Calculating Net Gravitational Field Strength Between a Planet and the Sun

[kingyof2thejring]

Hi there, everybody, ivgot a question here,
In a distant solar system, a planet (mass 4.13x1028 kg) is orbiting the sun (mass 7.67x1030 kg) with an orbit radius of 2.72x1011 m.

Calculate the magnitude of the net gravitational field strength midway between the planet and the sun, in N kg -1

so the Sun's gravitational field, is given by g = (G*Msun)/r^2sun
i get an aswer 1.48e-4 which is not correct of course, why not?.
please help, thnaks in advance.

 

[Kamataat]

Calculate the sun's grav. field magnitude at r/2 and then the planet's at r/2. Add these to get the total.

- Kamataat

 

[kingyof2thejring]

i get 1.489e-4 N kg-1 for the planets but
0.1125 N kg-1 for the suns
so wats wrong with the suns value

 

[Kamataat]

You sure you calculated correctly? For the sun I get
$$F=G\times\frac{M_{sun}}{(\frac{r}{2})^2}=6,672\cdot 10^{-11}\times\frac{7,67\cdot 10^{30}}{(\frac{2,72\cdot 10^{11}}{2})^2}=2,767\cdot 10^{-2}\frac{N}{kg}$$
- Kamataat

 

[kingyof2thejring]

yeh is see i have made a mistake there but if we add 1.48e-4 and 2.767e-2
i get an anwser still much greater than the riquired ansewr of 1.28e-4 infact my for the planet's gravitational field strength is greater than the net.

 

[Kamataat]

FS is vector directed away from the origin of the field. The magnitude of a resultant vector can be smaller that the magnitudes of the vectors being added.

- Kamataat

 

[kingyof2thejring]

hi, er... both answers (1.489e-4 & 2.767e-2) don't add-up to give me the net force of 1.28e-4.

Can someone please help me... I'm tired, stuck and utterly miserable.