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If we place a needle slowly on a water surface, it may float. Which of the following fluid property is responsible for this phenomenon?
1. Law of Floatation
2. Adhesive Force
3. Surface Tension
4. Equal Pressure distribution in fluids

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Correct Answer - Option 3 : Surface Tension

CONCEPT:

  • Surface Tension: The tendency of a liquid to reduce its surface area is called surface tension. 
    • The water molecules at the surface are pulled in by the cohesive force between themselves and molecules inside the bulk and thus have higher potential energy than those insides.

EXPLANATION:

  • If carefully placed on the surface, a small needle can be made to float on the surface of the water even though it is several times as dense as water. This is due to surface tension even if the density of iron nail is much higher than water.
  • The water surface tries to contract, It forms a skin-like sheet  This naturally-occurring skin is strong enough to hold the needle up and prevent it from sinking. 
  • If the surface is agitated to break up the surface tension, then the needle will quickly sink.

  • Law of Floatation: The body having a higher density than water will sink in water.
    • A body with a lower density than water will float. 
  • Adhesive Force: The force acting between fluid and other surfaces is called adhesive force.
  • Equal distribution of Pressure: The pressure offered at any horizontal level of liquid is the same throughout. 

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