# Gravitational field strength at latitude

## Main Question or Discussion Point

This figure is captured from my physics textbook and it is about the gravity at latitude

http://pix.gogobox.com.tw/out.php?i=567045_.JPG [Broken]

I want to know the difference between mg0 and mg(theta). Which one is the weight of the object? Thank you!

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Doc Al
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Looks to me like mg0 is the gravitational force on the object and mgθ is the apparent weight of the object at latitude θ. The apparent weight is the gravitational force minus the centripetal force. (If you hung an object from a string, the string would end up parallel to the apparent weight.)

Is gθ called 'apparent gravity'?

Doc Al
Mentor
Is gθ called 'apparent gravity'?
That's what I would call it, but it is also called the local value of g as a function of θ. It's somewhat a matter of semantics. What does your textbook call it? (What text are you using?)

It is a Hong Kong textbook called New Way PHYSICS for Advanced Level
In the book, gθ is called gravitational field strength at latitude θ
I think the author uses the wrong name
The gravitational field strength at latitude θ should be g0, right?

Here is the related page from my textbook

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Doc Al
Mentor
I think the author uses the wrong name
The gravitational field strength at latitude θ should be g0, right?
No, the author's terminology is reasonable. When they talk about 'gravitational field strength at latitude θ' they are lumping together the strictly gravitational component (g0) with the effect due to rotation. Think of it as the 'net' or 'apparent' gravitational field strength. (It is a bit confusing, but commonly done.)

gravity = g0
apparent gravity / gravity at latitude = gθ
weight = mg0
apparent weight = mgθ

I hope I don't understand wrongly

So when we measure our body weight, the reading of the balance = mgθ rather than mg0

Doc Al
Mentor
gravity = g0
apparent gravity / gravity at latitude = gθ
weight = mg0
apparent weight = mgθ

I hope I don't understand wrongly

So when we measure our body weight, the reading of the balance = mgθ rather than mg0
Looks good to me. (Of course, this is just a simple model of the earth as a sphere, ignoring various complications.)