# Does the moon have greater gravitational force than the Sun on the Earth?

by tunakdude
Tags: earth, force, gravitational, greater, moon
 P: 3 I'm going nuts trying to figure this out... in text books and online, everything i read says that the moon and earth have a much stronger gravitational force between them than the sun and the earth, and this is why the moon has greater effect on tides than the sun. They all say that this is because the moon, though much less massive than, is much closer to the earth than the sun. But... every time i do the calculations using the G(m1)(m2)/r^2, i get that the force between the Sun and the oceans is 8.37 x 10^18 N, and the force between the Moon and the oceans is 4.65 x 10^16 N so my calculations say that the Sun has a greater force on the oceans than the moon... if i changed out the value of the oceans' mass with the earths mass, i still get that the sun has a greater gravitational pull on the earth than the moon... so why do all my sources say that the earth-moon gravitational force is greater?????? thanks
 Mentor P: 13,691 You are confusing the gravitational force with tidal forces. The gravitational force varies with the inverse square of the distance between two bodies. Tidal forces result from the gradient of the gravitational force and thusly vary with the inverse cube of the distance. So while the gravitational force exerted by the sun on the earth is much greater than that exerted by the moon, the situation is reverse for tidal forces.
Mentor
P: 40,318
 Quote by tunakdude But... every time i do the calculations using the G(m1)(m2)/r^2, i get that the force between the Sun and the oceans is 8.37 x 10^18 N, and the force between the Moon and the oceans is 4.65 x 10^16 N
That equation gives you the gravitational force between the bodies, but that's not the tidal force. The tidal force depends on the change in gravitational strength at different points: It's the variation in pull on one side of the earth compared to the pull on the other that creates the tidal force. The tidal force is inversely proportional to the distance cubed.

Read this: Moon as Dominant Tidal Source

P: 3

## Does the moon have greater gravitational force than the Sun on the Earth?

 Quote by D H You are confusing the gravitational force with tidal forces. The gravitational force varies with the inverse square of the distance between two bodies. Tidal forces result from the gradient of the gravitational force and thusly vary with the inverse cube of the distance. So while the gravitational force exerted by the sun on the earth is much greater than that exerted by the moon, the situation is reverse for tidal forces.
thank you very much for the response...

but now i must ask, what is the "gradient of gravitational force" ??

why do you cube instead of square?
P: 3
 Quote by Doc Al That equation gives you the gravitational force between the bodies, but that's not the tidal force. The tidal force depends on the change in gravitational strength at different points: It's the variation in pull on one side of the earth compared to the pull on the other that creates the tidal force. The tidal force is inversely proportional to the distance cubed. Read this: Moon as Dominant Tidal Source
ah, this makes good sense to me...
so tell me, do we see the moon having a greater periodic effect on tides than the sun because as the points on the earth vary frequently with the moon, they experience a greater relative distance change with the moon than with the sun?

i thought that while searching around, i had read sources saying that gravitational force was greater, but they must have all said "tide-generating" potential or something like that (as does the earth science text book sitting in front of me)
 P: 24 First of all, hello, this is my first post in this forum. Second, excuse me everybody for opening such an old thread, but I have a question regarding Tidal forces. How exactly did the OP come to 8.37 x 10^18 and 4.65 x 10^16 N respectively? When I calculate the gravitational force of the Sun and of the Moon on the Earth I always get -3.5436^22 and -1.9830*10^20 rounded. Im using