In part (a) of the gure an electron is shot directly away from a uniformly charged plastic sheet, at speed v5 = 1.50 x 105 m/s. The sheet is nonconducting, at, and very large. Part (b) of the gure 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 t5 = 24.0 ps. D: The gure shows an electron with a negative charge which is travelling vertically upward. Below the electron a at large plastic sheet is placed horizontally with uniform positive charges along the top of the sheet. A graph shows a negative slope. The vertical axis plots v in units of 10 to the power of 5 meters per second with values ranging from negative v subscript s to v subscripts four units below and above the origin. The horizontal axis plots t in Pico seconds with values ranging from 0 to t subscript s, 14 units from the origin. A line begins from (0 comma, v subscript s) descends, cuts the horizontal axis at the point (0 comma, one half t subscript s) and ends at the point (t subscript s comma, negative v subscripts)