Assertion (A) : Collision between two particles is not necessarily associated with physical contact between them
Reason (R) : Only in physical contact momentum transfer takes place.

Collision between two particles is a term often used in physics to describe a specific type of interaction, but it doesn't always require physical contact in the macroscopic sense that we commonly think of.

In classical physics, a collision typically refers to a situation where two or more objects come into close proximity and interact in such a way that they exchange momentum and energy. This interaction can occur through various mechanisms, including:

1. **Direct Contact Collisions:** In many everyday situations, objects collide by physically touching each other. For example, when two billiard balls collide on a table, they make physical contact, and their interaction is described as a collision.

2. **Electromagnetic Interactions:** Particles with electric charge can interact with each other through electromagnetic forces, even without physical contact. For example, electrons repel each other due to their like charges, and this repulsion can be described as a collision in the context of electromagnetic interactions.

3. **Gravitational Interactions:** In celestial mechanics, massive objects like planets and stars can interact through gravitational forces. They can influence each other's trajectories and velocities without direct physical contact, which is also considered a form of collision in the context of gravitational interactions.

4. **Subatomic Collisions:** In the realm of subatomic particles and quantum mechanics, particles can collide and interact through fundamental forces such as the strong nuclear force or the weak nuclear force. These interactions don't involve macroscopic physical contact but are considered collisions in the particle physics sense.

So, while the term "collision" often conjures images of objects physically striking each other, it's important to recognize that in the broader realm of physics, collisions can involve a wide range of interactions, some of which don't require what we typically think of as physical contact at our everyday scale.

Momentum transfer takes place in all types of collisions provided the net external forece is zero.