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Mass, Gravity and Attraction

  1. Apr 10, 2010 #1
    From a Serious Newbie

    Have been interested in and fascinated by physics for a number of years now and am a total layman but watch every science/astronomy programme I can and have read a couple of very basic books on the subject, and have now steeled myself to read “A Brief History of Time”. I know this is going to sound very stupid but there is one simple thing I cannot get my head around (perhaps I should have taken up some other subject – maybe not being the type of mind to grasp these things! Knitting perhaps?)

    Regarding Newton’s first and second law (second page in of Chapter 2 of Stephen Hawking’s book):

    Is mass an object’s weight, density or size, or a combination of all three?
    A one ton ball of lead would be smaller than a one ton ball of rock – ignoring air resistance both would fall to the ground at the same rate, and I get that; but what about attraction?
    If they were both in space would the rock attract more than the lead because of its bigger size or the lead because of its greater density?

    Thanking you in advance, and please excuse my naivety
  2. jcsd
  3. Apr 10, 2010 #2
    in short:
    Mass does not change..it is fixed..
    But weight depends on gravitation pull (acceleration due to gravity = 9.8 m s-2).
  4. Apr 10, 2010 #3


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    Welcome to PF!

    Hi Ink Monitor! Welcome to PF! :smile:
    Two spheres of the same "weight" but of different size and density will exert the same gravitational attraction.

    Even a black hole and an ordinary star of the same mass have gravitational fields that are indistinguishable (outside the radius of the star, of course :wink:).
  5. Apr 10, 2010 #4

    jack action

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    Mass is the measure of the quantity of matter.

    Size (which I assume you mean "Volume") is the measure of the space occupied.

    Density is the ratio of the mass with respect to its volume.

    Weight is the gravitational force (or attraction) between two objects and its depends on the masses of BOTH objects and on the distance between them. Your mass is the same whether you are on earth or on the moon, but since the mass of the moon is a lot less than the mass of the earth, your weight (the gravitational force) is less on the moon than on the earth. This gravitational force is felt by both objects, but of course the one with a lower mass will accelerate a lot more under this force and that is why the big mass seems "stationary" with respect to the small mass.
  6. Apr 10, 2010 #5
    Welcome Ink monitor …

    Answer 1) The mass of an object is the measure of its "massiveness" hence the term mass. The more the mass means more massive the object is, In easier words the amount of matter contained in a object is called its mass. Although Mass depends on density more the density more the mass, but it is not density as density is mass per unit volume.

    Answer 2)The force of attraction between two bodies is directly proportional to the product of the mass of the objects and inversely proportional to the square of distance between them. So a ton of rock and ton of lead in space both will attract each other with the same amount of force …. as the total amount of mass in both objects is same.
  7. Apr 12, 2010 #6
    Thank you everyone for your replies, and between them all I think I've got it in my head now(!); and Rajini "acceleration due to gravity = 9.8 m s-2" means nothing to me at the moment - but thank you anyway.

    Now, where's that "Brief History of Time" sub title 'From the Big Bang to Black Holes' - Ummm, hold on to your hats, as I'm sure there will be more queries!
  8. Apr 12, 2010 #7
    Hi, why i used g here is..
    for e.g., a mass of 10 kg is equivalent to [tex]10\times9.8=98~{\rm N}[/tex] on earth.
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