Ball bearing is a device which usually converts :

Options

A. rolling friction into sliding friction
B. static friction into sliding friction
C. sliding friction into rolling friction
D. rolling friction into static friction

Answer

### Step-by-Step Solution:

1. **Understanding Ball Bearings**:
- A ball bearing is a mechanical device that helps reduce friction between moving parts. It consists of small metal balls that facilitate smooth movement.

2. **Friction in Motion**:
- When a block is placed on a surface, it experiences friction, which opposes its motion. This friction is known as static friction when the block is at rest.

3. **Application of Force**:
- To move the block, a force must be applied. If the friction is high, a greater force is needed to overcome it and initiate movement.

4. **Introduction of Ball Bearings**:
- By placing a layer of small metal balls (the ball bearings) between the block and the surface, the frictional force can be significantly reduced.

5. **Mechanism of Ball Bearings**:
- When the block is placed on the ball bearings and a force is applied, the balls roll instead of sliding. This rolling motion generates rolling friction, which is much lower than static friction.

6. **Comparison of Friction Types**:
- Static friction (when the block is stationary) is greater than rolling friction (when the block is moving over the rolling balls). This allows the block to move with less effort.

7. **Conclusion**:
- Therefore, ball bearings convert static friction into rolling friction, making it easier to move objects.

### Final Answer:
Ball bearings convert static friction into rolling friction.

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