Which condition is necessary for momentum to be conserved in a collision?

• A. No external forces act
• B. Masses are equal
• C. Collision is elastic
• D. Momentum is always conserved

Answer: (A)

Momentum is conserved in a collision when the system has no external impulse acting on it during the interaction. If there are no external forces (or their net effect over the collision time is zero), the total momentum before the collision equals the total momentum after. The collision itself involves internal forces between the bodies, which cancel in the total, so nothing outside the system changes the overall momentum.

This doesn’t require equal masses or an elastic collision. Momentum is conserved for both elastic and inelastic collisions as long as external forces are absent. The statement that momentum is always conserved isn’t true in the presence of external forces, since those forces can impart an impulse and change the total momentum.