The principle of conservation of linear momentum states that the linear momentum of a system

Options

A. cannot be changed
B. cannot remain constant
C. can be changed if only internal forces act
D. can be changed only if external forces act

Explanation

To solve the question regarding the principle of conservation of linear momentum, we will analyze the options provided and determine which one accurately reflects the principle.

### Step-by-Step Solution:

1. **Understanding the Principle of Conservation of Linear Momentum**:
- The principle states that the total linear momentum of a closed system remains constant if no external forces act on it. This means that the momentum can change only when external forces are applied.

2. **Analyzing the Options**:
- **Option 1**: "Cannot be changed"
- This option suggests that linear momentum cannot change at all, which is incorrect. Momentum can change if external forces are applied.

- **Option 2**: "Cannot remain constant"
- This option implies that momentum cannot remain constant, which is also incorrect. Momentum can remain constant if no external forces are acting on the system.

- **Option 3**: "Can be changed if only internal forces act"
- This option is misleading. Internal forces within a system cancel each other out due to Newton's third law, and thus do not affect the total momentum of the system. Therefore, this option is also incorrect.

- **Option 4**: "Can be changed only if external forces act"
- This option is correct. It aligns with the principle of conservation of momentum, which states that momentum changes only when external forces are applied.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Option 4**: "Can be changed only if external forces act."