* Elastic Collision: An elastic collision is one where kinetic energy is conserved. This means that the total kinetic energy of the system (the two helium atoms) before the collision is equal to the total kinetic energy after the collision.

* Momentum Conservation: In *any* collision, momentum is always conserved. This is a fundamental principle in physics known as the law of conservation of momentum. It states that the total momentum of a closed system (like our two helium atoms) remains constant.

Why Momentum is Conserved in Elastic Collisions

While kinetic energy is conserved in elastic collisions, momentum conservation applies to all collisions, regardless of whether they are elastic or inelastic. Momentum is a measure of an object's mass in motion and is calculated by:

* Momentum (p) = mass (m) x velocity (v)

Because momentum is a vector quantity (having both magnitude and direction), the total momentum of the system before the collision is the vector sum of the individual momenta of the two atoms. After the collision, the total momentum is again the vector sum of their individual momenta. The principle of momentum conservation requires that these two sums be equal.

In summary:

* In an elastic collision between two helium atoms, both momentum and kinetic energy are conserved.

* Momentum is always conserved in any type of collision.