Choose the correct statement(s) about the frictional force between two solid surfaces in contact

Options

A. Static friction is a variable force
B. `bar f_lim = mu_s N`
C. Kinetic friction is self adjusting
D. Both (1) and (2)

Answer

To determine the correct statements about the frictional force between two solid surfaces in contact, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Static Friction**:
- Static friction is the frictional force that prevents two surfaces from sliding past each other when a force is applied. It can vary depending on the applied force up to a certain limit.
- **Statement**: "Static friction is a variable force."
- **Analysis**: This statement is **correct**. Static friction can change in magnitude based on the applied force until it reaches its maximum value (limiting friction).

2. **Limiting Friction**:
- Limiting friction is the maximum static frictional force that can act before motion begins. It can be calculated using the formula:
\[ F_{\text{limiting}} = \mu_s \times N \]
where \( \mu_s \) is the coefficient of static friction and \( N \) is the normal force.
- **Statement**: "Limiting friction force is \( \mu_s \) times the normal force."
- **Analysis**: This statement is **correct**. It accurately describes the relationship between limiting friction, the coefficient of static friction, and the normal force.

3. **Understanding Kinetic Friction**:
- Kinetic friction occurs when two surfaces are sliding against each other. Unlike static friction, kinetic friction typically has a constant value that does not change with the applied force.
- **Statement**: "Kinetic friction is self-adjusting."
- **Analysis**: This statement is **incorrect**. Kinetic friction does not adjust itself; it remains constant for a given pair of surfaces and conditions.

4. **Conclusion**:
- Based on the analysis, the correct statements are:
- Static friction is a variable force.
- Limiting friction force is \( \mu_s \) times the normal force.
- The incorrect statement is that kinetic friction is self-adjusting.

### Final Answer:
The correct statements about the frictional force between two solid surfaces in contact are:
1. Static friction is a variable force.
2. Limiting friction force is \( \mu_s \) times the normal force.