Suppose the sun was replaced by a star that had twice as much mass. how would the gravitational force between the sun and its planets change?
a. the gravitational force would stay the same.
b. the gravitational force would be half as large.
c. the gravitational force would be twice as big.
d. the gravitational force would now depend on the square of the distance to each planet.

Hmm I am very much stuck between c and d, but my educated guess would be d since that makes most sense to me- a planet has to be within the gravitational field of the star but the further out it is, then the weaker the pull?

B. The gravitational force would be half as large.

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -