What is the correct formula for acceleration due to gravity at a certain height?
For more such problems, try my set Gravity
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Let a be the acceleration due to gravity, G be the gravitational constant and m be the mass of Earth. From the definition of g g = G R 2 m ⇔ G m = g R 2 and from Newton's law of universal gravitation, a = G ( R + h ) 2 m Substituting the (right side of the) first equation into the second gives a = g ( R + h ) 2 R 2 = g ( R + h R ) 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.
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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We have that:
m ′ a = ( R + h ) 2 G M m ′ ⟶ a = ( R + h ) 2 G M
We also have that:
m g = R 2 G M m ⟶ G M = g R 2
Substituting the second equation in the first:
a = ( R + h ) 2 g R 2
a = g ( R + h R ) 2