Gravitational force of the sun on earth

[tquiva]

Homework Statement

Consider the system of the Sun, Earth, and Moon, at a moment in time when they happen to be exactly in a line. (Assume that all distances d are measured between the centers of all bodies, and that all distances d are much greater than the radii of all bodies.
M(sun) = 1.99e30 kg
M(earth) = 5.97e25 kg
M(moon) = 7.35e22 kg
d(earth-sun) = 1.50e11 m
d(earth-moon) = 3.84e8 m
G = 6.67e-11 Nm^2/kg^2

Calculate the magnitude of the gravitational force of the Sun on the Earth. Also, what is the gravitational force of the Earth on the Sun?

Homework Equations

Fg = (Gm1m2) / (r^2)

The Attempt at a Solution

I know that all I have to do is plug in the value into the Fg equation above. However, I'm a little lost with the force of the Sun on the Earth, and the force of the Earth on the sun. Does the value of r change for both of these cases or stay the same?

 

[magwas]

If you forget Moon for a moment, then the force of Sun on Earth should be the same as the force of Earth on Sun.
If you do not forget Moon, those forces are still the same, but there is the force of the Moon as well, so the sum force acting on Earth is F(Sun-Earth) + F(Moon-Earth). In a vector sense, so you should substract the force by Moon if it is in the other side. I suspect from the wording that it is on the other side, but it is not exactly clear.

 

[Stonebridge]

The force of the Sun on the Earth is the same as the force of the Earth on the Sun. It must be, according to Newton's 3rd Law of Motion.