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Physics/ Rest mass energy

I was doing a past paper and came along this Question:
a) the maximum energy of a nitrogen atom was found to be 1.2MeV
Show that the maximum velocity of the atom is about 4x10^6ms^-1
(mass of nitrogen = 14(mu), mu=1.66x10^-27

This was only 3 marks so I assumed that the energy given was purely kinetic, and got the correct answer, however i was wondering why do you not account for the rest mass energy of the nitrogen atom ? e.g. 1.2MeV= rest mass energy + kinetic energy ?

Reply 1

TSR Jessica

Sorry you've not had any responses about this. :frown:

Are you sure you've posted in the right place? :smile:

Here's a link to our subject forum which should help get you more responses if you post there. :redface:

You can also find the Exam Thread list for A-levels here and GCSE here. :dumbells:

Just quoting in Puddles the Monkey so she can move the thread if needed :h:

Spoiler

Reply 2

Joinedup

Original post by Angus09

was it this question?

James Chadwick is credited with discovering the neutron in 1932. Beryllium was bombarded with alpha particles, knocking neutrons out of the beryllium atoms. Chadwick placed various targets between the beryllium and a detector. Hydrogen and nitrogen atoms were knocked out of the targets by the neutrons and the kinetic energies of these atoms were measured by the detector. (a) The maximum energy of a nitrogen atom wa found to be 1.2 MeV. Show that the maximum velocity of the atom is about 4 x 106 m/s. mass of nitrogen atom = 14u, where u = 1.66 x 10-27 kg

Reference https://www.physicsforums.com/threads/neutron-proton-collision-problem.662090/

if so it seems pretty clear that the question is referring to the kinetic energy of the ejected atoms

(edited 7 years ago)

Reply 3

Angus09

Original post by Joinedup

was it this question?

if so it seems pretty clear that the question is referring to the kinetic energy of the ejected atoms

yes thanks i must've missed it in the question, just to check though if it had not said kinetic energy would i have found the rest mass and subtracted it ?

Reply 4

Joinedup

Original post by Angus09

yes thanks i must've missed it in the question, just to check though if it had not said kinetic energy would i have found the rest mass and subtracted it ?

Well the context makes it look like kinetic - I'm not sure how Chadwick's detectors could have measured the energy equivalent of the mass of the atoms apart from by annihilating them with antimatter... which would have been a different type of experiment really.

The energy equivalent to the rest mass of a nucleus is enormous, about 938 MeV per nucleon... it couldn't be part of the energy figure given in the question because it's far bigger.