F=ma, but Gravity is a Force with no mass?

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Discussion Overview

The discussion revolves around the nature of gravity as a force, particularly questioning how gravity can be described in the context of Newton's second law of motion (F=ma) when gravity itself is said to have no mass. Participants explore the implications of gravity's dependence on mass, the concept of gravitational force, and the relationship between mass and acceleration in gravitational contexts.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants express confusion about gravity being described as a force, noting that gravity has no mass and questioning how it can be treated similarly to other forces.
  • Others clarify that gravity is a concept related to the mutual attraction between masses, as described by Newton's Law of Gravitation, and that concepts themselves do not possess mass.
  • One participant suggests that gravity is better understood as an effect of mass moving through curved space-time, rather than a force in the traditional sense.
  • Several participants discuss how gravitational force varies with mass, indicating that while the force experienced by different masses differs, the acceleration due to gravity remains constant for all objects in a given gravitational field.
  • There is mention of Newton's law of gravity and how it can be used to calculate the gravitational force on an object, linking it to Newton's second law to determine motion.
  • One participant emphasizes the action-reaction principle, explaining that the gravitational force is mutual between two masses, and that the force exerted by one mass on another is equal and opposite to the force exerted back.
  • Another participant raises a point about gravitational waves in general relativity, questioning how this relates to the initial misunderstanding of gravity's mass.

Areas of Agreement / Disagreement

Participants express differing views on the nature of gravity, with some advocating for a traditional force perspective and others suggesting a more conceptual understanding. There is no consensus on how to reconcile gravity's description as a force with its lack of mass, indicating ongoing disagreement and exploration of the topic.

Contextual Notes

Participants highlight various assumptions about gravity, including its dependence on mass and the complexities of gravitational interactions in different contexts, such as on the moon versus Earth. The discussion also touches on the distinction between contact and long-range forces, which remains unresolved.

KidWonder
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I've always wondered why Gravity was described as a force.

Force=mass*acceleration.

Gravity has no mass, the pull of gravity depends on the mass of the objects it's pulling on. Right?

Or does Physicist/Mathematicians see Force of Gravity depends on the mass of the objects it's pulling on.

Force of Gravity on every object in the universe should be the same right?

I would appreciate any help, sorry if I'm not so smart.
Thanks Guys.
God Bless
 
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What do you mean gravity has no mass?

I think you might have a misunderstanding of what the term "gravity" really means. Gravity is a concept. Two masses will feel a mutual attraction that can be determined by Newton's Law of Gravitation. The word "gravity" and "gravitation" is this concept. A concept can not have mass, it is not a physical thing.
 
Gravity is a perception of mass moving through a "curved space-time". It is not a force, it is an effect that we don't understand completely.

Paul
 
PaulS1950 said:
Gravity is a perception of mass moving through a "curved space-time". It is not a force, it is an effect that we don't understand completely.

Paul

Don't get into GR yet, the OP clearly has barely begun his studies and introducing relativity and space-time will not clear up a single thing.
 
KidWonder said:
Or does Physicist/Mathematicians see Force of Gravity depends on the mass of the objects it's pulling on.

The force will vary according to mass. So a 10-pound bowling ball will have twice the gravitational force ("F" in your equation) as a 5-pound bowling ball. But they will both have the same acceleration ("a" in your equation).

KidWonder said:
Force of Gravity on every object in the universe should be the same right?

Gravity follows the same laws everywhere, but the actual equations and numbers are a bit more complicated when you consider the entire universe. For example, you will experience a much lesser gravitational force walking on the moon, than on earth.
 
F=ma only gives a relation between the amount of force applied to a mass and the amount of acceleration that force produces.
 
KingNothing said:
The force will vary according to mass. So a 10-pound bowling ball will have twice the gravitational force ("F" in your equation) as a 5-pound bowling ball. But they will both have the same acceleration ("a" in your equation).



Gravity follows the same laws everywhere, but the actual equations and numbers are a bit more complicated when you consider the entire universe. For example, you will experience a much lesser gravitational force walking on the moon, than on earth.

Thank you for the simple explanation. I understand now.
 
What do you mean by gravity has no mass? I think in GR, propagating gravitational waves are also gravitating, but I guess that's not what you are talking about.

Possibly, you are asking about Newton's Gravitation, where gravity is proportional to both mass. So what do you mean by all the objects in the universe have the same gravity?
 
KidWonder said:
I've always wondered why Gravity was described as a force.

Force=mass*acceleration.

Gravity has no mass, the pull of gravity depends on the mass of the objects it's pulling on. Right?

Or does Physicist/Mathematicians see Force of Gravity depends on the mass of the objects it's pulling on.

Force of Gravity on every object in the universe should be the same right?
Forces come in pairs only: action - reaction. The fact the Earth is pulling on you is only half the story. You are also pulling on the earth. The force felt by both you and the Earth is a unique action-reaction between you and the earth, and the strength of that force is determined by the relative masses. The exact value of the earth/Kidwonder force is your weight. F = ma = mg = your weight. But you could turn a scale upside down, stand on it, and be measuring the force you exert on the earth. It will be equal to the force the Earth exerts on you, because of Newton's Third Law. It's an action-reaction, like any force pair. F = -F. Newton's third Law applies to forces of attraction as well as to collisions. You and the moon will have a completely different action-reaction force because the relative masses are different.

Gravity has no mass, but it only arises from mass. I have a textbook which neatly separates forces into "contact" forces and "long range" forces. A "long range" force is when one object is able to act upon another object without any apparent direct contact. Gravity is a long range force. So would electrical attraction and repulsion be. "Contact" forces are when one object interacts with another only by direct contact with it. The action - reaction is the same in both cases, long range or contact.
 
  • #10
Let's say You have a 1 kg object and you want to know what happens when you let go of it. First use Newtons law of gravity F=GMm/r^2. Here M is mass of Earth and m is your 1 kg object; r is the radius of the earth. This equation tells you the gravitational force on your object. Now, you write F=ma, putting that in for F. So GMm/r^2 = ma and you solve for the acceleration a.

So Newton's law of gravity tells you the force and then Newton's second law (F=ma) tells you how it will move.
 
  • #11
In F=ma, the mass considered is of the object on which it is acting and the acceleration is also on the object on which it is acting. It is not about the mass and acceleration of force itself.
Consider this. A train is pulling coaches with constant acceleration. You need to calculate force experienced by coaches. First, there are 5 coaches and in case 2- there are 7. If you are going to consider mass and acceleration of engine, you would get the same force in both cases. But, you know, it can't be same since more coaches are added.
 

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