A : Internal forces can change the kinetic energy but not the momentum of the system.
R : The net internal force on a system is always zero.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are falsė statements, then mark (4).

Answer

To solve the question, we need to analyze the assertion (A) and the reason (R) provided:

**Assertion (A):** Internal forces can change the kinetic energy but not the momentum of the system.
**Reason (R):** The net internal force on a system is always zero.

### Step-by-Step Solution:

1. **Understanding Internal Forces:**
- Internal forces are the forces that act between the particles within a system. For example, if we consider a system of two particles, the force exerted by particle A on particle B is an internal force, and vice versa.

**Hint:** Remember that internal forces are those that act within the system and do not include external influences.

2. **Net Internal Force:**
- When we consider a closed system, the internal forces acting on the particles of the system will cancel each other out. This means that the vector sum of all internal forces is zero.

**Hint:** Think about Newton's third law: for every action, there is an equal and opposite reaction. This leads to cancellation of internal forces.

3. **Effect on Momentum:**
- The momentum of a system is defined as the total mass of the system multiplied by its velocity. According to Newton's second law, the change in momentum (dP) of a system is related to the net external force acting on it. If the net internal force is zero, then the total change in momentum of the system is also zero.

**Hint:** Recall that momentum is conserved in an isolated system where no external forces are acting.

4. **Effect on Kinetic Energy:**
- While internal forces do not change the total momentum of the system, they can change the kinetic energy. For instance, in an explosion (a system of particles), the internal forces can cause the particles to move apart, thus increasing their kinetic energy, even though the total momentum remains constant.

**Hint:** Consider scenarios like explosions or collisions where internal forces can redistribute energy without affecting the overall momentum.

5. **Conclusion:**
- Since internal forces can change the kinetic energy of the system (as seen in explosive scenarios) but do not affect the total momentum (due to the net internal force being zero), both the assertion and the reason are correct. Moreover, the reason provided correctly explains the assertion.

**Final Answer:** Both assertion (A) and reason (R) are true, and R is the correct explanation for A.

### Summary:
- **Assertion (A)** is true: Internal forces can change kinetic energy but not momentum.
- **Reason (R)** is true: The net internal force on a system is always zero.
- Therefore, both A and R are true, and R is the correct explanation for A.