Question

Two relativistic particles with opposite velocities collide head-on and come to rest by sticking with each other. Which of the following quantities is/are conserved in the collision?

• A. Total momentum
• B. Total energy
• C. Total kinetic energy
• D. Total rest mass

Hint:

In relativistic inelastic collisions, total momentum and total energy are conserved, but not kinetic energy or rest mass separately.

Answer 1

The Correct Option is A, B

Step 1: Analyze conservation laws.
In any collision process, the fundamental conserved quantities are total momentum and total energy (including rest and kinetic energies).

Step 2: Relativistic collision condition.
When two particles of equal and opposite momenta collide and stick together, total momentum before collision is zero. Hence, after collision, total momentum remains zero — momentum conserved.

Step 3: Check kinetic and rest mass conservation.
Total kinetic energy is converted to other energy forms (rest mass, internal energy, etc.), so it is not conserved. However, total energy (including rest energy) is conserved. Rest mass, on the other hand, changes due to binding energy.

Step 4: Conclusion.
Hence, total momentum and total energy are conserved.