Which of the following statements is correct

Options

A. Work done by static friction is always zero
B. Work done by kinetic friction is always negative
C. The negative of the work done by the conservative internal forces on a system equals the change in kinetic energy
D. The work dony by all the forces on a system equals the change in kinetic energy

Answer

To solve the question regarding which statements are correct about work, energy, and friction, we will analyze each statement one by one.

### Step-by-Step Solution

1. **Evaluate the First Statement:**
- **Statement:** "Work done by static friction is always zero."
- **Analysis:** Static friction acts on an object that is at rest. While it is true that static friction does not do work when there is no displacement (the object does not move), it can do work in some scenarios where it prevents slipping or allows for movement without relative motion. Therefore, this statement is not always true.
- **Conclusion:** This statement is **incorrect**.

2. **Evaluate the Second Statement:**
- **Statement:** "Work done by kinetic friction is always negative."
- **Analysis:** Kinetic friction acts on an object that is in motion. It opposes the direction of motion. When calculating work done by kinetic friction, the force of kinetic friction acts in the opposite direction to the displacement. Hence, the work done by kinetic friction is indeed negative.
- **Conclusion:** This statement is **correct**.

3. **Evaluate the Third Statement:**
- **Statement:** "The negative of the work done by the conservative internal force on a system equals the change in kinetic energy."
- **Analysis:** Internal forces, including conservative forces, do not do net work on a system because they cancel each other out. The work-energy theorem applies to external forces. Therefore, this statement is incorrect.
- **Conclusion:** This statement is **incorrect**.

4. **Evaluate the Fourth Statement:**
- **Statement:** "The work done by all forces on a system equals the change in kinetic energy."
- **Analysis:** This statement is a direct reflection of the work-energy theorem, which states that the total work done by all external forces on a system is equal to the change in kinetic energy of that system.
- **Conclusion:** This statement is **correct**.

### Final Summary
- The correct statements are:
- **Second Statement:** Work done by kinetic friction is always negative. **(Correct)**
- **Fourth Statement:** The work done by all forces on a system equals the change in kinetic energy. **(Correct)**