Collision is a physical process in which two or more objects, either particle masses or rigid bodies, experience very high force of interaction for a very small duration. It is not essential for the objects to physically touch each other for collision to occur. Irrespective of the nature of interactive force and the nature of colliding bodies, Newton's second law holds good on the system. Hence, momentum of the system before and after the collision remains conserved if no appreciable external force acts on the system during collision.
The amount of energy loss during collision, if at all, is indeed dependent on the nature of colliding objects. The energy loss is observed to be maximum when objects stick together after collision. The terminology is to define collision as 'elastic' if no energy loss takes place and to define collision as 'plastic' for maximum energy loss. The behaviour of system after collision depends on the position of colliding objects as well. A unidirectional motion of colliding objects before collision can turn into two dimensional after collision if the line joining the centre of mass of the two colliding objects is not parallel to the direction of velocity of each particle before collision. Such type of collision is referred to as oblique collision which may be either two or three dimensional.
According to the definition of collision in paragraph I, which of the following physical process is not a collision?

Options

A. A projectile exploding into three fragments at its highest point.
B. Two soap bubbles coalescing to form a bubble of larger radius.
C. A vertically upward thrown particle changing direction at its highest point.
D. A piece of magnet thrown on a metallic surface.

Explanation

To determine which of the given physical processes is not a collision, we need to analyze each option based on the definition of collision provided in the question. A collision involves two or more objects experiencing a high force of interaction for a very short duration, and it is not necessary for the objects to physically touch each other.

### Step-by-Step Solution:

1. **Understanding the Definition of Collision:**
- A collision involves a high force of interaction between two or more objects for a very short duration.
- It is not necessary for the objects to physically touch each other for a collision to occur.

2. **Analyzing Each Option:**
- **Option 1:** **Projectile exploding into three fragments at the highest point.**
- When a projectile explodes, the fragments experience a force of interaction due to the explosion. This fits the definition of a collision.

- **Option 2:** **Two soap bubbles collapsing to form a bigger bubble of larger radius.**
- When two soap bubbles combine, there is an interaction force as they merge. This also fits the definition of a collision.

- **Option 3:** **A vertically upward thrown particle changing its direction at the highest point.**
- At the highest point, the particle does not interact with any other object; it is only influenced by gravity. There is no force of interaction with another object, so this does not fit the definition of a collision.

- **Option 4:** **A piece of magnet thrown on a metal surface.**
- When the magnet is thrown onto the metal surface, there is a force of interaction when it makes contact. This fits the definition of a collision.

3. **Conclusion:**
- The only option that does not fit the definition of a collision is **Option 3: A vertically upward thrown particle changing its direction at the highest point.** This is because there is no interaction force with another object during this process.

### Final Answer:
**Option 3: A vertically upward thrown particle changing its direction at the highest point is not a collision.**

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