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# F=GMm/r^2 help needed please?

I'm confused when with the variable 'r' in some questions in class it was the distance between two masses but in other questions we had to add the radius of the earth to r but i don't understand why/when i need to do this any help would be very much appreciated

I'm confused with the variable 'r'*

### 2 Answers

- PaulLv 76 years agoFavorite Answer
r is the center to center distance of the two masses.

If the distances between the two objects is very great i.e. the gravitational impact of the Sun on the Earth the radius of the Earth and the radius of the Sun is such a small portion of the distance between the Sun and the Earth that the error caused would be less than the margin of error in your measurements. For example if you say the distance between the Earth and the Sun is 93 million miles, who cares about a few thousand miles when the measurement of the distance is only to the nearest million miles? If you're measuring the surface gravity of the Earth you need to account for the distance to the center of the Earth.

- 6 years ago
the variable 'r' is the distance between two masses taken from the centre of the those masses.

In ur question, when u were adding the radius, you were doing the same i.e. adding the remaining distance frm the centre( i.e. radius) and plus that 'r' in the question must be the distance between the masses( and not between their radii) so u had to add he radius.

Hope this helps!