# Gravity; the very basics

1. Aug 14, 2015

### 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).

2. Aug 14, 2015

### gracy

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

3. Aug 14, 2015

### Cliff Hanley

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

4. Aug 14, 2015

### Qwertywerty

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 .

5. Aug 14, 2015

### gracy

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

6. Aug 14, 2015

### Cliff Hanley

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?

7. Aug 14, 2015

### Cliff Hanley

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

8. Aug 14, 2015

### gracy

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.

9. Aug 14, 2015

### Qwertywerty

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 .