Choose the correct statement.

Options

A. Work done by internal forces is equal to change in potential energy
B. net work done on the body is equal to change in total mechanical energy
C. net work done by all force other than conservative forces is equal to change in total mechanical energy
D. net work done on the system by internal forces is always zero

Answer

To solve the question, we need to analyze each of the statements provided and determine which one is correct based on the principles of physics, particularly the work-energy theorem and the concepts of conservative and non-conservative forces.

### Step-by-Step Solution:

1. **Understanding the Statements**:
- **Option A**: Work done by internal forces is equal to change in potential energy.
- **Option B**: Net work done on the body is equal to the change in total mechanical energy.
- **Option C**: Net work done by all the forces other than the conservative force is equal to the change in total mechanical energy.
- **Option D**: Net work done on the system by the internal forces is always zero.

2. **Analyzing Option A**:
- Internal forces do not do work that changes the potential energy of a system. Instead, potential energy changes are associated with conservative forces (like gravity). Therefore, this statement is **incorrect**.

3. **Analyzing Option B**:
- The net work done on a body is equal to the change in kinetic energy (Work-Energy Theorem). Total mechanical energy includes both kinetic and potential energy. Therefore, this statement is **incorrect**.

4. **Analyzing Option C**:
- According to the work-energy theorem, the net work done by non-conservative forces (like friction) is equal to the change in total mechanical energy. This statement is **correct**.

5. **Analyzing Option D**:
- While internal forces can do work, the net work done by internal forces in an isolated system is zero when considering the system as a whole. However, this statement is misleading as it does not account for the work done on individual particles within the system. Therefore, this statement is **incorrect**.

6. **Conclusion**:
- After analyzing all the options, the correct statement is **Option C**: Net work done by all the forces other than the conservative force is equal to the change in total mechanical energy.

### Final Answer:
The correct option is **C**.