not a force (in agreement with your physics teacher). It curves space and time and objects just move along the geodesics (straight lines on curved surfaces). So gravity gives rise to space time geometry. Once that happens, you can stop thinking about gravity as a force. Worth emphasizing again - IANAP.

geomark 5y ago
Right. Just like the force between opposite charges. That's how I've always thought of it. Small nit - I think it is actually called electrostatic force, at least when we are talking about charges at rest.
- hexane360 5y ago
  Is there a way to describe electrostatics as objects following inertial trajectories through curved space?
DangitBobby 5y ago
See, that makes sense. I've heard it said before that gravity is not a force, and my thoughts were just that it obviously is a force. And of course here we are, cutting through the pedantry to learn that they were in fact not communicating anything actually meaningful with the phrase. You could simply say that potential energy and kinetic energy are different and it would be just as meaningful as what people mean when they say gravity is not a force. It's misleading. A single particle creates a gravitational potential that exerts nothing on its own. Okay. I say this as someone with a BS in Physics. Thank you for your explanation.
- kjeetgill 5y ago
  If you haven't seen it yet, this thread hes devolved into a refutation of OPs recollection of their teacher's explanation. There are other people rice chimed in about why Gravity isn't a force: Basically because gravity is space-time curvature and it's objects in free fall that are at rest.

deleted 5y ago

[deleted]

jessermeyer 5y ago

In a Newtonian context, gravity is a force. In an Einsteinian context, forces are not even discussed.

Enginerrrd 5y ago
They are, in the context of F=ma where m is the inertial mass and an object is deviating from a spacetime geodesic, which explains why we feel a force from gravity as we sit here.
- lupire 5y ago
  Do you feel a force from gravity? I thought you feel the electromagnetic force (normal force) of your chair pushing you away from the Earth and stopping your free fall.
  - DangitBobby 5y ago
    If not for the gravitational force, the normal force would have nothing to exert. So I guess you feel the magnitude of the force from gravity via the normal force.
- tsimionescu 5y ago
  Well, if you are currently at rest in relation to the surface of the earth, that means you are moving with speed c towards the future (the ct direction in Minkowsky space time). The shortest path from the past to the future, near a large mass like the Earth, is curved towards the center of the Earth. So, because of inertia, your trajectory in spacetime will be curved towards the center of the Earth. However, the surface you are standing on is preventing this motion via an electromagnetic interaction.

im3w1l 5y ago

You can have a gravitational field with waves in it without any charges at all. Place a single test particle in this field and it be affected by the field

forgotpwd16 5y ago

Mathematically all interactions can be described as fields. Where gravity differs from other interactions/forces is that it lacks, according to standard model but hypothesized to exist, a carrier (boson). Instead gravity is described by general relativity as the result of curved spacetime. Edit: This is what article is about and how double copy helps with this. The previous is described in the article's second section.

criddell 5y ago
> Mathematically all interactions can be described as fields. Is there any other way a field can be described? What's the physical reality of a field?
- forgotpwd16 5y ago
  Particles as shown in most introductory particle physics courses. Because of particle/wave (alternatively particle/field) duality the question of whether particles or fields are the fundamental constituents remains open but the current consensus is the later (fields).
  - criddell 5y ago
    Describing a field in terms of particles seems circular if particles are excitations of a quantum field.
    - forgotpwd16 5y ago
      Indeed. That's why it is a hard question to answer which is fundamental.
DangitBobby 5y ago
How does it being a consequence of curved spacetime make it not a force? When we learn the more fundamental origins behind the other forces, will they also cease to be forces? It's not helpful, IMO.

teachingassist 5y ago

I say that 'weight is the force due to gravity' - that weight is the force and gravity is the abstract concept (in college-level physics terms). Same for 'EM force' and 'electromagnetism' Conflating the two is a type of 'metonym' that is common in every-day and journalistic speech as here. If it's obvious what aspect you mean, then it doesn't really matter.

_Microft 5y ago

Gravity is what we call it when objects interact via their masses. Other fundamental interactions are electromagnetism, the weak or the strong interaction (often also called "weak (nuclear) force", "strong (nuclear) force"). In the static case (i.e. time-independent), there is a (let's e.g. focus on electric / gravitational) potential. It's spatial change ("gradient" / "derivative in space") is its electric / gravitational field which is also the ratio of the force on an infinitesimally small charge/mass at the given distance to the charge/mass (it has to be negligibly small so that it does not influence the field). So there are few different concepts that usually all get mixed up and lead to some confusion. It hope the overview helps a bit. If you want to read more, look up the italic terms in Wikipedia.

climech 5y ago

Veritasium made a cool video about this some months ago: https://www.youtube.com/watch?v=XRr1kaXKBsU https://www.youtube.com/watch?v=XRr1kaXKBsU

alberth 5y ago

This video really helped me understand why it’s not a force. Also awesome physics channel as well. https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU

jerf 5y ago

In General Relativity, gravity is not a force. Despite the fact we know that GR is broken and can't be a description of reality, even educated physicists seem to persist in speaking as if it is absolutely true, even though they know better. See also all the talk about "what happens in a black hole", which should almost always be qualified with "in General Relativity". When we have a unified theory a lot of the crazy stuff, like rotating singularities leading to "other universes", will probably disappear. Is gravity going to be a force in the Unified Theory? Probably. I believe it is in our current best candidates. But they aren't right either yet, so I don't know. Edit: Some modders seem to be confused and are probably reading this as some sort of criticism of science itself or something. It is not. It is literally true. It is not a force in GR, but that doesn't make it "not a force", because GR is known to be false. It is an exceedingly good approximation to something, but we know it is not the underlying truth, which is why we are still seeking out a Unified Theory... precisely because GR isn't it. Thus, I find confident proclamations about whether or not gravity is or is not a force to be premature. We don't know. The thing it is definitely not a force in is known to be not the truth. So to the extent you are confused about why it isn't a force... well, stay tuned, because this whole area is ripe for reconsideration in the next few decades. There are also theories that have attempted to rewrite all of physics as "not a force"s like gravity too, turning all forces into geometry. While these are not currently favored, IIRC these are the orginal physics theories that added rolled-up dimensions. String theory built on those.

cygx 5y ago

Nope, that's not what a physicist means when they say gravity isn't a force. What is meant is that apparent gravitational acceleration can be understood as a consequence of Newton's first instead of his second law. According to Newton's first law, a body on its own (ie with no net external forces acting) will continue to move uniformly in a straight line. Now, drag a marker uniformly and in a straight line (from your perspective) across a spinning disc. It will trace out a curved line, meaning an observer sitting on the disc will conclude that a force must have been present as the line would have been straight otherwise. We call this particular apparent force the Coriolis force, one of the pseudo-forces (aka fictitious forces or inertial forces) present in rotating reference frames. Being accelerated by such a pseudo-force won't register on an accelerometer, and the force vanishes if we analyze the motion from an inertial reference frame. According to General Relativity, gravity is like that, a pseudo-force, except that there's generally no frame that can make gravitational (pseudo-)forces vanish in an extended region. Taking a differential-geometric perspective, we note that there's no inherent notion of "continuing on in the same direction" on arbitrary manifolds. We need additional structure such as a covariant derivative, which gives us a notion of velocity change along a trajectory. Gravity hooks into that, with bodies in free-fall moving in 'straight lines' according to the Levi-Civita connection of spacetime.

dilippkumar 5y ago
> ...Being accelerated by such a pseudo-force won't register on an accelerometer... Just checked the accelerometer on my phone. It reads around 9.8m/s/s pointing downwards.
- m4r35n357 5y ago
  _You_ are providing that acceleration. You need to drop it!
- cygx 5y ago
  Assuming you're standing on the surface of the earth, that's wrong: You're being accelerated upwards, not downwards. Cf the introductory paragraph and "Physical principles" section of https://en.wikipedia.org/wiki/Accelerometer https://en.wikipedia.org/wiki/Accelerometer

grae_QED 5y ago

This sums it up [1]. [1] https://www.youtube.com/embed/XRr1kaXKBsU https://www.youtube.com/embed/XRr1kaXKBsU

prof-dr-ir 5y ago

Others have answered your question rather well. Let me offer a perspective. I believe it was Feynman who said that theoretical physicists have a very simple goal: they just wants to predict the future. Now this might be impossible in full generality, but in controlled circumstances (which we usually call "experiments") they can often do a pretty good job of predicting the outcome. As far as we know, predicting the future requires mathematics which is therefore the physicists' main tool. In doing so it is practical to give a name, like 'force' or 'geodesic motion' or 'particle' or 'wave', to certain mathematical concepts. But in my view one should not get too hung up on the precise meaning of any of those words, simply because it is not productive if the mathematics is already clear enough. In fact, I think this is the most common misconception for people with a passing interest in physics. (See also discussions elsewhere on this page...) So allow me to stress this: these words really mean very little without the equations. Of course, mine is just a physicist's perspective. I imagine your question would be the bread and butter for a philosopher.

deleted 5y ago
[deleted]

vincent-toups 5y ago

Whether it is pedantic depends on your goals and perspective, but if you are interested in a history and philosophical development of Force that culminates in the observation that gravitational forces are fictitious, I can heartily recommend "Concepts of Force" by Max Jammer.

polynomial 5y ago
What about the opposite? A clear explanation of what the strongest evidence for gravity NOT being fictitious but rather being a real force is?
- vincent-toups 5y ago
  Such a point of view is pretty much incongruous with contemporary physics. I'm not sure how one would go about even formulating the case. Despite its central presence in physics 101 the notion of force is actually not all that clearly present in fundamental physics. Newton's innovation might be said to be the representation of gravity as a force, but even in the Principia he doesn't commit to the idea because of an intuitive rejection of the non-locality implied by such a conception. So the idea that gravity is a force of the same sort as you pushing on a cart full of apples (whatever _that_ is) was never a particularly strongly held idea. I'm not an expert on the history of physics (I have read the Pricipia in translation, though) and I suspect Newton, if questioned appropriately, might have himself admitted there are issues with the conception that go beyond just non-locality. Even at the level of Lagranian physics in the contemporary style (which despite that description is hundreds of years old) the idea of forces has an ad-hoc quality to it. The description of a completely closed system doesn't really contain forces unless you explicitly include them in the form of lagrange multipliers for the constraints, and then the forces that you calculate are, in a sense, provided "externally" by processes outside of the system itself. For instance, the easiest way to represent the motion of two particles held together by a perfectly rigid rod doesn't mention the force exerted by the rod to maintain that constraint and yet simulates the motion perfectly. If we are interested in how much force the rod must exert to maintain the constraint, we can add in a largrange multiplier, but what that quantity tells us is in a sense a summary of the non-represented physics of the rod itself, which is still excluded from the system. A force then is a kind of interface between that which you are considering in detail and that which is assumed to be part of the external world. If everything in the world were simulated, then nothing would deviate from its expected motion, and thus there would be no forces at all, since all a force is is a description of how something deviates from the path it would ordinarily take if not for the intervention. On this view Newton's first law is much more important and fundamental than his second: objects always move in straight lines but we must take great care in understanding what a straight line actually is for a given physical system. Really, the seed of general relativity and the geometric formulation of field theories is right there in the first law. This explains why the second law is of so much less importance in modern physics. I'm not a professional, just an interested failed physicist, so standard equivocations apply to the above. This video by an actual professional hits on some of these ideas: https://www.youtube.com/watch?v=IBlCu1zgD4Y https://www.youtube.com/watch?v=IBlCu1zgD4Y

agumonkey 5y ago

I thought it was an abstraction trick. Just like they derive the electric field of a charge from abstracting away the second charge from the classic electric force equation.

jhardy54 5y ago

I’d highly recommend this explanation: https://m.youtube.com/watch?v=XRr1kaXKBsU https://m.youtube.com/watch?v=XRr1kaXKBsU

kazinator 5y ago

Yes, gravity is "more like a field". But what kind of field? A force field. The right kind of object in the right kind of force field experiences a force. A mass experiences a force in a gravitational field; a charge experiences a force in an electric field. A point mass is difficult to reason about because a point mass has infinite density, and a gravitational field that gets arbitrarily large the closer that the point is approached. Non-point masses, like uniform spheres, have a gravitational field that increases up to their surface, and then decreases. A particle at the exact centre of a uniformly dense field experiences no net force. Also, a particle floating inside a hollow sphere of uniform thickness experiences no net force; the field sums to zero everywhere inside. This is why the field gets weaker toward the centre of a solid sphere. Every point inside a solid sphere can be regarded as simultaneously being inside a hollow sphere (experiencing no net force from that), and being just on the surface of a smaller interior sphere: that part of the sphere which is deeper than the particle.