Relativistic Momentum and Energy · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Energy hiding in mass itself

Science · Quantum & Modern Physics · ages 19-20
Name ______________________   Date ____________

  1. A lump of mass sits perfectly still. What energy does it hold?

    • None at all
    • Only heat
    • Rest energy m c squared

  2. A particle speeds up toward light speed. What happens to gamma m u?

    • It stays equal to m u
    • It grows past the classical m u
    • It falls below m u

  3. Where does rest energy show itself?

    • In fission, fusion, and annihilation, as lost mass becomes energy
    • Only in hot objects
    • Only in moving objects

  4. How are total energy, momentum, and rest mass tied?

    • Momentum equals energy divided by mass
    • E squared equals p c squared plus m c squared squared
    • E equals p plus m always

  5. In units with c set to 1, a photon carries energy 4. Type its momentum.

    Answer: ______________

  6. A slow electron and a fast one share the same mass. Why do their momenta differ more than their speeds?

    • Mass grows with temperature
    • Charge increases with speed
    • Gamma stretches the fast momentum beyond m u

  7. A fusion reactor turns a small mass of hydrogen fuel into helium plus a large energy output. What explains the extra energy?

    • Converted rest energy of the lost mass
    • Heat leaking in from outside
    • New protons created from nothing

  8. A powerful enough rocket could push a massive ship past light speed.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Energy hiding in mass itself W1-mt_zBq87F-j3B-s1

  1. Rest energy m c squared · Even at rest, mass holds m c squared of locked energy.
  2. It grows past the classical m u · Gamma climbs with speed, so fast momentum outgrows the classical product.
  3. In fission, fusion, and annihilation, as lost mass becomes energy · Converting mass in nuclear reactions releases its rest energy.
  4. E squared equals p c squared plus m c squared squared · One relation binds all three: E squared equals the momentum term plus the mass term.
  5. 4 · With E equals p c and c set to 1, momentum equals energy: 4.
  6. Gamma stretches the fast momentum beyond m u · The fast electron carries a gamma factor the slow one nearly lacks.
  7. Converted rest energy of the lost mass · The missing mass returns as energy through m c squared.
  8. False · Its energy and momentum would need to become infinite first.
Worksheet · LightMySky