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Homework Help: Relativistic Kinematics

  1. Nov 12, 2015 #1
    1. The problem statement, all variables and given/known data

    A body at rest in a frame of reference S disintegrates into two pieces moving in opposite directions. The masses of each fragment are 3.0kg and 4.0kg and their velocities 0.8c and 0.6c, respectively. Find the mass of the body before it disintegrated. (Answer: 10kg)

    2. Relevant equations

    1. Energy-momentum relation
    [tex]E^2 = (pc)^2 + (m_0 c^2)^2 [/tex]

    2. Relativistic kinetic energy equation
    [tex]E_{ki} = m_i c^2 \left(\frac{1}{\sqrt{1-\frac{v_i^2}{c^2}}} -1 \right) [/tex]

    3. Relativistic momentum equation
    [tex]p_i = \frac{m_i v_i}{\sqrt{1-\frac{v_i^2}{c^2}}} [/tex]

    3. The attempt at a solution

    First I tried using conservation of energy, by taking the energy of the body at rest (1.), with p=0, and equating it to the sum of the kinetic energies of the two fragments (2.), which looked like this

    [tex] m_0c^2 = m_1c^2 \left( \frac{1}{\sqrt{1 - \frac{v_1^2}{c^2}}} - 1 \right) + m_2c^2 \left( \frac{1}{\sqrt{1 - \frac{v_2^2}{c^2}}} - 1 \right) [/tex]

    This yields [itex]m_0 = 3 \ kg[/itex] which doesn't make sense.

    Then it occurred to me that since I already have the answer [itex](10 kg)[/itex], which was provided in the problem, and both fragments sum up to [itex]7 kg[/itex] then there's [itex]3 kg[/itex] of mass missing, so I'm clearly skipping something here.

    Any hints?
  2. jcsd
  3. Nov 12, 2015 #2
    As far as I can see, you seem to be ignoring the mass-energy of the two pieces that fly apart - I think you're only considering their kinetic energy.
  4. Nov 12, 2015 #3
    You're absolutely right! Thank you, tomdodd4598
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