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Gravity; the very basics

  1. Aug 14, 2015 #1
    1. Is 'surface gravity' the same as 'gravitational field strength'?

    2. It seems to me that the 'g' in the formula w = mg sometimes refers to acceleration due to gravity (measured in m/s^2) and sometimes refers to something else that I've not quite grasped yet (measured in N/kg) ; what is the something else? And do the values of each always match exactly, ie, acceleration due to gravity near the surface of the Earth is 9.8 m / s ^2 and the magnitude of the 'something else' is 9.8 N/kg (likewise for on or near the Moon; 1.6 m/s^2 and 1.6 N/kg).
  2. jcsd
  3. Aug 14, 2015 #2
    Ok.Let me help you.What is the SI unit of force?What is SI unit of mass?
  4. Aug 14, 2015 #3
    Thanks. The SI unit of force is Newtons (N). The SI unit of mass is kilograms (kg).
  5. Aug 14, 2015 #4
    No , it isn't . The first is gravitational field strength at the surface of the earth .

    The second is a general term , that refers to the force a body exerts on a unit mass , kept at some distance , say d .

    If mass of that body is M , then it's gravitational field strength at a distance d would be -
    GM / d2 .

    Hope this helps .
  6. Aug 14, 2015 #5
    I think you have got it !but anyways
    what is Force /mass ?
  7. Aug 14, 2015 #6
    Thanks. Is surface gravity therefore the strength of gravity on an object when *in contact* with the Earth? Is that the phenomenon that's measured in N/kg? And is g.f.s therefore the strength of gravity not *in contact* with the Earth, but some distance from it?
  8. Aug 14, 2015 #7
    F/m is..? Well, I know that F=ma (Newton's 2nd law of motion), so F/m=a, yes?
  9. Aug 14, 2015 #8
    Right ! so N/kg comes out to be acceleration .Acceleration has another unit i.e m/s^2.N/Kg and m/s^2 is one and the same thing.
  10. Aug 14, 2015 #9
    Well , I'd rather say it was the g.f.s close to , or on the surface of the earth . G.f.s is measured in N/kg , and as surface gravity is a special case of g.f.s , it has the same units ( Surface gravity is g ) .
    Not necessarily . I've answered this in the previous part of this post .
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