# Theoretically where the g factor came from? g=9.8

1. Jul 13, 2010

### hagopbul

to all

i need to know the theoretically where the g factor came from?
g=9.8

2. Jul 13, 2010

Re: g

3. Jul 13, 2010

### hagopbul

Re: g

i think it is based on test only ... i tried to find some theoretical concept but i couldnt find any ...

4. Jul 13, 2010

### Staff: Mentor

Re: g

Well, as g is the acceleration due to gravity, the two theories would be acceleration and gravity, both from Newton's laws. Can you think of a way to derive g from Newton's laws...?

5. Jul 14, 2010

### mikelepore

Re: g

The weight of an object = the gravitatonal force that the earth exerts on the object = G m_earth m_object / r^2, where r is the dictance to the center of the earth, and G is the universal gravitational constant. Factor it like this: (G m_earth / r^2) m_object, and then everything in parentheses is "g". Therefore the weight of any object of mass m will be mg.

6. Aug 25, 2010

### hagopbul

Re: g

trying trough newton but it is not working

the numbers are not matching

7. Aug 25, 2010

### Staff: Mentor

Re: g

Show your calculation and the numbers you used.

8. Aug 25, 2010

### HallsofIvy

Re: g

Strictly speaking, it didn't come "theoretically" from anything- it came from experimentation. "9.98" is the measured acceleration, in meters per second squared, of an object at the earth's surface. (Actually, 9.981 is a more accurate value- but the value varies from place to place on the earth.)

Given "$F= GmM/r^2= mg$, so that $g= GM/r^2$, we could then put r equal to the radius of the earth and M equal to the mass of the earth, we could calculate g, but, in fact, it is done the other way around.

9. Aug 25, 2010

### cjameshuff

Re: g

First...g is too easily mixed up with grams. I strongly prefer to use g or g0.
Second: g isn't 9.8. It's 9.8 m/s^2. The numerical value is completely arbitrary, and depends on the units you measure it in. 9.8 m/s^2, 32 ft/s^2, 71.3 gigafurlongs/fortnight^2...and it isn't any sort of universal constant. Earth's surface gravity is determined by how much matter has been piled into it and how close you are to said matter...Earth is a little lumpy and out of round, so that actually varies slightly depending on where you are. 9.8 m/s^2 is a common approximate value, and as stated, generally determined by measurement (it's much easier to measure gravitational acceleration than to directly measure Earth's mass and your distance from each particle of it).