Final answer:
In a uniform magnetic field, a moving positively charged particle like a proton experiences a force that is perpendicular to both its velocity and the magnetic field. This force results in a proton performing a circular motion, and according to the right-hand rule, the force acts in the y-direction.
Step-by-step explanation:
A proton moving in the -x-direction encounters a uniform magnetic field pointing in the +x-direction. According to the right-hand rule, if we point our thumb in the direction of the proton's velocity (-x-direction), and our fingers in the direction of the magnetic field (+x-direction), then our palm faces the direction of the force exerted on a positively charged particle like a proton. Thus, the magnetic force acts in the y-direction, either up or down depending on which hand you are supposed to use for the right-hand rule (either the actual right hand or a hypothetical left hand, if you imagine a 'left-hand rule' for positive charges). However, as protons are positively charged, we use the right-hand rule indicating that the force will be upwards in the y-direction if the proton's movement in the -x-direction is considered to be into the page.
The force experienced by a proton in a uniform magnetic field remains perpendicular to its velocity, which means the magnitude of the velocity does not change; instead, the proton undergoes circular motion. The direction of the force is at a right angle to both the magnetic field and the proton's velocity vector. Therefore, the resultant magnetic force acting on the proton is both perpendicular to its motion in the -x-direction and to the magnetic field directed in the +x-direction.