Surface Gravity vs. Gravitational Field Strength: What's the Difference?

[Cliff Hanley]

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).

 

[gracy]

sometimes refers to something else that I've not quite grasped yet (measured in N/kg

Ok.Let me help you.What is the SI unit of force?What is SI unit of mass?

 

[Cliff Hanley]

Ok.Let me help you.What is the SI unit of force?What is SI unit of mass?

Thanks. The SI unit of force is Newtons (N). The SI unit of mass is kilograms (kg).

 

[Qwertywerty]

Is 'surface gravity' the same as 'gravitational field strength'?

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 .

 

[gracy]

I think you have got it !but anyways
what is Force /mass ?

 

[Cliff Hanley]

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 .

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?

 

[Cliff Hanley]

I think you have got it !but anyways
what is Force /mass ?

F/m is..? Well, I know that F=ma (Newton's 2nd law of motion), so F/m=a, yes?

 

[gracy]

yes?

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.

 

[Qwertywerty]

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?

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 ) .

And is g.f.s therefore the strength of gravity not *in contact* with the Earth, but some distance from it?

Not necessarily . I've answered this in the previous part of this post .