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61. A balloon filled with Helium does not rise in air indefinitely. Why?

When the Helium gas filled balloon is set free, it rises at first. This is because the weight of balloon (acting downwards) is less than the upthrust provided by air (acting upwards).

As the balloon goes on rising, i.e. the height of the balloon goes on increasing, the density of atmospheric air goes on decreasing. Thus the balloon will be facing lesser and lesser upthrust as it goes on rising. Hence, there comes a condition when the weight of balloon just becomes equal to the weight of air displaced at that height, it stops to rise further.
 

 

62. The force required by a man to move his limbs immersed in water is smaller than the force for the same movement in air, why?

Itís evident that when anything is immersed in water, there is a force called upthrust which acts in the upward direction. So when a person moves his limbs in water, due to the density of the water, it applies upthrust on the body which will be in the same line of action as of the movement of the personís limbs.

But in case of air, the upthrust given to the body is very low as the density of air is very low. So itís considered to be negligible. Thus the person feels easy to move his limbs in water than in the air.

[Had it been the case between water and mercury, the effects will be different as mercury is denser than water. Thus the person feels much easier to move his limbs in mercury than in water. The other fluids can also be considered likewise.] 

 

63. The blood pressure in human is greater at the feet than at the brain, why?

The blood pressure is due to the pressure exerted by the blood in the walls of the blood vessels in the body of human beings. There is a known fact that the fluid pressure (i.e. of liquid or gas) increases with the depth of the liquid column. This is also evident from the formula P = ρhg which shows that the pressure is directly proportional with the depth of the liquid column.

So is the case in human body where the depth of blood in the feet region is more than that in the head region, due to which the blood exerts more pressure in the feet region than in the head region or the region of the brain.
 

 

64. Does a ship sink more in river water or in seawater?

The sinking of any object in any fluid depends on the volume of the object itself and the density of the fluid it is sinking in. In our case the same object is sinking in the two different liquids. So the dependency of sinking is restricted to the density of the fluid alone.

The upthrust of a fluid is directly proportional to its density. Thus the object feels more upthrust when sunk in the sea water than in the river water as the density of the sea water is more than that of the river water. Thus it sinks more in the river water than in the sea water.
 


65. Lead has a greater density than iron and both are denser than water. Is the buoyant force on a lead greater than, less than or equal to the buoyant force on iron of same volume?

The buoyant force means the upthrust given by the fluid on the body fully or partially immersed in it. It depends on the volume of the body that has sunk on the liquid as well as the density of the fluid. Here the liquid is same i.e. water while the body sinking in it is different. So the dependency of the sinking is confined to the volume of the body sinking in it. But in this case the volume of the both bodies is same and the liquid used is also the same for both bodies. So the upthrust on the body, lead and iron is same.  

 

66. If one material has a higher density than another, does this mean that the molecules of the first must be heavier than those of second? Explain.

If one material has density higher than the other, it does means that the density of the first material is greater than that of the other. This is because each material is composed of the component molecule and the individual property in the molecule is expressed as the property of the material as a whole. In other words, the single property of any matter is the composite property of its component molecules. Heaviness is also one of such types of properties which are also determined by the property of the molecules contained in that matter. Thus for the matter to be heavy, the molecule of the matter needs to be heavy.  

 

67. A steel ball is floating in a trough of mercury. If we fill the empty part of the trough with water, what will happens to the steel ball?

The steel ball floats in the mercury because it's less dense than mercury. However it's denser than the water. In case if it is immersed in the water, it gets sunk in it. But here the case is something else. Here a part of it is in the mercury while a part is in water. due to the density of mercury, it floats on it as it gets some upthrust and when water is poured over the setup, then it gets some additional upthrust from water too. Due to this the steel gets raised to some extent however it's not floated in the water due to its density.  

 

 

68.  A piece of ice floats on water. What will happen to the level of water when the ice melts completely?

The floating of ice or any other body is governed by the principle of floatation which states that the weight of the floating body is equal to the weight of the liquid displaced by it. So the sunken portion of the ice displaces some amount of the water. And the weight of that amount of water is equal to the wholesome weight of the ice floating in it. When  ice melts giving water, the system does not get any thing from outside, no addition, no subtraction. So the weight of the displaced water is same, i.e. there will be no rise or fall in the level.

 

69. A piece of ice is floating inside a vessel containing water and inside the ice is a bubble of air. Will the level of water in the vessel be altered when the ice melts?
The floating of ice is governed by the principle of floatation which states that the weight of the floating body is equal to the weight of the liquid displaced by it. So the sunken portion of the ice plus air bubble displaces some amount of the water. And the weight of that amount of water is equal to the wholesome weight of the ice plus air inside air bubble. When ice melts giving water the air enclosed inside air bubble escapes from it. As a result, the level of the water in the vessel falls. So, there is alteration in the level of the vessel when the ice melts.

 

70. A man is sitting in a boat floating in a pond. If the drinks some water from the pond, what will happen the level of water in the pond?

A man sitting in a boat floating in a pond & if he drinks some water from the pond then the level of water will not change. On drinking the water (say m gm), the man displaces some more water equivalent to 'm' gm tending to raise the level. But this much amount of water had just been used. Since the addition and deduction of water is same, the level of water will be same. 

 

 

71. When air is blown in between two balls suspended close to each other, they are attracted towards each other. Why?
When the air blows the balls, the velocity of air increases and hence according to Bernoulli's Law, the pressure there will decrease compared to other regions. So the balls will face an inner directing force from other regions and they seem to attract each other.

When the suspended balls were in the undisturbed condition, there was some amount of pressure exerted by the air on the balls which exists due to the existence of the air in the atmosphere. When the air is blown between the balls, the velocity of the air in between increases and the pressure decreases. There is a creation of a partial vacuum in between the balls so to fill the space the balls move towards each other. Hence, when air is blown in between two balls suspended close to each other, they are attracted towards each other. 

 

 

72. A balloon filled with Helium does not rise in air indefinitely. Why?

The balloon filled with helium doesn't rise indefinitely because the helium is a slightly denser gas than the hydrogen which is usually filled in the balloons. Itís density is near to the density of the atmospheric air. And as the height increases or as we go up from the earth surface, the atmosphere becomes rarer. So at some height, the density of the air becomes equal to the density of the helium gas filled in the balloon.

Thus that forms a homogeneous system. And the air can no longer provide the upthrust to the balloon filled with helium because only the denser medium can provide upthrust to the body rarer than the medium. So after attaining equilibrium, the balloon will neither rise nor falls.  

 

 

73. Ice floats in water with about nine-tenth of its volume submerged. What is the friction volume submerged for an iceberg floating on a fresh water lake of a (hypothetical) planet whose gravity is ten times that of that of the earth?

According to the principle of floatation, any floating object displaces the amount of liquid (in which itís floating) which is equal to itís own weight.

In case of the ice here, nine-tenth fraction of its volume is under the water. So that much amount of water is displaced. Thus the wholesome weight of the ice is given by that amount of water.

We can write it as 9/10V g=mg (where 9/10V is the fractional volume of the submerged ice, is the density of the liquid, m is the mass of the ice and g is the acceleration due to gravity.). On analyzing, we see that 9/10V=m/ . Thus the fractional volume of the floating object is found to be independent of the acceleration due to gravity. Even though the weight of the floating object and the amount of water displaced by the submerged portion changes. But the net effect is neutral on the fractional volume. Thus no change is seen in the fractional submerged volume of ice on such planet provided that the mass of ice taken is of same mass and the density of the liquid used be equal to the density of water on earth.  

 

74. A bucket containing water is rotated in a vertical circle. Why does not water fall?

A bucket containing water is rotated in a vertical circle. The water does not fall because the weight of the water is balanced by centrifugal force. When the bucket containing water is rotated in a vertical circle then centripetal as well as centrifugal forces come into play. When the bucket is upside-down then the tension T will be minimum and the resultant of the tension and total weight of bucket plus water becomes equal to centripetal force whose direction is always towards the center of that circular motion. At the instant, the centripetal force is balanced by centrifugal force whose magnitude is equal to that of the centripetal force and direction is always opposite to that of the centripetal force. Hence, fall of water from bucket is prevented by the balanced in the centripetal & centrifugal force. As a result, water does not fall.

 

 

75. Why does banking a pilot not fall down, when the aeroplane loops a vertical loop?

By banking a pilot does not fall down even when the aeroplane loops a vertical loop. It is because the weight of the plane plus pilot is balanced by centrifugal force as the motion of aeroplane during banking is circular motion where centripetal as well as centrifugal forces come into play. When the pilot is upside-down then the resultant of the tension and total weight of aeroplane plus pilot becomes equal to centripetal force whose direction is always towards the center of that circular motion which is opposed by centrifugal force whose magnitude is equal to that of the centripetal force. Hence, falling of pilot from aeroplane is prevented by the balanced in the centripetal & centrifugal force. As a result, a pilot does not fall down even when the aeroplane loops a vertical loop.

 

 

76. How does banking of road reduce wear and tear of the tires?

If the roads are not banked then the centripetal force for circular motion is provided by the sidewise frictional force exerted by the road on the tyres which causes the wear & tear of the tyres. When the roads are banked where it takes a curve then the normal reaction of the vehicles splits into two components i.e. vertical component & horizontal component. The vertical component balances the weight of that vehicle & the horizontal component provides necessary centripetal force for uniform circular motion. As a result, no sideways frictional force comes into play to provide centripetal force as the necessary centripetal force is already provided by the horizontal component of normal reaction. So, there is no wear & tear of the tyres when the roads are banked.

 

77. Why are passengers of a car rounding a curve thrown outward?

As the force acting on passengers of a moving car is always opposite to the direction in which acceleration of the car acts. When a car is rounding a curve then the centripetal force acts on that car & direction of centripetal acceleration is towards the center of a car. Hence, the force acting on the passengers of a car is away from the center. So, passengers of a car rounding a curve are thrown outward.

 

 

78. What will provides necessary centripetal force to car taking turn on a level road?

If the roads are not banked then the centripetal force for circular motion is provided by the sidewise frictional force exerted by the road on the tyres. When the roads are banked where it takes a curve then the normal reaction of the vehicles splits into two components i.e. vertical component & horizontal component. The vertical component balances the weight of that vehicle & the horizontal component provides necessary centripetal force to car taking turn on a level road.

 

 

79. Curved railway tracks are banked. Why?

If the curved railway tracks are not banked then the centripetal force for circular motion is provided by the rails exerting a sidewise force on the inner rims of the wheel which may cause dangerous accident which may take thousands of lifes. When the curved railway tracks are banked then the normal reaction of the train splits into two components i.e. vertical component & horizontal component. The vertical component balances the weight of train & the horizontal component provides necessary centripetal force for uniform circular motion. As a result, no sideways frictional force comes into play to provide centripetal force as the necessary centripetal force is already provided by the horizontal component of normal reaction of train. In order to prevent accident & to provide safe driving, curved railway tracks are banked.

 

80. The handle of a screw is made wide, why?

In rotational dynamics, the torque required for revolution is defined as the product of the force F and the perpendicular distance from the axis of rotation i.e. T = Fr. To produce a given turning effect, if the distance from the axis of rotation r is large, less force F will be enough. Hence, the handle of the screw is made wide.

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