The fact that the magnetic field generates a force perpendicular to the instantaneous velocity of the particle has implications for the work that the field does on the particle. As a consequence, if only the magnetic field acts on the particle, its kinetic energy will .
__ increase over time
__ decrease over time
__ remain constant
__ oscillate

The kinetic energy will remain constant

Explanation:

Charged particles in a magnetic field feel a force perpendicular to their velocity,  since the force is F = qvB, and a charged particle feels a force of constant magnitude always directed perpendicular to its motion which causes circular motion in the magnetic field but the speed and kinetic energy of the particle will remain constant.

Therefore, if only the magnetic field acts on the particle, its kinetic energy will remain constant.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.