3 ms·

Ok, great opportunity for me to ask a dumb question that's been bothering me for a while, for practical reasons I won't go into. How is gravity like a force at

by sfink 6y ago

Ok, great opportunity for me to ask a dumb question that's been bothering me for a while, for practical reasons I won't go into. How is gravity like a force at all, even in Newtonian physics? It seems like mismatched units. Gravity is an acceleration, not a force. F=ma, right? So if gravity were a force, it would produce an acceleration that was dependent on the mass, and it doesn't, so it seems to me like the only sense in which gravity is a force is if you define force as "something you can't see that makes things move", which is a pretty useless definition.

bananabreakfast 6y ago
Gravity is a force that acts due to mass just as the electromagnetic force acts due to charge. Since gravity acts in a magnitude directly proportional to the mass being acted upon, it just so happens that it manifests as an acceleration since the mass cancels out, unlike EM where acceleration due to charge does not cancel out the mass.
aflag 6y ago
According to Newton's equation, gravity's force is F=(GmM) / r^2. Those are the gravitational masses. The mass in F = ma is the inertial mass. The fact that they are the same is remarkable. If it weren't for that, different mass objects would feel different accelerations due to gravity.
caf 6y ago
In Newtonian physics it's a force with a magnitude that is proportional to the mass of the body. For two different objects at the same point in the same gravitational field, the difference in gravitational force due to their differing mass exactly cancels out the difference in acceleration due to their differing mass, so they both accelerate at the same rate.
- sfink 6y ago
  Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water. My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known acceleration, and that means you need to multiply by the mass. Clearly different masses will just cancel, so the resulting acceleration is the same. I'm fine with saying "force due to gravity" or even "gravitational force". Which is what you're describing in your first sentence, and I have no disagreement with that. "Force of gravity" starts to sound a little off, and I bet if I ask "what is gravity at Earth's surface?" I'll get back "9.8 m/s^2", which is an acceleration not a force.
  - caf 6y ago
    Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water. I don't see how. A "force between two bodies with a magnitude proportional to the product of their masses and inversely proportional to the square of the displacement between them" seems no different in principle from a "force between two bodies with a magnitude proportional to the product of their electric charges and the inversely proportional to the square of the displacement between them". My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known acceleration, and that means you need to multiply by the mass. Clearly different masses will just cancel, so the resulting acceleration is the same. We're talking Newtonian physics, a model of the world where an acceleration is the result of an unbalanced force applied to a mass. You don't have to "reverse-engineer it from the known acceleration", you can calculate it as Gm₁m₂/r². I'm fine with saying "force due to gravity" or even "gravitational force". Which is what you're describing in your first sentence, and I have no disagreement with that. "Force of gravity" starts to sound a little off, and I bet if I ask "what is gravity at Earth's surface?" I'll get back "9.8 m/s^2", which is an acceleration not a force. You can equally describe the magnitude of the gravitational field as 9.8 Nkg⁻¹.

by sfink 6y ago

bananabreakfast 6y ago

Gravity is a force that acts due to mass just as the electromagnetic force acts due to charge. Since gravity acts in a magnitude directly proportional to the mass being acted upon, it just so happens that it manifests as an acceleration since the mass cancels out, unlike EM where acceleration due to charge does not cancel out the mass.

aflag 6y ago

According to Newton's equation, gravity's force is F=(GmM) / r^2. Those are the gravitational masses. The mass in F = ma is the inertial mass. The fact that they are the same is remarkable. If it weren't for that, different mass objects would feel different accelerations due to gravity.

caf 6y ago

In Newtonian physics it's a force with a magnitude that is proportional to the mass of the body. For two different objects at the same point in the same gravitational field, the difference in gravitational force due to their differing mass exactly cancels out the difference in acceleration due to their differing mass, so they both accelerate at the same rate.

sfink 6y ago
Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water. My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known acceleration, and that means you need to multiply by the mass. Clearly different masses will just cancel, so the resulting acceleration is the same. I'm fine with saying "force due to gravity" or even "gravitational force". Which is what you're describing in your first sentence, and I have no disagreement with that. "Force of gravity" starts to sound a little off, and I bet if I ask "what is gravity at Earth's surface?" I'll get back "9.8 m/s^2", which is an acceleration not a force.
- caf 6y ago
  Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water. I don't see how. A "force between two bodies with a magnitude proportional to the product of their masses and inversely proportional to the square of the displacement between them" seems no different in principle from a "force between two bodies with a magnitude proportional to the product of their electric charges and the inversely proportional to the square of the displacement between them". My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known acceleration, and that means you need to multiply by the mass. Clearly different masses will just cancel, so the resulting acceleration is the same. We're talking Newtonian physics, a model of the world where an acceleration is the result of an unbalanced force applied to a mass. You don't have to "reverse-engineer it from the known acceleration", you can calculate it as Gm₁m₂/r². I'm fine with saying "force due to gravity" or even "gravitational force". Which is what you're describing in your first sentence, and I have no disagreement with that. "Force of gravity" starts to sound a little off, and I bet if I ask "what is gravity at Earth's surface?" I'll get back "9.8 m/s^2", which is an acceleration not a force. You can equally describe the magnitude of the gravitational field as 9.8 Nkg⁻¹.