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Fluid Mechanics: Fluid properties · Density and Specific Gravity

How density, specific weight and specific gravity describe fluids, and why they explain floating, sinking and layering.

Fluid Mechanics · 25 questions

Try 3 questions

1. In fluid mechanics, what does the continuum hypothesis assume?

  1. Fluids keep the same density everywhere
  2. Fluid flow never stops or reverses
  3. Fluid properties vary smoothly, ignoring molecules
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Answer: Fluid properties vary smoothly, ignoring molecules

Engineers treat a fluid as continuous matter so density, pressure and velocity can be defined at every point, even though it is really made of discrete molecules.

2. When does the continuum hypothesis stop working well?

  1. When molecular spacing rivals the flow's size
  2. Whenever the fluid is a liquid, not a gas
  3. When the fluid moves faster than walking speed
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Answer: When molecular spacing rivals the flow's size

In very thin gases (the upper atmosphere) or tiny channels, the distance molecules travel between collisions becomes comparable to the geometry, so averaging over molecules fails.

3. At roughly what temperature is liquid fresh water at its densest?

  1. Right at 0 °C
  2. About 20 °C
  3. About 4 °C
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Answer: About 4 °C

Water is unusual: as it cools below about 4 °C, hydrogen bonds begin forming an open, ice-like structure, so it expands slightly before it freezes.

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