VE09. Ujuva keha tihedus

Ülesanne. Keha hõljub vees, tema ruumalast 80% on veepinnast madalamal. Mis on keha tihedus?

Lahendus.

Paneme kirja andmed:
rho subscript v e s i end subscript equals 1000 fraction numerator k g over denominator m cubed end fraction minus negative negative negative negative negative negative negative negative rho subscript k e h a end subscript minus ?

Kui keha hõljub vees, siis on kehale mõjuv raskusjõud tasakaalus vee üleslükkejõuga:
m subscript k e h a end subscript g equals rho subscript v e s i end subscript g V subscript v e s i end subscript
Asendame keha massi tema tiheduse ja ruumalaga. Lisaks arvestame, et vee ruumala, mille keha on enda alla võtnud, on 80% keha kogu ruumalast:
m subscript k e h a end subscript g equals rho subscript v e s i end subscript g V subscript v e s i end subscript rho subscript k e h a end subscript V subscript k e h a end subscript g equals rho subscript v e s i end subscript g V subscript v e s i end subscript rho subscript k e h a end subscript V subscript k e h a end subscript equals rho subscript v e s i end subscript V subscript v e s i end subscript rho subscript k e h a end subscript V subscript k e h a end subscript equals 1000 fraction numerator k g over denominator m cubed end fraction times 0 comma 8 times V subscript k e h a end subscript rho subscript k e h a end subscript equals 1000 fraction numerator k g over denominator m cubed end fraction times 0 comma 8 rho subscript k e h a end subscript equals 800 fraction numerator k g over denominator m cubed end fraction

 

VASTUS: Keha tihedus on 800 kg/m3.