An object cannot remain at rest unless which of the following holds? the net force acting on it is zero. the net force acting on it is constant. there are no forces at all acting on it. there is only one force acting on it.

An object cannot remain at rest unless the net force acting on it is zero.  It also can't move with constant, uniform motion (in a straight line at constant speed).

If the net force on it is anything other than zero, then the object must accelerate.  (Either its speed, or its direction, or both, must be changing.)  

volume: 22

density: 7.04

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