Which of the following sentences is true for frictional force?

Options

A. Frictional force brings a ball down when we throw it up in air.
B. Frictional force helps to change the direction of moving objects.
C. Frictional force always opposes motion.
D. Frictional force is sometimes attractive and sometimes repulsive.

Answer

To determine which sentence is true for frictional force, we will analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding Frictional Force**:
- Frictional force is a force that opposes the relative motion between two surfaces in contact. It acts when one object tries to move over another.

2. **Analyzing Option A**:
- **Statement**: "Frictional force brings a ball down when thrown up in the air."
- **Analysis**: When a ball is thrown up, the force that brings it down is gravity, not friction. Therefore, this statement is **incorrect**.

3. **Analyzing Option B**:
- **Statement**: "Frictional force helps to change the direction of the moving object."
- **Analysis**: While friction can affect the motion of an object, it does not change the direction of motion by itself. Instead, it opposes the motion. Hence, this statement is also **incorrect**.

4. **Analyzing Option C**:
- **Statement**: "Frictional force always opposes motion."
- **Analysis**: This statement is true because frictional force acts in the opposite direction to the motion of the object. Therefore, this statement is **correct**.

5. **Analyzing Option D**:
- **Statement**: "Frictional force is sometimes attractive and sometimes repulsive."
- **Analysis**: This description is more applicable to magnetic forces rather than frictional forces. Friction does not have attractive or repulsive characteristics; it simply opposes motion. Thus, this statement is **incorrect**.

### Conclusion:
After analyzing all the options, we find that the correct statement about frictional force is:
- **C: Frictional force always opposes motion.**