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第80页

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2.4

小

$ 解：V_水=\frac {m_水}{ρ_水}=\frac {150\ \text {g}}{1\ \text {g/cm}^3}=150\ \text {cm}^3$

$ V_石=V-V_水=250\ \text {cm}^3-150\ \text {cm}^3=100\ \text {cm}^3$

$ m_石=ρV=2.5\ \text {g/cm}^3×100\ \text {cm}^3=250\ \text {g}$

$ 需要\frac {250\ \text {g}}{5\ \text {g}}=50个小石子$

$ 解：V_球=V_总-V_水=80\ \text {cm}^3-50\ \text {cm}^3=30\ \text {cm}^3$

$ V_铁=\frac {m_铁}{ρ_铁}=\frac {158\ \text {g}}{7.9\ \text {g/cm}^3}=20\ \text {cm}^3$

$ V_铁<V_球，所以是空心的$

$ V_空=V_球-V_铁=30\ \text {cm}^3-20\ \text {cm}^3=10\ \text {cm}^3$

$ 解：(1)ρ_液=\frac {∆m }{∆V}=\frac {(107-86) g}{(58-37) cm^3}=1\ \text {g/cm}^3$

$ m_1=ρ_液V_{1}=1\ \text {g/cm}^3×37\ \text {cm}^3= 37\ \text {g}$

$ m_容=m_{总1}-m_{1}=86\ \text {g}-37\ \text {g}=49\ \text {g}$

$ (2)由(1)知ρ_液=1\ \text {g/cm}^3$