Explain the following observations very briefly. (a) A firki does not rotate in a closed area. (b) The arrow of weather cock points towards a particular direction at a particular moment. (c) An empty glass in fact is not empty. Breathing through mouth may harm you.

• Solution:

• In a closed area, there is a lack of air movement which does not support the rotation of a firki. A firki usually rotates when placed in an open area due to the moving air present all around.
• A weather cock is an instrument used to indicate the direction of the It consists of an arrow, mounted at its centre of gravity for it to move freely about a vertical axis.

Hence, the arrow points towards a particular direction at a particular moment to show the latest direction of the wind movement.

• An empty glass is not empty it is filled with air. It can be proved with a simple experiment in which an empty bottle is turned upside down in a beaker filled with water. The water does not enter the bottle when it is pushed in an inverted position because there is no space for air to escape. When the bottle is tilted, the air is able to escape out in the form of bubbles and water fills up the empty space. This shows that even an empty glass is filled with the air.

• Yes, breathing through mouth may harm us. Some gases, water vapour and dust particles may be present in air. When we inhale air through nostrils, the fine hair and mucus present in the nose prevent the dust particles from entering the respiratory But, if we breathe through mouth, the harmful dust particles may enter our body and make us ill. Thus, breathing through mouth may harm us.