You are Here: Home >< Physics

# Physics/ Rest mass energy

Announcements Posted on
Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
1. 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 ?
2. Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.

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

Just quoting in Puddles the Monkey so she can move the thread if needed
Spoiler:
Show
(Original post by Puddles the Monkey)
x
3. (Original post by Angus09)
x
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

if so it seems pretty clear that the question is referring to the kinetic energy of the ejected atoms
4. (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 ?
5. (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.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 17, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Who is getting a uni offer this half term?

Find out which unis are hot off the mark here

### Brexit: Why you should study law

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.