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Properties of Matter question

2021 · 18 Mar · Shift 2 · Q65
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Properties of Matter question

2021 · 18 Mar · Shift 2 · Q65

JEE MainPhysicsProperties of MatterNumerical+4 / −1
Consider a water tank as shown in the figure. It's cross-sectional area is 0.4 m2. The tank has an opening B near the bottom whose cross-section area is 1 cm2. A load of 24 kg is applied on the water at the top when the height of the water level is 40 cm above the bottom, the velocity of water coming out the opening B is v ms-1. The value of v, to the nearest integer, is ‾\underline{\hspace{2cm}}​. [Take value of g to be 10 ms-2] JEE Main 2021 (Online) 18th March Evening Shift Physics - Properties of Matter Question 187 English
Numerical answer
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Correct answer: 3

  1. Given data
  • Cross-sectional area of tank:
    A=0.4 m2A = 0.4\,\text{m}^2A=0.4m2
  • Area of opening near bottom:
    a=1 cm2=10−4 m2a = 1\,\text{cm}^2 = 10^{-4}\,\text{m}^2a=1cm2=10−4m2
  • Load applied on top:
    m=24 kgm = 24\,\text{kg}m=24kg
  • Height of water above opening:
    h=40 cm=0.4 mh = 40\,\text{cm} = 0.4\,\text{m}h=40cm=0.4m
  • Acceleration due to gravity:
    g=10 m s−2g = 10\,\text{m s}^{-2}g=10m s−2
  1. Pressure applied by the load

The load on the top surface produces extra pressure on the water:

Pextra=mgAP_{\text{extra}} = \frac{mg}{A}Pextra​=Amg​

Substituting:

Pextra=24×100.4=2400.4=600 PaP_{\text{extra}} = \frac{24\times 10}{0.4} = \frac{240}{0.4} = 600\,\text{Pa}Pextra​=0.424×10​=0.4240​=600Pa

  1. Equivalent water head due to this pressure

Using

P=ρgheqP = \rho g h_{\text{eq}}P=ρgheq​

so

heq=Pextraρg=6001000×10=0.06 mh_{\text{eq}} = \frac{P_{\text{extra}}}{\rho g} = \frac{600}{1000\times 10} = 0.06\,\text{m}heq​=ρgPextra​​=1000×10600​=0.06m

  1. Total effective head

The outlet speed is due to:

  • actual water height: 0.4 m0.4\,\text{m}0.4m
  • extra equivalent head from the load: 0.06 m0.06\,\text{m}0.06m

Hence,

H=0.4+0.06=0.46 mH = 0.4 + 0.06 = 0.46\,\text{m}H=0.4+0.06=0.46m

  1. Apply Bernoulli/Torricelli theorem

Since tank area is much larger than outlet area,

v=2gHv = \sqrt{2gH}v=2gH​

Thus,

v=2×10×0.46=9.2v = \sqrt{2\times 10\times 0.46} = \sqrt{9.2}v=2×10×0.46​=9.2​

v≈3.03 m s−1v \approx 3.03\,\text{m s}^{-1}v≈3.03m s−1

  1. Nearest integer

v≈3v \approx 3v≈3

Therefore, the required integer answer is:

3\boxed{3}3​

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