Final answer:
The magnetic force acts on a proton moving perpendicular to a magnetic field and is perpendicular to both the proton's velocity and the magnetic field direction.
Step-by-step explanation:
When a proton moves perpendicular to a magnetic field, the force that acts on it is the magnetic force. This force follows from the Lorentz force law, which states that a charged particle, like a proton, experiences a force when moving in a magnetic field. The direction of this force is perpendicular to both the velocity of the proton and the magnetic field, according to the right-hand rule.
For example, if a proton moves in the negative x-direction (-x) and encounters a uniform magnetic field in the positive x-direction (+x), then the magnetic force acting on the proton would also be perpendicular to both the velocity and the magnetic field. This magnetic force would cause the proton to undergo circular motion, as the force would serve as a centripetal force, keeping the magnitude of the proton's velocity constant while changing its direction.