Show that for the diamond structure the Fourier component U-G of the crystal potential seen by an electron is equal to zero for G=2A, where A is a basis vector in the reciprocal lattice reffered to the conventional cubic cell

Answer (d) the universe had cooled enough for atoms to combine, protons and neutrons were too far apart to form nuclei;

I'm having some difficulty seeing the diagram clearly.but the question says there's a net inward force on the object.so we know right away,  from newton's 2nd law of motion, thatthat baby is accelerating.  it doesn't even matter which way thenet force is pointing.  if there's any net force, then either theobject's speed or else its direction must be changing.