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Question

The surface area of water in a circular well with vertical wall is 30 $m^2$. When an idol made of a material of density 2 $g/cm^3$ is fully immersed, the water level rises by 10 cm. What is the mass of the idol?

The correct answer is
6000 kg

Calculating Idol Mass from Water Displacement

The problem asks for the mass of the idol based on the surface area of water in a well and the water level rise upon immersion. The key principle is that the volume of water displaced is equal to the volume of the submerged idol.

Volume of Displaced Water

The volume of water displaced ($V_{displaced}$) is calculated by multiplying the surface area of the water ($A$) by the rise in water level ($h$).

  • Surface Area, $A = 30 \ m^2$.
  • Water level rise, $h = 10 \ cm$. Convert this to meters: $h = 10 \ cm = 0.1 \ m$.

Therefore, the volume of displaced water is:

$ V_{displaced} = A \times h $

$ V_{displaced} = 30 \ m^2 \times 0.1 \ m = 3 \ m^3 $

Since the idol is fully immersed, the volume of the idol ($V_{idol}$) is equal to the volume of displaced water:

$ V_{idol} = V_{displaced} = 3 \ m^3 $

Calculating Idol Mass

The mass of the idol ($m_{idol}$) can be found using its density ($\rho_{idol}$) and its volume ($V_{idol}$).

  • Density of idol material, $\rho_{idol} = 2 \ g/cm^3$.
  • Volume of idol, $V_{idol} = 3 \ m^3$.

First, convert the density from $g/cm^3$ to $kg/m^3$ for consistency:

$ \rho_{idol} = 2 \ \frac{g}{cm^3} = 2 \times \frac{10^{-3} \ kg}{(10^{-2} \ m)^3} = 2 \times \frac{10^{-3}}{10^{-6}} \ \frac{kg}{m^3} = 2 \times 10^3 \ kg/m^3 = 2000 \ kg/m^3 $

Now, calculate the mass using the formula $Mass = Density \times Volume$:

$ m_{idol} = \rho_{idol} \times V_{idol} $

$ m_{idol} = 2000 \ kg/m^3 \times 3 \ m^3 $

$ m_{idol} = 6000 \ kg $

The mass of the idol is 6000 kg.

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Important Questions from Mensuration 3D (Notes)

  1. On a spherical balloon of 10 cm radius, a circular colour patch has an area of 25 cm². If the balloon is uniformly expanded to a sphere of 50 cm radius, the area of the colour patch in cm² would be
  2. A block of marble 5 m x 4 m x 2 m in size is cut into rectangular tiles of 1 m x 0.5 m size having thickness of 10 cm. Assuming 10% wastage in cutting, how many tiles will be made?
  3. The height of a cylinder is 14cm and its curved surface area is 264cm². The volume of the cyclinder (in cm³) is:
    ($\pi=\frac{22}{7}$)
  4. What is the volume of a 6 m deep tank having rectangular shaped top 6m X 4 m and bottom 4 m X 2 m? (use mean-area method).
  5. The surface area of the solid generated by revolving the curve $x = e^t \cos t, y = e^t \sin t$ about y-axis $0 \leq t \leq \pi/2$ is
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