Choose the correct statement (s) :

Options

A. Totalworkdonebyinternalforcesofasystemonthesystemisalwayszero.
B. Totalworkdonebyinternalforcesofasystemonthesystemissometimeszero.
C. Totalworkdonebyinternalforcesactingbetweentheparticlesofarigidbodyisalwayszero.
D. Totalworkdonebyinternalforcesactingbetweentheparticlesofarigidbodyissometimeszero.

Answer

To solve the question regarding the correctness of the statements about work done by internal forces, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Statement 1**: "Total work done by internal forces of a system is always 0."
- **Analysis**: This statement is incorrect. The total work done by internal forces can be non-zero if there is a net external force acting on the system that causes a change in kinetic energy. For example, if a block is being pushed and there is kinetic friction opposing the motion, the work done by the applied force can exceed the work done by friction, resulting in a net positive work done on the system.
- **Conclusion**: **Wrong**

2. **Statement 2**: "Work done by internal forces of a system is sometimes zero."
- **Analysis**: This statement is correct. If the net external forces acting on the system are balanced (e.g., applied force equals friction), there will be no change in kinetic energy, and thus the work done by internal forces can be zero.
- **Conclusion**: **Correct**

3. **Statement 3**: "Total work done by internal forces acting between the particles of a rigid body is always zero."
- **Analysis**: This statement is correct. In a rigid body, internal forces are equal and opposite (Newton's third law). For every internal force doing positive work, there is an equal internal force doing negative work, leading to a total work of zero.
- **Conclusion**: **Correct**

4. **Statement 4**: "Total work done by internal forces acting between the particles of a rigid body is sometimes zero."
- **Analysis**: This statement is incorrect. As established in the analysis of Statement 3, the work done by internal forces in a rigid body is always zero due to the nature of internal forces being equal and opposite.
- **Conclusion**: **Wrong**

### Final Conclusion:
The correct statements are **B** and **C**.