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JEE Advanced 1980 · Mechanical Properties of Fluids

A man is sitting in a boat which is floating in a pond. If the man drinks some water from the pond, the level of the water in the pond decreases.

Answer: False

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Written solution

Step 1: Analyze the initial equilibrium of the floating boat.

For a floating boat of mass MboatM_{\text{boat}} containing a man of mass MmanM_{\text{man}}, the buoyant force equals the total weight of the system.

ρVdg=(Mman+Mboat)g\rho V_d g = (M_{\text{man}} + M_{\text{boat}}) g

The initial volume of water displaced by the boat, VdV_d, is:

Vd=Mman+MboatρV_d = \frac{M_{\text{man}} + M_{\text{boat}}}{\rho}

Step 2: Express the initial water level of the pond.

Let VwaterV_{\text{water}} be the volume of the water in the pond, and AA be the horizontal cross-sectional area of the pond. The initial water level hh is given by:

h=Vwater+VdAh = \frac{V_{\text{water}} + V_d}{A}

Step 3: Determine the changes after the man drinks water.

When the man drinks a mass mm of water from the pond, two changes occur:

  1. The volume of the liquid water in the pond decreases:
Vwater→Vwater−mρV_{\text{water}} \to V_{\text{water}} - \frac{m}{\rho}
  1. The total mass of the boat-man system increases by mm, which increases the displaced volume:
Vd→Mman+Mboat+mρ=Vd+mρV_d \to \frac{M_{\text{man}} + M_{\text{boat}} + m}{\rho} = V_d + \frac{m}{\rho}

Step 4: Calculate the new water level.

Substitute the new values into the expression for the new water level h′h'.

h′=(Vwater−mρ)+(Vd+mρ)Ah' = \frac{\left(V_{\text{water}} - \frac{m}{\rho}\right) + \left(V_d + \frac{m}{\rho}\right)}{A} h′=Vwater+VdA=hh' = \frac{V_{\text{water}} + V_d}{A} = h

The change in water level is:

Δh=0\Delta h = 0

The water level remains unchanged.

Answer: The given statement is False (the level of the water in the pond remains the same).

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