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Problem Sets:
Problem
There are two identical containers A and B on a table, each containing a solid wooden sphere and a solid copper sphere of equal volume. The containers are filled with equal masses of water and alcohol, respectively. The density relationship among water, alcohol, the wooden sphere, and the copper sphere is:
$\rho_wood < \rho_alcohol < \rho_water < \rho__copper$.The wooden sphere is placed into the water, and the copper sphere is placed into the alcohol (no liquid overflows). After the system comes to rest, the pressures exerted by containers A and B on the table are $p_1$ and $p_2$, respectively, and the buoyant forces on the wooden sphere and the copper sphere are $F_1$ and $F_2$, respectively.
- Compare the magnitude of $p_1$ and $p_2$
- Compare the magnitude of $F_1$ and $F_2$
Hints
Hint 1:
[Q1] $p_1 < p_2$ [Q2] $F_1 < F_2$
[Q1] Compare the magnitude of $p_1$ and $p_2$
The pressure exerted by a container on the table is given by $p = \frac{W_{total}}{A_{base}}$, where $W_{total}$ is the total weight of the system and $A_{base}$ is the area of the container's base. Since the two containers A and B are identical, their base areas are equal ($A_1 = A_2$). Therefore, the comparison of pressures reduces to a comparison of the total weights of the two systems.
The total weight of system A (container, water, wooden sphere) is:
$$W_1 = (m_c + m_w + m_{wood})g$$The total weight of system B (container, alcohol, copper sphere) is:
$$W_2 = (m_c + m_{alc} + m_{cu})g$$We are given that the masses of the container ($m_c$) are identical and the masses of the liquids are equal ($m_w = m_{alc}$). The comparison of weights thus depends on the masses of the spheres, $m_{wood}$ and $m_{cu}$. The masses are related to density ($\rho$) and volume ($V$) by $m = \rho V$.
$$m_{wood} = \rho_{wood}V_{wood}$$ $$m_{cu} = \rho_{cu}V_{cu}$$Given that the volumes are equal ($V_{wood} = V_{cu} = V_{sphere}$) and the densities are related by $\rho_{wood} < \rho_{cu}$, we can conclude that $m_{wood} < m_{cu}$.
Therefore, the total weight of system A is less than that of system B:
$$W_1 < W_2$$Since the base areas are equal, the pressures are related as:
$$p_1 < p_2$$[Q2] Compare the magnitude of $F_1$ and $F_2$
The buoyant force $F_1$ acts on the wooden sphere in water. According to the given density relationship, $\rho_{wood} < \rho_{water}$, which means the wooden sphere will float. For a floating object, the buoyant force is equal to the object's weight.
$$F_1 = W_{wood} = m_{wood}g = \rho_{wood}V_{sphere}g$$The buoyant force $F_2$ acts on the copper sphere in alcohol. Since $\rho_{cu} > \rho_{alcohol}$, the copper sphere will sink and be fully submerged. The buoyant force on a submerged object is equal to the weight of the fluid it displaces (Archimedes' principle).
$$F_2 = \rho_{alcohol} g V_{submerged}$$Since the sphere is fully submerged, the volume of displaced alcohol is equal to the volume of the sphere, $V_{submerged} = V_{sphere}$.
$$F_2 = \rho_{alcohol} g V_{sphere}$$Now we compare $F_1$ and $F_2$:
$$F_1 = \rho_{wood} g V_{sphere}$$ $$F_2 = \rho_{alcohol} g V_{sphere}$$We are given the density relationship $\rho_{wood} < \rho_{alcohol}$. Since $g$ and $V_{sphere}$ are the same in both expressions, it follows that:
$$F_1 < F_2$$