____is friction that acts on objects when they are sliding over a surface

Options

A. Sliding friction
B. Static friction
C. Rolling friction
D. Fluid friction

Answer

The answer to the question "____ is friction that acts on objects when they are sliding over a surface" is **sliding friction**.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the type of friction that occurs when objects slide over a surface.

2. **Identifying Types of Friction**: There are different types of friction:
- **Static Friction**: This type of friction acts on objects that are not moving. It prevents motion until a certain force is applied.
- **Sliding Friction**: This occurs when two surfaces slide against each other. It is the friction that opposes the motion of an object sliding over a surface.
- **Rolling Friction**: This type of friction occurs when an object rolls over a surface, like a wheel or a ball.
- **Fluid Friction**: This occurs when an object moves through a fluid (liquid or gas).

3. **Analyzing the Options**: Based on the definitions:
- Since the question specifically mentions "sliding over a surface," we can eliminate static friction, rolling friction, and fluid friction.
- The only type of friction that applies to sliding objects is sliding friction.

4. **Conclusion**: Therefore, the correct answer to fill in the blank is **sliding friction**.

### Final Answer:
**Sliding friction**