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Properties of Matter question

2020 · 4 Sep · Shift 1 · Q50
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Properties of Matter question

2020 · 4 Sep · Shift 1 · Q50

JEE MainPhysicsProperties of MatterMCQ+4 / −1
A air bubble of radius 1 cm in water has an upward acceleration 9.8 cm s–2. The density of water is 1 gm cm–3 and water offers negligible drag force on the bubble. The mass of the bubble is (g = 980 cm/s2).
  1. A
    1.52 gm
  2. B
    4.51 gm
  3. C
    3.15 gm
  4. D
    4.15 gm
View written solutionFree

Correct answer: D

  1. Given data
  • Radius of air bubble: r=1 cmr = 1\ \text{cm}r=1 cm
  • Density of water: ρ=1 g cm−3\rho = 1\ \text{g cm}^{-3}ρ=1 g cm−3
  • Upward acceleration of bubble: a=9.8 cm s−2a = 9.8\ \text{cm s}^{-2}a=9.8 cm s−2
  • Acceleration due to gravity: g=980 cm s−2g = 980\ \text{cm s}^{-2}g=980 cm s−2
  • Drag force is negligible.
  1. Forces acting on the bubble

Since the bubble is moving upward, the forces are:

  • Upthrust (buoyant force) upward: Fb=ρVgF_b = \rho V gFb​=ρVg
  • Weight of bubble downward: W=mgW = mgW=mg

Net upward force is: Fb−mg=maF_b - mg = maFb​−mg=ma

So, ρVg=m(g+a)\rho V g = m(g+a)ρVg=m(g+a)

Hence, m=ρVgg+am = \frac{\rho V g}{g+a}m=g+aρVg​

  1. Volume of the bubble

Since the bubble is spherical, V=43πr3=43π(1)3=4π3 cm3V = \frac{4}{3}\pi r^3 = \frac{4}{3}\pi (1)^3 = \frac{4\pi}{3}\ \text{cm}^3V=34​πr3=34​π(1)3=34π​ cm3

Using π≈3.14\pi \approx 3.14π≈3.14, V≈4×3.143=4.1867 cm3V \approx \frac{4 \times 3.14}{3} = 4.1867\ \text{cm}^3V≈34×3.14​=4.1867 cm3

  1. Substitute in the formula

m=(1)(4.1867)(980)980+9.8m = \frac{(1)(4.1867)(980)}{980+9.8}m=980+9.8(1)(4.1867)(980)​

m=4102.966989.8m = \frac{4102.966}{989.8}m=989.84102.966​

m≈4.15 gm \approx 4.15\ \text{g}m≈4.15 g

  1. Match with options
  • A: 1.52 g1.52\ \text{g}1.52 g
  • B: 4.51 g4.51\ \text{g}4.51 g
  • C: 3.15 g3.15\ \text{g}3.15 g
  • D: 4.15 g4.15\ \text{g}4.15 g

Therefore, the correct option is: D: 4.15 gm\boxed{\text{D: } 4.15\ \text{gm}}D: 4.15 gm​

  1. Comparison with stored correct answer

Stored correct answer: D

Our derived answer is also D, so they agree.

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