Chapter 3, Problem 9Q

If $\stackrel{\to }{F}$ = q( $\stackrel{\to }{v}$ × $\stackrel{\to }{B}$) and $\stackrel{\to }{v}$ is perpendicular to $\stackrel{\to }{B}$, then what is the direction of $\stackrel{\to }{B}$ in the three situations shown in Fig. 3-24 when constant q is (a) positive and (b) negative?

Figure 3-24 Question 9.