Saturday, March 5, 2016



What floats – what sinks?
Ayesha was waiting for dinner. Today Ammi was making her favourite food – puri and spicy potatoes. Ayesha watched as her mother rolled out the puri and put it in the hot oil. She saw that at first the puri sank to the
bottom of the pan.

As it puffed up, the puri came up and started floating on the oil. One puri did not puff up and did not float like the others. On seeing this, Ayesha took some dough and rolled it into a ball. She flattened it and put it in
a bowl of water.  Alas! it sank to the bottom and stayed there.

In the evening Ayesha went for a bath. She had just come out when her mother called, “Ayesha, you have dropped the soap in the water again.

Take it out and put it in the soap case.” Ayesha was in a hurry and the soap case fell out of her hands. It started floating on water. Ayesha
gently put the soap in the soap case. She saw that the case continued to float, even with the soap in it.

A wooden boat in water will float. But a needle will sink! Why does this happen?

Let me think... An iron ship will also float, though its’ much heavier
than my boat!

Archimedes' Principle of Buoyancy:
Archimedes' principle indicates that the upward  buoyant force that is exerted on a body immersed in a  fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.

Because The reason that a ship floats is that it displaces a lot of water. The displaced water wants to return to it's original location, where the ship is now, and this pushes the ship upwards. The force which pushes the ship up is called the buoyancy force.

Archimedes continued to do more experiments and came up with a buoyancy principle, that a ship will float when the weight of the water it displaces equals the weight of the ship and anything will float if it is shaped to displace its own weight of water before it reaches the point where it will submerge.

This is kind of a technical way of looking at it. A ship that is launched sinks into the sea until the weight of the water it displaces is equal to its own weight. As the ship is loaded, it sinks deeper, displacing more water, and so the magnitude of the buoyant force continuously matches the weight of the ship and its cargo.

But a needle, light as a leaf, thin as a pin, will sink right in! Why does this happen?

The density of nail (as of iron) is much larger than the water. So it sinks easily. The weight of the water displaced by the ship is equal to its weight, so it floats. Whereas the weight of the water displaced by the iron nail is less than its weight so the iron nail sinks.

Have you seen that some thing float on water while others sink? Think how this happens! The poem here raises such questions.

Dead Sea:
All oceans and seas have salty water. The saltiest of all is the Dead Sea. How salty? Imagine 300 grams of salt in one litre of water! Would you be able to even taste such salty water? It would be very bitter.

Interestingly, even if a person does not know how to swim, she would not drown in this sea. She will float in water, as if lying down on it!
Remember the lemon you floated in salty water?

What dissolved, what did not?
On Sunday Ayesha’s cousin brother Hamid came to her house
to play. As soon as he came he asked his aunt to make his favourite shakkarpara (a sweet dish).

Ammi said, “Let me come back from the market, then I will make some for you. Why don’t you help me? Take two glasses of water and put a bowl of sugar in it. Mix it till it dissolves.” Hamid thought, “Let me
finish this work fast. Then I will watch TV”.

Dandi March:
This incident took place in 1930, before India became independent. For many years the British had made a law that did not allow people to make salt themselves.

They had also put a heavy tax on salt. By this law people could not
make salt even for use at home. “How can anybody live without salt?” Gandhiji said, “How can a law not allow us to use freely what nature has given!” Gandhiji, with several other people, went on a yatra (long walk) from Ahmedabad to the Dandi seashore in Gujarat, to protest against
this law.

Do you know how salt is made? The sea water is collected in shallow beds dug in the sand. Water is allowed to dry in the sun. After
the water dries the salt remains on the ground.


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