(a) FALSE
Yes, force is a vector, but these two expressions describe only the
magnitude of the force. The expression for Fgrav is
always positive.
(b) FALSE
Felec is positive when the charges repel, i.e., when
q1 and q2 are either both positive
or both negative.
(c) FALSE
We can either think of r as the scalar distance between the two
masses or charges, or if we prefer, we can think of r2
as the scalar product of the appropriate displacement vector with itself.