JEE Physics 2026 Question with Solution

Given: The position vector changes as $\vec r \rightarrow -\vec r$.

Find: Which vector does not change sign under this transformation.

From the solution, if position changes sign, then velocity and acceleration also change sign:

$$\vec v = \frac{d\vec r}{dt} \rightarrow -\vec v, \qquad \vec a = \frac{d^2\vec r}{dt^2} \rightarrow -\vec a$$

Now check each option.

Linear momentum:

$$\vec p = m\vec v \rightarrow -\vec p$$

So linear momentum changes sign.

Velocity changes sign.

Acceleration changes sign.

Angular momentum is

$$\vec L = \vec r \times \vec p$$

Under sign change,

$$\vec L \rightarrow (-\vec r) \times (-\vec p) = \vec r \times \vec p$$

So angular momentum remains unchanged because the cross product of two vectors that both reverse sign stays the same.

Therefore, the correct option is B.

Given: $\vec r \rightarrow -\vec r$.

Find: The vector that does not flip sign.

Use the idea that quantities directly proportional to $\vec r$ or its time derivatives will reverse sign, but a cross product of two vectors that both reverse sign will not.

Since angular momentum is

$$\vec L = \vec r \times \vec p$$

and both $\vec r$ and $\vec p$ change sign, we get

$$(-\vec r) \times (-\vec p) = \vec r \times \vec p$$

Hence angular momentum does not flip.

Therefore, the correct option is B.