Final answer:
The magnetic force on a positively charged particle at rest in a uniform magnetic field is zero because the magnetic force is dependent on the particle having velocity.
Step-by-step explanation:
If a positively charged particle is momentarily at rest in a uniform magnetic field, there will be no magnetic force acting on the particle. The magnetic force on a charged particle is given by the Lorentz force law, which states that the force on the particle is equal to the charge times the cross product of the velocity of the particle and the magnetic field. Since the magnetic force depends on the velocity of the particle relative to the magnetic field, if the particle is not moving (velocity is zero), then the magnetic force is also zero, regardless of the orientation of the magnetic field.
In other words, for a magnetic force to exist on a charged particle, the particle must be moving within the magnetic field. The direction of this force is always perpendicular to both the direction of the magnetic field and the direction of the particle's motion, following the right-hand rule. Therefore, since the positively charged particle in question is at rest, it experiences no magnetic force.