Why the needle of iron swims on the water surface when it is kept gently?
(1) It will remain under the water when it will displace more water than its weight
(2) the density of the needle is less than that of the water
(3) due to surface tension
(4) due to its shape
Answer: (3) According to Archimedes principle and we can note that the amount of water displaced by a needle is lesser than the amount of water moved out by the needle (that is displacement of the needle). So the needle initially stays on the surface of the water and then sinks into the water. Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in the floating of some objects on the surface of the water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by the cohesion of similar molecules and is responsible for many of the behaviours of liquids.
