Inspired by Parag Zode

What is the correct formula for acceleration due to gravity at a certain height?

  • Details and assumptions:
    • g = acceleration due to gravity (on the surface)
    • R = radius of the Earth
    • h = the height above the surface of the Earth.

For more such problems, try my set Gravity

Image credit: Wikipedia Arteaga Douglas
g ( R R + h ) 2 g\cdot(\frac{R}{R+h})^2 g R + h R g\cdot\frac{R+h}{R} g ( ( 1 g ) + ( 1 + h ) ) 2 g\cdot((1-g)+(1+h))^2 g ( R + h R ) 2 g\cdot(\frac{R+h}{R})^2

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3 solutions

Diego Barreto
Feb 8, 2015

We have that:

m a = G M m ( R + h ) 2 a = G M ( R + h ) 2 m'a=\frac { GMm' }{ { (R+h) }^{ 2 } } \longrightarrow a=\frac { GM }{ { (R+h) }^{ 2 } }

We also have that:

m g = G M m R 2 G M = g R 2 mg=\frac { GMm }{ { R }^{ 2 } } \longrightarrow GM=g{ R }^{ 2 }

Substituting the second equation in the first:

a = g R 2 ( R + h ) 2 a=\frac { g{ R }^{ 2 } }{ { (R+h) }^{ 2 } }

a = g ( R R + h ) 2 \boxed{a=g{ \left( \frac { R }{ R+h } \right) }^{ 2 }}

Caleb Townsend
Feb 8, 2015

Let a a be the acceleration due to gravity, G G be the gravitational constant and m m be the mass of Earth. From the definition of g g = G m R 2 G m = g R 2 g = G\frac{m}{R^2}\Leftrightarrow Gm = gR^2 and from Newton's law of universal gravitation, a = G m ( R + h ) 2 a = G\frac{m}{(R + h)^2} Substituting the (right side of the) first equation into the second gives a = g R 2 ( R + h ) 2 = g ( R R + h ) 2 a = g\frac{R^2}{(R+h)^2} = \boxed{g(\frac{R}{R+h})^2}

Note that this is not a strictly correct formula, even by pre-relativity standards, as Earth is not a point mass or a perfect sphere of uniform density. It is simply a better approximation.

Parth Kohli
Mar 17, 2015

Now, of course, everyone who's studied even a little about classical mechanics knows this and even remembers the answer without even doing any work. This question is an interesting one because it allows us to use only a little reasoning and eliminate all the other choices. Here's the solution:

The choices leaving the correct one all imply that force of gravity increases with height, which we know does not.

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