Answer
(a) decrease
(b) $1.05\times 10^5m/s$
Work Step by Step
(a) We know that $r=\frac{mv}{qB}$. This equation shows that the separation $r$ and $v$ are directly proportional. Thus, if we want the separation to become zero, then we should decrease the speed.
(b) We can find the required initial speed as follows:
$v=\frac{qB(r_{238})}{m}$
We plug in the known values to obtain:
$v=\frac{(1.6\times 10^{-19})(0.75)(0.34)}{3.95\times 10^{-25}}$
$v=1.05\times 10^5m/s$
