3.
[tex]m = 800 \ g = 0,8 \ kg\\g = 10\frac{N}{kg}\\F = ?\\\\F = m\cdot g\\\\F = 0,8 \ kg\cdot10\frac{N}{kg}\\\\\boxed{F = 8 \ N}[/tex]
4.
[tex]1 \ kg = 1000 \ g\\1 \ g = 0,001 \ kg\\1 \ m^{3} = 1 00 \ cm \times 100 \ cm \times 100 \ cm = 1 \ 000 \ 000 \ cm^{3}\\1 \ cm^{3} = 0,000001 \ m^{3}\\\\19,3\frac{g}{cm^{3}} = 19,3\cdot\frac{0,001 \ kg}{0,000001 \ m^{3}} = 19,3\cdot1000 \ \frac{kg}{m^{3}} = 19 \ 300 \ \frac{kg}{m^{3}}[/tex]
