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

  1. Feb 18, 2012 #1
    I've always wondered why Gravity was described as a force.


    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
  2. jcsd
  3. Feb 18, 2012 #2


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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.
  4. Feb 18, 2012 #3
    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.

  5. Feb 18, 2012 #4


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    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.
  6. Feb 18, 2012 #5
    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.
  7. Feb 18, 2012 #6
    F=ma only gives a relation between the amount of force applied to a mass and the amount of acceleration that force produces.
  8. Feb 18, 2012 #7
    Thank you for the simple explanation. I understand now.
  9. Feb 19, 2012 #8
    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?
  10. Feb 19, 2012 #9
    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.
  11. Feb 19, 2012 #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.
  12. Feb 19, 2012 #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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