When selecting seeds of crops or trees, "salt water seed selection" is usually used. Xiaoming and his classmates participate in the practical activities of agricultural scientists using salt water to select seeds. (1) (1) salt water seed selection is based on the density of the seeds in the water buoyancy is different, so that the seeds will be selected out, so the use of physical knowledge of density, buoyancy.

(2) The density of salt water is high. This is because the salt water in the glass to the measuring cylinder when the pouring is not complete, so that the volume of the salt water to reduce the measured salt water, and therefore the density of the salt water measured on the high side.

(3) Let the volume of the sample of brine for V,

In accordance with the meaning of the question: V = V water + V salt

Total mass of brine: m = m water + m salt = ρ water V water + ρ salt V salt

By the significance of the salt content of brine can be seen, the mass of the salt: m salt = TV = ρ salt V salt

T=

m salt

V

=

ρ Salt V Salt

V

= < td>V

ρSalt (V?V water)

And the volume of water in the brine: V water=

=

m water

ρ water

ρSaline Water V?TV

ρWater

=

ρ Salt Water?T

ρWater

V

Substituting into the above equation collapses the equation to give: T = ρSalt

ρSalt Water?ρWater

ρSalt?ρWater

Density of sampled brine: ρBrine =

1.05kg

10?3m3

= 1.05×103kg/m3

Sample brine content:T=

2.7×103Kg/m3(1.05×103kg/m3?1.0×103kg/m3)

2.7×103kg/m3?1.0×103kg/m3

≈79kg/m3

Answer: (1) The physics used is density and buoyancy;

(2) It is large because the water can't be poured all the way through, so the measured volume of water is small;

(3) (3) The salt content of the prepared salt water is small. ) The salt content of the prepared brine is 79 kg/m3.