My Science School

                  It is a well-known fact that a solid block of iron which may be as small as a needle readily sinks in water but something as big as a ship floats on water. It is interesting to know why it so happen?

                   According to Archimedes principle, the apparent loss in weight of a body totally or partially immersed in a liquid is equal to the weight of the liquid displaced. The centre of gravity of the body and that of the displaced liquid (centre of buoyancy) must lie in the same vertical line which is called the centre line of the body. In the case of a solid iron block, the weight of the liquid displaced is less than the weight of the block — so it sinks. 

 

 

 

 

                   The case is different in case of an iron ship due to its special construction. Its body is shaped in such a manner that it displaces a large quantity of water, and therefore, experiences an upward thrust greater than its weight. Therefore, when it floats on water, its weight is equal to the weight of water displaced by its immersed portion. In other words, when the ship enters the water the volume of the water displaced is much greater than the volume of actual iron immersed, and as a solid it cannot displace more than its own weight of a liquid. The ship sinks only to the extent the weight of the displaced water is equal to the weight of the ship. Thus the remaining portion of the ship stays out of water.

 

               When we hear the sound of a police car speeding past us with its siren blaring or a train roaring past another train, we experience something strange happening to the pitch of the sound. The sound seems to get higher as the car approaches and lower as it goes past. This despite the fact that the actual pitch of the sound remains the same; it just seems so because sound waves reach us faster as the car gets nearer. The effect is known as the Doppler Effect after the Austrian physicist Christian Doppler who first studied it in 1842.

               The Doppler Effect is thus described as the apparent change in frequency of sound, light or radio waves caused by the motion of the source, observer or medium. The basis of the ‘Doppler Effect’ is the fact that sound travels in the form of waves. The pitch of a sound depends on its frequency. The frequency is the number of sound waves striking the ear every second. When the source of the sound is approaching, each wave sent out by the source has a shorter distance to travel than the wave that was sent out earlier from a longer distance. Each wave reaches the listener a little sooner than it would have if the source had not been moving. The waves seem to be more closely spaced. They have a higher frequency or higher pitch. As the train passes away the observer, each wave starts a little further away. Each wave seems to be longer than it would ordinarily be. Hence the pitch is lowered.

               

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The word ‘detergent’ means any substance that cleans things. But today the word is usually used to mean synthetic or man-made detergents such as washing powders.

A detergent is an organic substance composed of carbon, oxygen, sulphur and hydrogen compounds. When combined with water it helps to clean soiled materials. The ordinary soap is a type of detergent, but it has a different chemical composition. The household detergents, used mainly for cleaning clothes and utensils, come in powder, flake or liquid form.

The first detergent was developed in 1916 by a German scientist called Fritz Gunther. Since then their use has been on the constant increase.

All the detergents contain a basic cleaning agent called a surfactant or surface-active agent. The surfactant molecules attach themselves to dirt particles in soiled materials like cloth etc. They pry the dirt particles from the cloth and surround the particles with a layer of water that allows them to be carried away. The surfactants that are made by treating beef fat or tallow with various chemicals increase the wetting ability of water by lowering its surface tension. The surface tension is the force that keeps water molecules separate and help to move deeper into soiled materials. This helps remove deep-seated dirt particles in fabrics. For example, surfactants also help detergents create lather and suds. Contrary to the popular belief, lather and suds have very little to do with the cleaning ability of a detergent.

Most of the detergents contain many other agents besides surfactants, including bleaches, fabric brighteners, builders and stabilizers. They also contain anti-redeposition agents that prevent removed dirt particles from returning to the cleaned material.

The surfactants can be divided into three main groups: anionic, which become negatively charged ions when in solution; cationic - which form positive ions in solution; and non-ionic which do not become ionized. Detergents may be anionic, cationic or non-ionic or mixture of two or more type of surfactant.

 

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