Defined as how much energy it takes to perform a specific amount of work.

a.) machine
b.) friction
c.) efficiency
d.) mechanical advantage

Defined as how much energy it takes to perform a specific amount of work.

B. Friction

I think the best answer from the choices listed above is option B. Friction force is defined as how much energy it takes to perform a specific amount of work since it is the only force among the choices listed above. Hope this answers the question.

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 -