Assertion When a charged particle moves perpendicular to a uniform magnetic field then its momentum remains constant.
Reason Magnetic force acts perpendicular to the velocity of the particle.

Options

A. If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If Assertion is false but Reason is true.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that when a charged particle moves perpendicular to a uniform magnetic field, its momentum remains constant.

**Hint:** Consider the definition of momentum and how it relates to velocity.

### Step 2: Analyze the Momentum
Momentum (p) is defined as the product of mass (m) and velocity (v):
\[ p = m \cdot v \]
When a charged particle moves in a magnetic field, it experiences a magnetic force that acts perpendicular to its velocity. This force causes the particle to move in a circular path, meaning its direction changes continuously.

**Hint:** Think about what happens to the velocity of the particle as it moves in a circular path.

### Step 3: Determine the Effect on Momentum
Since the velocity of the particle is changing due to the circular motion (even though the speed may remain constant), the momentum is also changing. This means that the assertion is false.

**Hint:** Reflect on how changing velocity affects momentum.

### Step 4: Understand the Reason
The reason states that the magnetic force acts perpendicular to the velocity of the particle. This is indeed true, as the magnetic force is given by:
\[ \mathbf{F} = q (\mathbf{v} \times \mathbf{B}) \]
where \( q \) is the charge of the particle, \( \mathbf{v} \) is its velocity, and \( \mathbf{B} \) is the magnetic field.

**Hint:** Recall the properties of the cross product and how it determines the direction of the force.

### Step 5: Conclusion
Since the assertion is false (momentum does not remain constant) and the reason is true (magnetic force acts perpendicular to velocity), the correct answer is that the assertion is false but the reason is true.

**Final Answer:** The fourth option is correct: "Reason is false but assertion is true."