A particle of mass $10 \mathrm{~g}$ and charge $80 \mu \mathrm{C}$ moves through a uniform magnetic field, in a region where the free-fall acceleration is $-9.8 \hat{\mathrm{j}} \mathrm{m} / \mathrm{s}^{2} .$ The velocity of the particle is a constant $20 \mathrm{i} \mathrm{km} / \mathrm{s}$. which is perpendicular to the magnetic field. What, then, is the magnetic field?