To solve the question, we need to analyze each of the given statements regarding friction and determine which one is correct.
### Step-by-Step Solution:
1. **Understanding Static Friction**:
- The first statement claims that the work done by static friction on a body will always be positive.
- Static friction can act in the direction of motion (doing positive work) or against it (doing negative work), or it can be zero if there is no displacement. Therefore, this statement is incorrect.
2. **Analyzing Work Done by Static Friction**:
- Consider a scenario where a block is on top of another block. If the top block moves with the bottom block, static friction acts in the direction of the top block's motion, doing positive work.
- Conversely, if the bottom block is pulled forward and the top block does not move, static friction acts backward on the bottom block, doing negative work.
- Thus, static friction can do positive, negative, or zero work. This means the statement that work done by static friction may be positive, negative, or zero is correct.
3. **Understanding Kinetic Friction**:
- The second statement suggests that the work done by kinetic friction on a body will always be negative.
- Kinetic friction always acts in the opposite direction to the motion of the body. Therefore, the angle between the force of kinetic friction and the displacement is 180 degrees, resulting in negative work. This statement is also correct.
4. **Conclusion**:
- After analyzing the statements, we find that:
- Statement A is incorrect (work done by static friction is not always positive).
- Statement B is correct (work done by kinetic friction is always negative).
- Statement C is incorrect (work done by static friction is not always zero).
- Statement D is correct (work done by static friction can be positive, negative, or zero).
- Therefore, the correct statements are B and D.
### Final Answer:
The correct statements are:
- **B**: Work done by kinetic friction on a body will always be negative.
- **D**: Work done by static friction on a body may be positive, negative, or zero.
