A 15 g piece of Styrofoam carries a net charge of -0.5 μC and floats above the center of a very large horizontal sheet of plastic that has a uniform charge density on its surface.
The acceleration of gravity is 9.8 m/s2 and the permittivity of free space is 8.85419 × 10-12 C2/N/m2.
What is the charge per unit area on the plastic sheet?
Answer in units of μC/m2.