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Homework Help: Entery level Mass-energy equivalence questions

  1. Apr 30, 2013 #1
    1. The problem statement, all variables and given/known data
    The diagram shows a proton moving with Ek of (0.178)(10-12)J towards a stationary nucleus X. The proton hits X and is absorbed.

    The resulting nucleus splits into two alpha particles, which move off in the directions shown with equal Ek of (1.481)(10-12)J

    1. Calculate the mass equivalence of the net energy release during the collision.
    2. Calculate the rest mass of the nucleus X. The mass of a proton is (1.673)(10-27) kg and the mass of an alpha particle is (6.645)(10-27)kg

    2. Relevant equations

    3. The attempt at a solution
    First question I have no idea how to do that.

    second question, I simply added the proton mass + alpha particle mass (which is a wrong answer since energy is used for binding energy)

    I sort of get the concepts but can't do them mathematically...
    Please help me step by step.
    1. 3.09X10-29
    2. 11.6X10-27
  2. jcsd
  3. Apr 30, 2013 #2
    The guiding principle is mass/energy conservation.

    initial (rest mass energy + kinetic energy)= final (rest mass energy + kinetic energy)

    Question 1 is just concerned with the increase of kinetic energy expressed as a mass equivalence.
    Question 2 just use the above equation. Be careful to include all rest masses and kinetic energies and express them all in the same unit.
  4. May 2, 2013 #3
    Sorry, I really do not know how to do these questions...
  5. May 2, 2013 #4
    For question 1 find the difference between the initial kinetic energy and the final kinetic energy. Then use E=mc^2 to convert energy to a mass equivalent.
  6. May 2, 2013 #5
    Thank you I've finally got and understood the first question.
    I still don't know how to solve the second question... :/
  7. May 2, 2013 #6
    The increase in kinetic energy must have come from a reduction in rest mass. In other words the mass of the two alphas is less than the mass of X and the proton. You have values for the rest mass of the alphas and the proton and the mass equivalent of the kinetic energy gained.
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