Physics Paper 2, Nov/Dec. 2012  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 5

  Question 5
A needle floating on the surface of water sinks when a drop of detergent is added to the water.  Explain this observation.      


   Most candidates could not relate the observation ( i.e sinking of the needle) to the concept of surface tension.  They were using “upthrust” to explain the observation which was wrong.  Another group of candidates lost marks for not mentioning net/resultant downward force (since there are more than one force acting) which was the cause of the needle sinking.
The expected answer is
The needle floats on the surface of water due to surface tension . The addition of detergents reduces / weakens surface tension of the water; hence there is a resultant downward force causing the needle to sink.

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