Yeah yeah quantum mechanics is wrong too, because it's foundation was laid by classical mechanics.

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I'm simply saying, I apply gravity as space-time curve when I apply the Einstein's relativity in action. If had explained the question in the first scenario using relativity, I would've mentioned gravity as space time curve

Okay do me a favour, workout a classical problem considering gravity as curved space time. Simple pendulum, compound pendulum, linear Harmonic oscillator two body problem anything.

Well yeah Newton's doesn't explain Mercury's precession. I were sticking the context of the questioner and I had mentioned defining it classically, you need to think what it is when required. You can't just tell all the concepts that neglected friction, air resistance are wrong, for the classical assumption gravity has to be assumed as force. Relativity got better explanation apsidal precession and all at the end of the day approximately g = 9.81 m/s². Not lower than that or higher than that.

The comment above still fits, You can obtain mass of the earth by doing the simple pendulum method to obtain g from the equation √g = 2π√L/T

g = earth's gravity L = length of the pendulum T = time period

Note that simple pendulum give adequate answers for small angles only, if you swing the pendulum to wide you won't br arriving at the earth's gravity.

then equate it with g = GM/r²

Yeah right, the only change would depends whether spaceship or the object is bound to the system or not ( planet's atmosphere then the distance between them would be radius of the planet ), if it's extra planetary then the r would be distance between them, pretty much the same.

Are you speaking about Newton's law of gravitation? It's used to obtain force of attraction between two planets or it's satellites, which is deduced from f_g = G m_1 m_2/r²

f_g is force of attraction,

G is universal gravitational constant

m_1 mass of the planet

m_2 mass of the other planet

r is distance between them.

The above problem is otherwise called two body problem as well which turns into complicate when you apply classical mechanics

The formula for gravity of the planet obtained from the above equation as folloing as

We know that f = mg and also f = GmM/r²

Here g is gravity due to acceleration G is gravitational constant. m is mass of the object in the earth. M is mass of the earth.

Since the object is bound to the planet the r distance between them would be radius of the planet itself Thus r is radius of the earth

mg = GmM/r²

And you get g = GM/r²

I would suggest you to look into two body problem and barycentre you might find it relevant.