A magnetic field B is along the +y axis and a proton moves along the +x axis. The direction of the magnetic force on the proton is:

1. Force on a positive charge: $\mathbf{F} = q(\mathbf{v} \times \mathbf{B})$. 2. Here $\mathbf{v} = v\hat{i}$ and $\mathbf{B} = B\hat{j}$. 3. Cross product: $\hat{i} \times \hat{j} = \hat{k}$, so $\mathbf{v} \times \mathbf{B}$ is along $+z$. 4. For a positive proton the force is along the $+z$ axis (a negative charge would feel it along $-z$). _Source: syllabus Physics Ch 4 "Moving Charges and Magnetism", p.136_