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Difficulty: MediumPhysics Fundamentals and Everyday Applications

Which of the following statements regarding fluid dynamics, surface tension, and viscosity in everyday phenomena are correct?

  1. Detergents added to water decrease its surface tension, enabling the solution to wet fabrics and loosen dirt particles more effectively.Answer
  2. The dynamic viscosity of liquids generally decreases as temperature increases, causing warm liquids like syrup or motor oil to flow more easily.Answer
  3. C
    A small steel sewing needle resting horizontally on calm water floats primarily because of buoyant force equal to the weight of displaced water.
  4. D
    Cooking oil spreads out into a thin film on cold water because the surface tension of cooking oil is significantly greater than that of cold water.

Answer

The correct statements are the statement regarding detergents decreasing water's surface tension to enhance wetting, and the statement regarding the decrease of liquid viscosity with increasing temperature.
Detergents lower water surface tension to improve wetting and cleaning efficiency. Furthermore, increasing temperature reduces liquid viscosity by weakening intermolecular cohesion, allowing warm liquids to flow faster.

Step-by-Step Solution

1
Analyze the action of detergents on surface tension.
Detergents act as surfactants, disrupting hydrogen bonding between water molecules and lowering surface tension to allow penetration into fabric pores.
Lower surface tension facilitates contact angle reduction and better cleaning.
2
Examine the effect of temperature on liquid viscosity.
Increasing temperature increases molecular kinetic energy and reduces cohesive forces in liquids, decreasing viscosity.
Viscosity in liquids depends inversely on temperature.
3
Evaluate the mechanism supporting a floating steel needle.
The needle rests on an indented surface film created by surface tension forces acting upward along the contact line, rather than static fluid displacement (Archimedes' principle).
Density of steel exceeds water density, making buoyancy insufficient.
4
Evaluate oil spreading behavior on water.
Water possesses a higher surface tension than oil ( 72 mN/m~72\text{ mN/m} for water vs  32 mN/m~32\text{ mN/m} for oil), creating a net outward force that pulls the oil film thin.
Spreading coefficient requires the supporting liquid to have higher surface tension than the spreading liquid.

Key Concept

Surface Tension, Viscosity, and Fluid Properties in Everyday Applications
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Topics

Physics Fundamentals and Everyday Applications

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