arthham

JEE Advanced 1986 · Mechanical Properties of Fluids

A block of ice with a lead shot embedded in it is floating on water contained in a vessel. The temperature of the system is maintained at 0∘C0^\circ\text{C} as the ice melts. When the ice melts completely the level of water in the vessel rises.

Answer: False

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Written solution

Step 1: Set up the equilibrium condition for the floating block.

The buoyant force balances the total weight of the ice block and the embedded lead shot.

ρwVdispg=(mi+ml)g\rho_w V_{\text{disp}} g = (m_i + m_l) g

Step 2: Express the initial displaced volume.

Solve for the volume of water displaced by the floating system, VdispV_{\text{disp}}.

Vdisp=mi+mlρwV_{\text{disp}} = \frac{m_i + m_l}{\rho_w} Vdisp=miρw+mlρwV_{\text{disp}} = \frac{m_i}{\rho_w} + \frac{m_l}{\rho_w}

Step 3: Determine the final volume after the ice melts.

The melted ice becomes water of volume VmeltV_{\text{melt}}, while the lead shot sinks and displaces its own volume VleadV_{\text{lead}}.

Vmelt=miρwV_{\text{melt}} = \frac{m_i}{\rho_w} Vlead=mlρlV_{\text{lead}} = \frac{m_l}{\rho_l} Vf=miρw+mlρlV_f = \frac{m_i}{\rho_w} + \frac{m_l}{\rho_l}

Step 4: Compare the initial and final volumes.

Calculate the difference between the initial displaced volume and the final volume.

Vdisp−Vf=(miρw+mlρw)−(miρw+mlρl)V_{\text{disp}} - V_f = \left(\frac{m_i}{\rho_w} + \frac{m_l}{\rho_w}\right) - \left(\frac{m_i}{\rho_w} + \frac{m_l}{\rho_l}\right) Vdisp−Vf=ml(1ρw−1ρl)V_{\text{disp}} - V_f = m_l \left(\frac{1}{\rho_w} - \frac{1}{\rho_l}\right)

Since the density of lead is greater than the density of water (ρl>ρw\rho_l > \rho_w), the difference is positive.

1ρw−1ρl>0\frac{1}{\rho_w} - \frac{1}{\rho_l} > 0 Vdisp>VfV_{\text{disp}} > V_f

Since the total volume occupied decreases, the water level in the vessel falls.

Answer: The water level falls (the statement that the water level rises is FALSE).

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