Multiple choice

A solids sphere (diameter 6 mm) is rising through oil (mass density 900 kg/m3, dynamic viscosity 0.7 kg/ms) at a constant velocity of 1 cm/s. What is the specific weight of the material from which the sphere is made? (Take g = 9.81 m/s2)

  1. 4.3 kN/m3

  2. 5.3 kN/m3

  3. 8.7 kN/m3

  4. 12.3 kN/m3
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For a sphere moving at constant velocity, the drag force (Stokes' Law) plus buoyancy equals the weight. Solving for the specific weight of the sphere material gives the result.

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