Explain why static charges do not build up on a conductor

Because it is conducted away IF the conductor is connected to earth;otherwise as the first feller said it will remain where it is and act as in a capacitor.Example is in the gold leaf electroscope where two gold pieces of gold leaf with the same charge visibly separate from one another;but if the electroscope is earthed the charges disappear and the leaves fall back into contact.As to hospital operating theatres such a spray of fine droplets would be attracted to surfaces especially if they were 
charged and would kill germs in unreachable places.A bit like electrostatic painting of car bodies during manufacture where the paint drops ae electrostatic-ally drawn to all surfaces of a car body which is at high voltage of electrostatic charge.  hope that helped

Static charge builds on anon moving object so the charges in a conductor are moving so it cannot build up

Answer: fn = wcos50 = 38.89(9.81)(cos50) = 245.23 n