Spherical balls of radius 'r' are falling in a viscous fluid of viscosity 'η' with a velocity 'v'. The retarding viscous force acting on the spherical ball is:
Correct answer: B. Directly proportional to both radius 'r' and velocity 'v'
- A. Inversely proportional to 'r' but directly proportional to velocity 'v'
- B. Directly proportional to both radius 'r' and velocity 'v'
- C. Inversely proportional to both radius 'r' and velocity 'v'
- D. Directly proportional to 'r' but inversely proportional to 'v'
Explanation
Stokes' law for low Reynolds number gives drag F = 6 π η r V, which is directly proportional to radius r and to velocity V.
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About Fluid Dynamics
Fluid flow is described through speed, pressure and density, including steady flow and the equation of continuity for conserving mass. Bernoulli's equation relates pressure, speed and height, while fluid drag and terminal velocity explain why a falling object eventually moves at constant speed when drag balances its weight.
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