If `u_(1), u_(2)` and `v_(1), v_(2)` are the initial and final velocities of two particles before and after collision respectively, then `(v_(2)-v_(1))/(u_(1)-u_(2))` is called ___________.

Answer

To solve the question, we need to identify what the expression \((v_2 - v_1) / (u_1 - u_2)\) represents in the context of collisions between two particles.

### Step-by-Step Solution:

1. **Understanding the Variables**:
- Let \(u_1\) and \(u_2\) be the initial velocities of the two particles before the collision.
- Let \(v_1\) and \(v_2\) be the final velocities of the two particles after the collision.

2. **Identifying the Terms**:
- The term \((v_2 - v_1)\) represents the relative velocity of separation after the collision. This indicates how fast the two particles are moving apart after they collide.
- The term \((u_1 - u_2)\) represents the relative velocity of approach before the collision. This indicates how fast the two particles were moving towards each other before they collided.

3. **Understanding the Concept**:
- The ratio of the relative velocity of separation to the relative velocity of approach gives us a measure of how elastic the collision is. This ratio is defined as the coefficient of restitution.

4. **Defining Coefficient of Restitution**:
- The coefficient of restitution \(E\) is defined as:
\[ E = \frac{v_2 - v_1}{u_1 - u_2} \]
- This coefficient quantifies the elasticity of the collision. If \(E = 1\), the collision is perfectly elastic; if \(E = 0\), the collision is perfectly inelastic.

5. **Conclusion**:
- Therefore, the expression \((v_2 - v_1) / (u_1 - u_2)\) is called the **coefficient of restitution**.

### Final Answer:
The answer is **coefficient of restitution**.