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162. If a wet cloth is rubbed between two hands during washing, washing is found to be more effective. Justify this statement.
Ans: The height of liquid that rises in a tube due to surface tension is given by . This applies to any channels, not only the channels that are vertically standing. If the channels are small ('r' small), they are able to travel more heights, or more distances. When they are rubbed, the channels available are compressed, or they become small. So water can reach to every nook and corner. This helps in better cleansing action.


163.  Explain how does the wick of an oil lamp regulate oil for continuous burning of the lamp?
Ans: The continuous burning of an oil lamp requires small, regulated and uniform supply of oil from the main mass of oil to the wick, for which it has to rise against gravity to the height where the flame is burning. this is facilitated by the structure of the wick which has many small channels in between the cotton fibers. these small channels have very small diameter and according to the relation , the oil will rise to sufficient height up to the region of the flame.


164. A cotton towel is used for absorbing water from wet body. Explain.
Ans: The absorption of water requires that water travel from the wet places or substances to the cotton, which requires the path. cotton is porous in nature because of which it has a lot of small channels in it. Thus their radius is small and according to the relation , water can travel to different regions as much as and where required. This removes water from the bodies and take them to cotton.


165. The fibers of a painting brush spread when dipped in a liquid and cling together when taken out. Explain.
Ans: The spreading of the brush in water is due to the upthrust force. The fibers are weak enough and so the upthrust raises them from their 'clung' state. However, when they are taken out, most water particles drops from them. The remaining water however act as bridges between the two fibers. Since water has high surface tension, their surface is tight enough. So they hold two fibers near enough. When this phenomena spreads across all the fibers, all the fibers stay near each other and so they appear clung to each other.


166. .
Ans:
 
 
167. Ploughing of the field helps in retaining the moisture of the soil. Why?
Ans: When the soil is not ploughed or is unattended, they become tight, i.e. the soil particles are held close together with a lot of thin channels in between. These channels will act as capillary tubes and the liquid rises according to the relation . Due to closeness of the particles, the radius of such channels will be very small. The liquids will then rise all the way to the surface and get evaporated away. In order to prevent that the channels have to be wither widened or cut off, the best way being ploughing.
 

168. Oil spreads on cold water; however, it may remain a drop in hot water. Explain why?

Ans: Water has high value of surface tension, the value is even more in cold water, since surface tension increases with reduction in temperature. when oil is placed on it, it will feel a strong tight surface, and so spreads to achieve uniformity. however, if water is hot, its surface tension is less, that means the surface is now weak and soft. so when oil is placed on it, the surface get depressed. Oil then can not spread from that position, and remains as a drop or globule.


169. Water remains cool in an earthen pot in summer. Explain how?
Ans: An earthen pot is made of soil particles tightened by baking in kilns. So the particles are held enough close together with a lot of thin channels in between. These channels will act as capillary tubes and the liquid rises according to the relation . Due to closeness of the particles, the radius of such channels will be very small. The liquids will then rise all the way to the surface and get evaporated away. In order to prevent that the channels have to be wither widened or cut off, the best way being 'Ploughing'.  

 
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171. Sand appears drier than clay, why?
Ans: Sand has, by nature large grains of particles. So the gaps between their particles will also be wide. However, clay has very fine particles and the gaps between them will also be less. The gaps in sand will act as wide channels whereas those in clay will act as narrow ones. Then according to the relation , water will climb from deeper grounds to sufficient height in clay, making them appear wet. In case of sand, this is not possible and it will appear dry.
 

172. A steel needle may float on water but sinks when a small amount of detergent is added to water. Why?
Ans: Water has high value of surface tension compared to any other substance. So when steel needle is placed slowly the surface of water holds it. However, if detergent is added, their molecules come in between water molecules and so the surface tension decreases. When the steel needle is now placed, water surface can no longer hold it and it sinks.
 

173. Hot soup tastes better than cold one. why?
Ans
: The whole phenomena is based on how well soup (hot or cold) spreads over the taste buds of the tongue. When temperature is low, the surface tension of water is high, so water molecules are held back, which prevents their reach to the taste buds. This makes the taste incomplete. However, increase of temperature reduces the surface tension. This makes the water (soup) spread better, increasing their access to the taste buts. So the taste is good and complete.
 

174. A beaker containing glycerin and another containing water are stirred vigorously and placed on a table, Which one comes to rest first?
Ans:
As evident, glycerine has high coefficient of viscosity compared to water. So Glycerine would come to rest first.

 

175. Why do the clouds seem floating in the sky?
Ans:
Clouds are actually not floating, like every other substances, they are also falling. But the way they fall is different. Cloud is basically made of tiny water drops plus dust particles associated with them. However, because of their small size, the terminal velocity attained by them is also very low according to the relation . Such velocity is so less that for an observer down on the earth, they just seem like floating.
 
 
 
176. Why does a flag flutter on strong wind?
Ans: Fluttering of flags means rising of the flag and its subsequent flapping according to the motion of air. This is due to the pressure difference created by the movement of air over and below the flag. In general, velocity of air is high at grater heights and less close to the ground (remember velocity gradient).
 
Then according to the relation (Bernoulli's Theorem), , higher velocity leads to low pressure. This gives low pressure at height and high below, creating a resultant upward force and pressure. So the flag is lifted from its drifted position and start to flutter. 

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