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Edexcel Physics Unit 4 Question

Hi, I'm stuck on a particular question.
I don't understand the concept of this question, could someone kindly explain it to me please?

A radium nucleus decays by emitting an alpha particle. The speed of the recoiling nucleus is small compared to the speed of the alpha particle. This is because...
A. Force acting on the recoiling nucleus is smaller than the force acting on the alpha particle.
B. Momentum is mainly concentrated in the alpha particle.
C. Momentum of the recoiling nucleus is smaller than the momentum of the alpha particle.
D. Recoiling nucleus has a much larger mass than the alpha particle.
It all stems from the fact the law of conservation of momentum. As the initial momentum was 0. The two nuclei have equal momentum. let m = mass of alpha nucleus (small m to show smaller mass) and analogously, M for radium/radium. Define the speeds as V similarly.

now mV(alpha) = MV(recoiling/radium). then the speed of the recoiling = [mV(alpha)]/M. = (m/M)*V(alpha). now as M > m (M is much bigger than m) this implies that a small fraction of the alpha's speed V, is the speed of the recoiling nucleus.
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andyl95
OP
Original post by hecandothatfromran
It all stems from the fact the law of conservation of momentum. As the initial momentum was 0. The two nuclei have equal momentum. let m = mass of alpha nucleus (small m to show smaller mass) and analogously, M for radium/radium. Define the speeds as V similarly.

now mV(alpha) = MV(recoiling/radium). then the speed of the recoiling = [mV(alpha)]/M. = (m/M)*V(alpha). now as M > m (M is much bigger than m) this implies that a small fraction of the alpha's speed V, is the speed of the recoiling nucleus.


Thank-you for your help, so you're saying that the answer is D because the recoiling nucleus has a greater mass than the alpha particle. I don't understand why this is though?
Erm, sorry but i can't do much better than that sorry mate...

because moment is conserved (and it was initialy 0), the momentum of the two nuclei are equal. but because the mass of the radium is much greater than the alpha, the speed of the radium has to be smaller to make it equal.

Better? sorry, if i can't help.
Reply 4
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andyl95
OP
Yeah I understand it a lot better now, thank-you very much for your help :smile:
This is a past paper question right?
Could you tell me which paper it is please?

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