# Gre Problem # 64

1. Sep 6, 2004

### quantumworld

Dear forum contributer,

The binding energy of a heavy nucleus is about 7 Mev per nucleon, whereas the binding energy of a medium-weight nucleus is about 8 Mev per nucleon. Therefore, the total kinetic energy liberated when a heavy nucleus undergoes symmetric fission is most nearly
(A) 1876 Mev
(B) 938 Mev
(C) 200 Mev
(D) 8 Mev
(E) 7 Mev

I found that this problem needs more info in order to be solved, I mean for example, if they said the heavy nucleus is Uranium of atomic weight 238, or something like that, I would have been able to calculate the energy liberated, by saying, the heavy nucleus will become medium nucleus by delivering 1 Mev per nucleon, so with 238 the energy liberated willl be around 200 Mev, which is the correct answer, but with only the given info, I would not be able to solve it

Many thanks

2. Sep 6, 2004

### nrqed

Well, you are right, more is needed than what is given. This is often the case with GRE questions. But what is required is simply basic knowledge that anybody studying physics can be expected to have. This problem is a good example. Nuclei which undergo fission are heavy nuclei, typically above iron. So they won't have only a handful of nucleons, which rule out the 7 or 8 MeV answers. On the other hand, there is no nucleus with a thousand or more nucleons, so this rules out the large answers. The only number that makes any kind of sense is the 200 MeV.

So I agree that extra knowledge is necessary, but it's basic knowledge. You have to expect that kind of situation, where extra knowledge is assumed, in GRE questions.

Pat