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

Uranium fission within a nuclear reactor produces high-speed neutrons. These must be slowed down for them to trigger a chain reaction within the uranium. In a heavy-water reactor the neutrons of mass m make multiple elastic collisions with deuterium nuclei of 2m, losing speed at each collision.

(a) Suppose a fast-moving neutron of speed v makes a head-on elastic collision with a stationary deuterium nucleus. Show that its speed is reduced to v/3.

2. Relevant equations

p=mv - We probably have to use this

E_{k}=0.5mv^{2}- Not sure if we have to use this, but the question IS in a section about kinetic energy. It just seems logical to use p=mv because using kinetic energy makes no sense.

3. The attempt at a solution

Ok it took me absolutely AGES to do this, but I don't think it's right:

P_{D1}= mv = 2m x 0 = 0 N s

P_{N1}= mv = 1m x v = mv

P_{D1}+ P_{N1}= P_{D2}+ P_{N2}if no external forces are applied

0 + mv = P_{D2}+ P_{N2}

mv = P_{D2}+ P_{N2}

P_{D2}= mv - P_{N2}

P_{D2}= P_{N2}- not sure

Substitute above two equations together to show P_{N2}:

P_{N2}= mv - P_{N2}

2P_{N2}= mv

P_{N2}= mv/2

v_{N2}= P_{N2}/2m

v_{N2}= (mv/2)/2m

v_{N2}= mv/4m - my rearranging may be wrong

v_{N2}= v/3m

But the answer is just v/3 not v/3m. Is this still right? Or is it just coincidence I got an answer so similar?

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# Homework Help: Momentum in elastic collision

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