A friend and I have been discussing what we think is an error in our textbook (either that or a misunderstanding on our part). The book gives an example in which two protons are initially traveling parallel to one another in the same direction with equal velocity. We think that since the protons are at rest with respect to one another, initially they will feel no magnetic force--only electrostatic repulsion. Then, once the protons begin to move away from one another, one proton will generate a magnetic field because of its velocity with respect to the other--but neither proton will feel a magnetic force because the component of the magnetic fields are zero on the axis along which the protons have velocity. However, the book says that the protons DO feel a magnetic force because they have a velocity (this velocity is with respect to some seemingly arbitrary coordinate system). How can this be true? If it were, different observers in different inertial reference frames would see different magnetic fields acting on these protons; for example, a person traveling with the protons would see no magnetic field due to their initial velocity and would predict that the protons will only be influenced by electrostatic forces, whereas a person moving in the opposite direction of the protons would predict that, because the protons have a velocity, they will exert magnetic forces in opposition to the electrostatic--this leads to different proton trajectories depending on which reference frame you're in--clearly not possible!