(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A nuclear reactor (see below) contains nuclei that fission when bombarded by slow neutrons. The fission reaction produces fast neutrons which need to be moderated (i.e. slowed down) in collisions before they can cause further fissions. In this problem we consider a reactor which uses graphite (i.e. carbon) as moderator. Assume the neutron and carbon nuclear masses are 1.67 × 10^{–27}kg and 20.0 × 10^{–27}kg respectively.

Consider a head-on collision between a neutron with speed 10.0 Mm/s and a stationary carbon nucleus.

(i) Calculate the speed of the neutron after the collision

(ii) Calculate the speed of the carbon nucleus after the collision

3. The attempt at a solution

(i) I assume this is a perfectly elastic collision so, momentum before = momentum after

m_{n}v_{n,i}= (m_{n}+m_{c})V

10 × (1.67 × 10^{–27}) = [(1.67 × 10^{–27})+(20.0 × 10^{–27})]V

V=4.5 Mm/s

But the correct answer is 8.46 Mm/s. How did they get that?

(ii) What do they mean by "the carbon nucleus"? And how do we obtain its speed? (by the way it has to be 1.54 Mm/s)

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Collisions homework help

**Physics Forums | Science Articles, Homework Help, Discussion**