1. The problem statement, all variables and given/known data 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 2. Relevant equations http://edugen.wiley.com/edugen/courses/crs1650/art/images/halliday8019c13/image_t/tfg045.gif 3. 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???