Which sentence is true for frictional force?

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Frictional Force**: The frictional force is a force that opposes the motion of an object when it is in contact with a surface. It acts in the opposite direction to the movement of the object.

**Hint**: Remember that friction always works against the direction of motion.

2. **Identifying True Statements**: We need to evaluate the statements provided to determine which one is true regarding frictional force.

3. **Analyzing Each Statement**:
- **Statement 1**: "Frictional force always opposes motion."
- This statement is true because friction acts against the direction of movement.

- **Statement 2**: "Frictional force brings a ball down when we throw it up in the air."
- This statement is false because the force that brings the ball down is gravitational force, not frictional force.

- **Statement 3**: "Frictional force helps to change the direction of moving objects."
- This statement is also false. Friction does not change the direction of motion; it primarily works to slow down or stop the object.

- **Statement 4**: "Frictional force is sometimes attractive and sometimes repulsive."
- This statement is false. Friction is a contact force and does not exhibit attractive or repulsive characteristics.

4. **Conclusion**: After analyzing all the statements, we conclude that the first statement is the only true statement regarding frictional force.

**Final Answer**: The true statement about frictional force is: "Frictional force always opposes motion."

---