(A) : When a body moves on rough surface the mechanical energy is not constant
(R) : Friction is non conservative force

Options

A. Both (A) and (R) are true and (R) is the correct explanation of (A)
B. Both (A) and (R) are true and (R) is not the
C. (A) is true but ( R) is false
D. Both (A) and (R ) are false

Answer

To solve the assertion and reason question, we will analyze both statements step by step.

### Step 1: Understanding the Assertion
The assertion states that "when a body moves on a rough surface, the mechanical energy is not conserved."

**Explanation**:
- Mechanical energy consists of kinetic energy (energy of motion) and potential energy (stored energy due to position).
- On a rough surface, the presence of friction opposes the motion of the body.
- As the body moves, some of its mechanical energy is transformed into other forms of energy, primarily heat and sound, due to the work done against friction.

### Step 2: Understanding the Reason
The reason states that "friction is a non-conservative force."

**Explanation**:
- A conservative force is one where the work done is independent of the path taken and depends only on the initial and final positions. Examples include gravitational and elastic forces.
- In contrast, a non-conservative force, like friction, dissipates energy and the work done by friction depends on the path taken. This means that energy is not fully recoverable, leading to a loss in mechanical energy.

### Step 3: Connecting Assertion and Reason
Now, we can connect the two statements:
- Since friction is a non-conservative force, it causes mechanical energy to be converted into other forms of energy (like heat and sound) when a body moves on a rough surface.
- Therefore, the assertion that mechanical energy is not conserved when a body moves on a rough surface is true because of the presence of friction, which is a non-conservative force.

### Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion.

### Final Answer
Both assertion (A) and reason (R) are true, and R is the correct explanation of A.

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