The jeans equations: (a) consider the dark matter density profile p(r) (5) 4πgr2 which gives rise to constant rotation curves mal" and isotropic suppose that the dark matter is "isother- so that the 1d velocity dispersion o2 = a^0,d the jeans equation analog for "hydrostatic equilibrium" in spherical coordinates and =const. wvrite down v2/2 for this system show o2 galaxy at large radius r 3.5, but they contribute mass to the system. again using the jeans equations (but using gravity set (b) next consider their distribution is approximately spherical with p. c population of stars which orbit the same a negligible by the dark matter of equation 5), solve for o2

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

conservation of mass

mass before something happens = mass afterwards