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
20°C.
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 moon’s surface.
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