In a collision where two equal masses collide head-on and stick together, is momentum conserved?

• A. No; momentum is not conserved in inelastic collisions.
• B. Yes; momentum is conserved and the final velocity is 1 m/s in the original direction.
• C. Yes; momentum is conserved but the final velocity is 2 m/s in the original direction.
• D. Momentum is conserved only when external forces are zero.

Answer: (B)

Momentum stays the same for a system when there’s no external push on it. If two equal masses collide and stick, the total momentum before the collision equals the total momentum after they become a single 2m object.

If one mass is moving at 2 m/s and the other is at rest, the initial momentum is m(2) + m(0) = 2m. After sticking, the combined mass is 2m, so the final velocity is p divided by total mass: v_final = (2m) / (2m) = 1 m/s in the original direction. That shows momentum is conserved and gives a final speed of 1 m/s, which matches the stated answer.

The other options misstate either the conservation idea (inelastic collisions still conserve momentum) or rely on a specific final speed that doesn’t follow from the given scenario, or introduce external forces as a requirement.