Gravitation multiple particle system

[Puchinita5]

Homework Statement

The three spheres in Fig. 13-45, with masses ma=80g ,mb=10g and mc=20g, have their centers on a common line, with L=12 cm and d=4cm. You move sphere B along the line until its center-to-center separation from C is d=4cm. How much work is done on sphere B (a) by you and (b) by the net gravitational force on B due to spheres A and C

Homework Equations

http://edugen.wiley.com/edugen/courses/crs1650/art/images/halliday8019c13/image_t/tfg045.gif

The Attempt at a Solution

I figured if W=Fd...then I started with trying to figure out the Force on particle b.
F= Gm1m2/rr so...
-G(.01)(.08)/(.04^2) + G(.01)(.02)/(.08^2) which gave me -3.13e-11...i then multiplied this by the distance d=.04...this gave me = -1.25e-12 J...

however the answer is .50 pJ

that's for net gravitational force, I'm not really sure how they mean for me to calculate the force by me?

 

[alphysicist]

Homework Statement

The three spheres in Fig. 13-45, with masses ma=80g ,mb=10g and mc=20g, have their centers on a common line, with L=12 cm and d=4cm. You move sphere B along the line until its center-to-center separation from C is d=4cm. How much work is done on sphere B (a) by you and (b) by the net gravitational force on B due to spheres A and C

Homework Equations

http://edugen.wiley.com/edugen/courses/crs1650/art/images/halliday8019c13/image_t/tfg045.gif

The Attempt at a Solution

I figured if W=Fd.

No, I don't believe that will work here. That is the formula for work done by a constant force, but here the force on the moving particle is changing as it moves.

I would suggest using an energy approach here. What is the change in energy of the moving particle? And think about these questions: How is that energy change related to the work done? What is the work done by a conservative force?