In a perfectly inelastic collision, which quantity is conserved?

• A. Neither momentum nor kinetic energy is conserved.
• B. Both momentum and kinetic energy are conserved.
• C. Only momentum is conserved.
• D. Only kinetic energy is conserved.

Answer: (C)

In a collision, momentum is conserved when there are no external horizontal forces acting during the impact. A perfectly inelastic collision is when the two objects stick together, so they move with a common final velocity. The total momentum before the collision equals the total momentum after: m1v1 + m2v2 = (m1 + m2)*V. This is why momentum is conserved in this scenario.

Kinetic energy, on the other hand, is not generally conserved in inelastic collisions. Some of the initial kinetic energy is transformed into other forms of energy—such as heat, sound, and permanent deformation of the objects—so the final kinetic energy is typically less than the initial kinetic energy.