JEE Main 2026 Jan 21 Shift 1, Physics Q8: Motion of a Charged Particle in a Magnetic Field

Inside a (vertically-oriented) solenoid the magnetic field $\vec{B}$ is along its axis. The particle starts from rest and falls under gravity, so its velocity $\vec{v}$ is also along the axis — parallel (or anti-parallel) to $\vec{B}$.

The magnetic force ${\vec{F}}_B=q\vec{v}\times \vec{B}=0$ when $\vec{v}\parallel \vec{B}$. Hence only gravity acts on the particle, giving $a=g$.