Which of the following is not a perfectly inelastic collision

The correct option is Striking of two glass balls.

To understand why this is the correct answer, let's break down the physical concepts of collisions:
1. Perfectly Inelastic Collision: A collision in which the colliding bodies stick together after the impact and move with a common velocity. In such collisions, the maximum possible kinetic energy is lost (converted into heat, sound, or deformation energy), though total momentum remains conserved.
2. Inelastic Collision: A collision where kinetic energy is not conserved, but the objects do not necessarily stick together. Some kinetic energy is lost.
3. Elastic Collision: A collision in which both momentum and kinetic energy are conserved. The bodies separate after the collision and tend to regain their original shape and size.

Let's analyze the given options based on these definitions:

• A bullet striking a bag of sand: The bullet gets embedded in the sand bag, and they move together (or remain at rest together). This is a classic example of a perfectly inelastic collision.
• An electron captured by a proton: The electron and proton combine to form a single entity (a hydrogen atom), meaning they stick together. This behaves as a perfectly inelastic collision at the quantum level.
• A man jumping onto a moving cart: Once the man jumps onto the cart, he moves along with the cart at the same common velocity. They stick together, representing a perfectly inelastic collision.
• Striking of two glass balls: When two glass balls collide, they do not stick together; instead, they rebound after impact. Since glass is relatively rigid, this collision is near-elastic (with a high coefficient of restitution), meaning it is definitely not a perfectly inelastic collision.

Therefore, the striking of two glass balls is not a perfectly inelastic collision.