(a) Charge qA is at rest at the origin in system S; charge qB flies by at speed v on a trajectory parallel to the x axis, but at y = d. What is the electromagnetic force on qB as it crosses the y axis?

(b) Now study the same problem from system S, which moves to the right with speed v. What is the force on qB when qA passes the y axis? [Do it two ways: (i) by using your answer to (a) and transforming the force; (ii) by computing the fields in S and using the Lorentz force law.]