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41. If you could journey to the centre of the Earth, what would be the sensation of gravity at various points on the way down, and at the centre?

This problem piqued the curiosity of no less a physicist than Isaac Newton, who of course solved it in his Principia (Book 1, theorem 33). If you are at the centre of the Earth you are pulled equally in all directions, so you are in fact weightless. Higher up, at radius R from the centre, Newton found that the attractions of the materials in the hollow spherical shell of radius greater than R will all cancel one another out--a beautiful mathematical consequence of the fact that gravity decreases as the square of the distance. You feel only the pull of the mass in the sphere below you.

Newton showed that its combined pull is simply proportional to the inverse square of the distance R from the centre. The mass of this sphere is proportional to its volume, that is, R3. So, the weight you would feel, if you were foolhardy enough to descend through a homogeneous planet, would decrease in direct proportion to R3/R2 (which is equal to R) as you moved inwards, reaching zero at the centre.

In fact, the central parts of the Earth are much more massive than the outer parts, mostly dense iron, so your weight would decrease a bit more gradually at first and more rapidly as you penetrated the core.  


42. Why do birds fly in formation shaped like the alphabet' V'? Which bird works the hardest in such a formation and why?

The bird in the lead (at the forward point of the 'V' formation) is working the hardest by being the first to 'break through' the air, which is offering resistance to its flight. Just as a boat leaves a V-shaped wake of smoother water behind it, the lead bird leaves a V-shaped wake of slightly 'smoother' air behind it (actually the lead bird creates a trail of air turbulence that helps lift along the V-shaped direction), and it is a bit easier for the other birds to fly in the wake of the lead bird. If you watch a V-formation carefully, you'll notice that the lead bird does not stay in that position for very long and will drop back into the formation, while another, not-as-tired bird takes the lead, breaking through the air first.  


43. When liquid boils what TYPE of gases are in the bubbles?

The gas in the bubbles you see in the liquid is that liquid in a gaseous state.

For example the gas in the bubbles of boiling water is simply steam. Boiling is a physical change not a chemical change, Therefore only the bonds between the molecules would break not the bonds between the elements. If you want to separate the Hydrogen from the Oxygen in H2O you would have to run an electric current though the water. Here's a hint put something in the water that will ionize, table salt or HCl will work, the electricity will flow easier with the ions. (Note form the editor: be careful this can be dangerous since the gases you produce may explode under certain conditions!).


44. Why is the gap in the ozone layer the worst above Antartica, given that the highest emission of CFC's happens in the Northern Hemisphere?

Well now this is a really great question. I often wondered about this same fact. Before I explain this I would like to first explain tides to you. If I asked you what makes the tides, I might expect you to tell me the moon, but could you explain why there are two high tides and two low tides a day even though the moon only circles once a day?

If we look at the situation where we have a lunar eclipse, there is a high tide on the side of the earth that the sun and moon are, and there is another high tide on the opposite side of the earth than the sun and the moon. The gravitational pull of the sun and moon cause the high tide on the side of the earth that they are on but what causes that other high tide? The second high tide is caused by the centrifugal force cause by the earth orbiting the sun. These two high tides stay right where they are, and as the earth spins, we see the tides 'go around the earth'. The fact that extra water is pulled to these two places makes low tides everywhere else (including the north and south pole).

So why did I tell you about the tides? You see the atmosphere does the same thing that the water does. This means at the north and south pole, the atmosphere is thinner than anywhere else on earth, because the air is pulled away from there. Ozone is in the same percentage all around the earth (because gases diffuse evenly). So it is natural that there is less at the poles. This is why the 'gap' in the ozone is thinner at Antarctica than in the populated region of the northern hemisphere.  


45. What causes high tide and low tide and why?

The high and low tide are caused by the gravitational forces between the earth and the moon. However, the source of the real effect takes some explaining to grasp. The incorrect way of thinking is that the moon attracts all the water to itself, therefore causing a high tide on the side of earth close to the moon, and a low tide on the side far from the moon. However, if this was the case, there would be a high tide once per day. But there are TWO high tides a day. The reason is, the part of the earth both AWAY from the moon, and CLOSE to the moon BOTH get high tides...

Now we need to explain why this is so. The earth and moon rotate around each other, each pulling the other towards itself. The moon attracts every piece of matter on earth. Since gravity is inversely proportional to the square of the distance, this force is greater on the side of the earth closer to the moon, and lesser on the side of the earth further from the moon. Since the earth is quite a rigid object, this difference in forces fails to deform the earth (much). However, it succeeds quite well in deforming the oceans -- which are not as rigid. Since the waters on the moon side are attracted more strongly than average, they tend to bulge TOWARDS the moon, hence causing a high tide. The waters on the opposite side of the moon, since they are attracted less strongly than average, tend to 'lag behind' the rigid earth, and bulge AWAY from the moon, which in this case, is also AWAY from the earth, again, causing a high tide. Low tide occurs at about right angles to the moon, where the force on the waters match the average pull of the moon on the earth closely.

The question that usually follows this is, why doesn't the sun cause any tides (comparable to that of the moon) although its pull on the earth is larger? The answer is, although the gravitational pull of the sun on the earth is larger than that of the moon, due to the much greater distance, the force changes very little from one end of the earth to the other. Since it is the difference in the force than the average magnitude of the force that matters for creating tides, the net effect is much less than that for the moon.  





47. Assume there is an ice cube in a glass of water. When the ice cube melts, what will happen to the water level? Why?

Lets see now. Some intuitive center in my brain is screaming 'it will remain the same', but we will try to think it over.

In the first state, we have an ice cube of mass m floating in the water. If it is floating (in equilibrium), it will have to displace enough water to support its weight. How much is that? It is just Volume = m/d , where m is the mass of the ice cube, and d is the density of water.

In the second state, where the ice has melted, it turns into water of volume.... Volume = m/d! exactly the same volume as it displaced before. So the added volume is the same, so the level of the water will not change.

As a matter of fact, as long as objects are floating (i.e. they don't rest on the bottom) they displace enough water to support their mass. Since by turning from solid to liquid, the mass of water does not change (well, maybe it does, due to mass-energy equivalence, but that's _really_ negligible) it will keep displacing the same amount of water.

However, note that this may not apply to everything. If you had solid alcohol floating in water, when it melts, the level would drop, because water and alcohol mix at the molecular level; i.e. water filling spaces among alcohol molecules.  


48. If Newton's law of motion, which states that any object in motion will remain in motion, is true, then why is it that as a comet gets close to the sun and it melts, and a tail forms, and why wouldn't it be just a big ball of melted stuff?


Newton's First Law of Motion actually states that a body in motion will remain in motion UNLESS ACTED UPON BY AN OUTSIDE FORCE. In the case of a comet, there are a number of outside forces acting upon it.


The primary force on a comet, of course, is gravitational as it orbits the Sun. That force, however, acts more or less uniformly on all parts of the comet (ignoring tidal forces). Other forces, however, include the Solar Wind (a stream of subatomic particles emitted by the Sun) and sunlight itself. While these forces are small and have little effect on the large mass of the comet, they are large enough to influence the smaller mass of its tenuous tail. The small particles of dust and gas that form as the comet's material is heated near the sun are 'blown' outward, away from the sun and the comet's main body, because their small mass is more easily accelerated by the small forces mentioned above.



49. What is escape velocity?


If you throw an object straight up, it will rise until the the negative acceleration of gravity stops it, then returns it to Earth. Gravity's force diminishes as distance from the center of the Earth increases, however. So if you can throw the object with enough initial upward velocity so that gravity's decreasing force can never quite slow it to a complete stop, its decreasing velocity can always be just high enough to overcome gravity's pull. The initial velocity needed to achieve that condition is called escape velocity.


From the surface of the Earth, escape velocity (ignoring air friction) is about 7 miles per second, or 25,000 miles per hour. Given that initial speed, an object needs no additional force applied to completely escape Earth's gravity.  



50. Why can't we put our hands through a table?


If you try to put your hand through a table, you'll feel a force stopping you from doing it. But what force is it? There are 4 fundamental forces in the universe and everything can be attributed to them, and this case is no exception.


The four forces are Gravity, Electromagnetism, the Strong Force and the Weak Force. Clearly gravity isn't the force in question, firstly gravity is only ever attractive, secondly gravity is very very weak and only comes into play when you consider celestial size bodies. The Strong and Weak Forces only every occur on the atomic scale, and are very rarely encountered in day to day life. This leaves one force to be the force preventing you from putting your hand through a table, electromagnetism.

As you will know, everything around us is made of atoms. Atoms are made of very small nuclei (which, in turn, are made of positively charged protons and electrically neutral neutrons) with tiny negatively charge electrons orbiting around them. Now when you try to get close to the table, the electrons orbiting the atoms in your hand, come in very close proximity to the electrons orbiting the atoms in the table. You can bring your hand very close to the table, until you think you are touching it. In fact, you aren't actually in contact with the table. The electrons in your hand are being repelled by the electrons in the table, creating a very strong repulsive force, which you think of as the 'hardness' of the table. It can be quite strange to think you aren't actually touching the table, you're nanometers away from it, and you can't get any closer! You can't put your hand through the table because the really strong electromagnetic force is causing the electrons in your hand and the table to repel! When you try to get closer, the table will move away, and this is due to the repulsion of the electrons being enough to cause a change in momentum of the table!


This is the case in every day life. Electromagnetism is the most common force we encounter (apart from gravity keeping us on the planet). And it can become quite infuriating if you think about it too much and realize you are never actually, truly touching anything!


51. If you hover any distance above the earth (in a helicopter, for example) for a day or so, why aren't you and the helicopter occupants on the other side of the world if the Earth is going through its normal revolution?

If we are inside a helicopter which is landed on the earth's surface then both are sharing the same horizontal velocity of the earth. Under gravitational field intensity, if we hover any distance above the earth by flying from that helicopter for a day or so, we aren't occupants on the other side of the world because the horizontal component of velocities of both remains unchanged even if the earth is going through its normal revolution. So, we are moving with the same horizontal velocity as that of the earth.


52. A stationary van is filled with 1 tone of bees sitting down. Is it true that the overall weight of the van is the same if bees are flying instead of sitting?


Of course this is true! How else could the bees push up if there were not an equal, but opposite, force pushing down? This is an example of Newton's Third Law. This Law of the universe says that for every force there is an equal and opposite force.

You can try an experiment that is close to the bee example for this law. You will need someone to help you. Get a bathroom scale and have your helper watch the scale as you stand on it. Have your helper record your weight. Now, bend your knees and jump up and off of the scale. (You might ruin your scales if you land back down on them, but, you can explain to your mother that it was done in the name of science!) At the exact same instant that you push down on the scales to jump up have your helper read the scale. You will find that you have exceeded your weight in order to jump. This is because in order to push up an equal and opposite force had to be pushed down. Now, if you were a bee with wings you would exceed your weight only for an instant at take-off and then while you hovered the force you would have to push down with would equal your weight.

I hope this helps you to better understand Newton's Third Law. Most people get confused when they try to understand this Law. this is easy to do because people often forget that the two forces that are equal and opposite act on different objects. There is a famous story about a farmer and his horse. The horse is a lazy beast who spends his time eating hay and lazily reading science books. One day the farmer attaches a heavy cart to the horse and demands that the horse pull the cart. 'Well,' says the horse, 'I cannot pull the cart because, according to Newton's third law, if I apply a force to the cart the cart will apply an equal and opposite force on me. The net result will be that I cannot pull the cart since all the forces will be balanced. And, as you know, Newton's first law says that if there are equal forces acting on an object the net result is no change in motion. Therefore, it is impossible for me to pull this cart.' The farmer was very up set! Do you know what he could say to convince the horse to move?

Another answer

Yes. For an object to be supported by air, its weight needs to be countered with an upward force of equal magnitude. Newton's third law of motion also says that if the air provides an upward force on the bees, it feels an opposite force of the same magnitude pushing down on it. In your example, the downward force on the air results in a downward force on the floor of the van equal to the weight of the bees.

If you replace the bees with grasshoppers which don't fly, but get into the air by pushing off from the van's floor, however, the situation is different. The air is not supporting them as it does the flying bees (jumping can still occur in a vacuum). So if all the grasshoppers jump at the same time, it results in the van temporarily 'weighing' MORE, then less, until they all returned to the floor. You can do this experiment yourself by getting on a bathroom scale and jumping up. While in the air, the scale registers zero. Just before, and upon landing, it registers MORE than your weight.


53. Why is it impossible to pump water from very deep in the ground with a surface pump?


The key to understanding this is realizing that suction is not a force, but simply removing an opposing force to the force of air pressure which is already there.

When you stick a pipe down a deep hole into a pool of water at the bottom of a well, air inside the pipe is pushing down on the water in the pipe, and air outside the pipe is pushing down on the water outside the pipe, which in turn pushes up on water inside the pipe. All is in balance.

But now lets say you suck out the air inside the pipe. The water is pushed up the the same as it was before, but there is no counter acting force pushing the water down, so it begins to rise inside the pipe.

So far so good, but why does the water stop rising? Well the water is pulled down by gravity; the more water in the pipe the more it weighs. Since the force of the air outside the pipe is not changing, eventually the weight of the water is equal to the air pressure outside the pipe, and everything is in balance again.


Another answer

Water is pumped from a well by creating a partial vacuum above the water by the pump. The amount of vacuum, in inches of mercury, is equal to the weight of the column of water from the water table to the surface.

Atmospheric pressure at sea level is approx. 76 cm of mercury. This is equivalent to a column of water 3approx. 10.3 m). Therefore, a total vacuum could only pump water from a depth of just under 10.3 meters.

Actually, a total vacuum cannot be created over water. As the pressure is reduced, the boiling point of the water is lowered, producing a layer of water vapor between the water's surface and the pump. The water vapor reduces the ultimate vacuum and the maximum pumping depth, but only by about 1.8cm at 20C.

54. Why are there no tides in rivers and lakes?


Tides are caused by the gravitational pull of the moon and, to a lesser extent, the sun on bodies of water. The earth's water bulges/moves toward and away from the moon(/sun), while it becomes shallower in areas perpendicular to the pull. Thus, there are two complete tide cycles per day.

Tides occur mainly in oceans because that is basically one huge body of water that is free to move all over the earth. Lakes and rivers do not cover enough area to have their water be moved significantly by gravity, or in other words, to have tides.

55. If you had an iceberg in a bathtub and it melted, what would happen to the water level in the bathtub, and why?

An iceberg in a bathtub? It depends on whether you are considering a regular sized iceberg or a VERY small one in a normal bathtub. That's because it depends on whether the iceberg is floating or resting on the bottom. If the bathtub is large and deep enough so that the iceberg is floating, the water level won't change.

A floating object displaces an amount of water equal to its own weight. Since water expands when it freezes, one ounce of frozen water has a larger volume than one ounce of liquid water. A completely submerged ice cube weighing one ounce, for example, displaces MORE than one ounce of liquid water. The cube will rise until the volume remaining under the surface displaces only one ounce of water.


If you could remove the ice cube and leave a 'hole' in the water where the cube used to float without disturbing the surrounding water, that hole would take exactly one ounce of liquid water to fill. Let the ice cube melt. Since it is now one ounce of liquid water, putting it back into the 'hole' will exactly fill it and leave the remaining water undisturbed.

If the iceberg is sitting on the bottom of the bathtub and NOT floating, then the water level will rise as the ice melts.

56. Why is centrifugal force called a fictitious force? (Why is the centrifugal force called as unreal force?)

A particle moving uniformly along a circle has an acceleration since its velocity is changing by direction. To keep that particle rotating in circular path, it is believed that there is existence of an equal but opposite force to the centripetal force. It always acts away from the center as the centripetal force acts towards the center. We always consider existence of normal reaction a non real force balancing the weight of the particle. In the same way, centrifugal force is called a fictitious force or unreal force as it is a reaction force of centripetal force in an inertial frame.



57. If earth shrinks suddenly, how will be the duration of day affected?

If earth shrinks suddenly, its radius will be decreased which causes to decrease moment of inertia (I). In the absence of external torque, the angular momentum L of the system remains conserved i.e. L = I w = constant (where I is moment of inertia & w be the angular velocity). As, the moment of inertia decreases, angular velocity has to be increased according as L = I w = constant. We know that the angular velocity is inversely proportional to the time period T as . So, time period T will be decreased causing to decrease the duration of the day. Hence, if earth shrinks suddenly, the duration of day is decreased.

58. What is the maximum length of a vertical straw with which you can drink cola?


59. Why is there no hydrogen in the earth's atmosphere?


60. Explain why moon has no atmosphere?

Moon has no atmosphere because the value of acceleration due to gravity g on its surface is small. Hence, the escape velocity is only about 2.5 km/sec. The atmosphere gas molecules have thermal velocities greater than this velocity. As a result, the molecules escape and hence there is no atmosphere on moons surface.



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