Why do we need to know the value of 'g' to such accuracy?

1. Feb 4, 2014

Jon.G

I was looking through a textbook, and read that the value of g was determined to be exactly 9.80665 ms-2.
I just started wondering why do we need to know it to that accuracy? Was it necessary for certain experiments? Or did people just want to know?

Then I started to think about why we might need to know the value of g, and I was quite surprised when I couldn't really think of anything. What are the applications of knowing the value of g?
My first thought was that of rockets etc. but then as their altitude increases, g will vary, so knowing the value at ground/sea level will not be the whole story.

:) Thanks

2. Feb 4, 2014

Let a block of mass 1kg,be at 1m from sea level,And assume that g is $10m/s^2$
Weight of the block is found as $weight=mass*g$
So,Weight=10N,
If the value of g is given as 9.8,Weight is 9.8N
If it is 9.80665,Weight is calculated as 9.80665.
g is also necessary for calculating the time taken for the fall,Distance traveled etc...
So the more accurate the value of g,the more accurate the calculation.

3. Feb 4, 2014

D H

Staff Emeritus
The value of g has not been determined to be exactly 9.80665 m/s2.

That is the defined value.

There is no one value for g that applies worldwide. The value varies with latitude, altitude, and the makeup of the stuff underfoot. The acceleration due to gravity in Mexico City is 9.776 m/s2. In Oslo, Norway it's about half a percent higher, 9.825 m/s2.

4. Feb 4, 2014

D H

Staff Emeritus
One use is in US customary units. We have a pound mass and a pound force. The pound force is exactly that needed to make an object with a mass of one pound accelerate at 9.80665 m/s2.

Another use is in a metric unit that is deprecated but nonetheless is widely used in some engineering domains. The same factors that drive some engineers in the US to use pounds mass and pounds force drive their non-US counterparts to use the kilogram and kilogram-force. The kilogram-force is exactly that needed to make a one kilogram object accelerate at 9.80665 m/s2.

5. Feb 4, 2014

Moidaki

Gravity surveys especially in mineral exploration requires g to me measured accurately in order to detect the small gravity anomalies caused by density variations.

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6. Feb 4, 2014

olivermsun

Even more amazing, most of the 'known' bathymetry of the ocean floors is inverted from gravity surveys.

7. Feb 4, 2014

Meir Achuz

Does anyone know how that number was decided, since it wouldn't be measured to be
9.80665 m/s2 almost anywhere on Earth? If it is the result of a measurement, I want to go there so I'll know how fast I would fall.

8. Feb 4, 2014

olivermsun

Well, according to Wikipedia: