A proton is fired from origin with velocity $\overrightarrow{\mathrm{v}}={\mathrm{v}}_0\hat{\mathrm{j}}+{\mathrm{v}}_0\hat{\mathrm{k}}$ in a uniform magnetic field $\overrightarrow{\mathrm{B}}={\mathrm{B}}_0\hat{\mathrm{j}}$.

In the subsequent motion of the proton

The path of the particle will be helix as shown in figure Clearly x-coordinate is always negative except at points A,C . x and z coordinate will be zero at the same time at points A, C etc, at all other points z coordinate is negative .

$\mathrm{y}={\mathrm{v}}_0\mathrm{t}\Rightarrow \mathrm{y}\propto \mathrm{t}$