In part (a) of the figure an electron is shot directly away from a uniformly charged plastic sheet, at speed vs = 2.80×105 m/s. The sheet is nonconducting, flat, and very large. Part (b) of the figure gives the electron's vertical velocity component v versus time t until the return to the launch point. What is the sheet's surface charge density? Assume ts = 14.0 ps. (a) (b) Number Units