Okay i'm totally stuck and nobody i know really gets it either, so i've turned to yahoo for : )
basically there are four aliens on a planet. they want to go on a sea-saw. the end seats on the sea-saw are 2m from the pivot, and the middle seats are 1m from the pivot. the aliens way 400n, 300n, 200n and 100n - which two can sit on the sea-saw (on any of the seats) and make it balance? it'd be a huge if you at least me work out the answer! : )

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia