21. I often get confused between the terms centrifugal force and
centripetal force could you please enlighten me?
Both terms describe forces
associated with circular motion, but let's start with a straight
line analogy. If you are in a car whose speed is increasing,
the car is being accelerated by a force applied in the direction
of travel. Inside the car, you feel as though a force is pushing
you back in your seat toward the rear of the car. Those two
forces, one forward and one backward, are the straight line
equivalents of centripetal and centrifugal force in circular
If the car described
above is moving at a constant speed, but changing direction
as it travels around a curve, it ALSO is accelerating. Acceleration
occurs anytime VELOCITY changes, and velocity is defined as
a combination of speed AND direction. The car is changing
direction because of a force (supplied by friction between
road and tires) directed toward the center of the curve. That
is the centripetal force, which is always directed toward
the center of the curve. When you swing a weight around your
head at the end of a string, your hand supplies the centripetal
force to keep the weight moving in a circle.
Back inside the car,
you feel a force pushing you AWAY from the center of the curve.
That force is called centrifugal force, which is always equal
in magnitude but opposite in direction from centripetal force.
You have a lot of company in your confusion. A lot of people
get centrifugal and centripetal force mixed up. Here is the
way I keep them straight. 'Centripetal' comes from the Latin
word for 'center seeking' so the direction that centripetal
force acts in is towards the center of a circle. Think of
the 'p' as the first letter of 'pushed', as in 'being pushed
towards the center'.
'Centrifugal' has two
Latin roots in it. You can see the same 'centri' from centripetal,
so the idea of 'center' is a part of this word. The other
Latin root is 'fugere', which means 'to flee'. So, 'centrifugal'
means 'to flee from the center'. Think of the 'f' as the first
letter of flee as in 'fleeing from the center of the circle'.
I presume you know that
of these two, only centripetal is the real force. For something
to go in a circle it must be being pushed or pulled (there's
the 'p' again!) towards a center point; otherwise it would
be going in a straight line. Centrifugal 'force' is really
a function of the inertia of the object being pushed into
a circle. It is not really a force at all, it is simply the
tendency of an object to go in a straight line.
22. Is it easier for an airplane to travel
East to West or West to East?
The ease with which an
airplane may travel east or west depends on your definition of
'ease'. In all cases, the key is the direction of the airplane
relative to the wind.
In terms of safety, both scenarios are equally safe during
normal flight at cruising altitude. It is safer to land and
takeoff, however, into the wind - more lift is provided at a
given ground speed, which is advantageous for the relatively low
ground speeds at which planes take off and land.
In terms of time and expense of travel at a given air speed,
it is clearly advantageous to travel with the wind, since
airplanes expend energy pushing backwards against the air
through which they are flying but progress towards their
destination is measured relative to the ground. An airplane with
an airspeed of 550 mph traveling in a 100 mph tailwind is
traveling 650 mph relative to the ground, allowing it to reach
its destination more economically. Conversely, traveling into a
headwind at a given airspeed is more expensive. It also takes
more time at a given airspeed (there is a maximum airspeed
determined by law and/or air traffic controllers that may not be
exceeded on commercial flights).
The prevailing winds are from west to east for most
locations in the United States. Therefore, it is more economical
to fly to the east, taking advantage of the prevailing tail
winds, for a cheaper and faster flight over a given distance.
By the way - Since the earth rotates about 1000 mph near the
equator and about 700 mph at 45 degrees latitude, from west to
east, it may be tempting to think that this affects the relative
ease with which the plane may travel in a particular direction.
However, the airplane has inertia, one interpretation of which
means the airplane assumes the earth's frame of reference,
moving right along with the earth's surface, and does not gain
any advantage from this motion in travel to another point within
this same frame of reference. However, rockets being launched
into orbit AROUND the earth do gain an energetic advantage from
this rotation, and are therefore launched towards the east
almost without exception. Have you ever noticed that the space
shuttle always curves out over the Atlantic, to the east,
shortly after its initially vertical take-off?
23. How can you measure the mass of the
Earth or any other planet?
Wow, what a good question. Do
you know what is so good about it? You ask about the mass of the
Earth and not about its weight. I suppose that is because you
already know that as the Earth orbits the Sun it is weightless.
But it certainly is not massless.
Because the Earth has mass it exerts a force of gravity and
the magnitude of this force of gravity is determined by the mass
of the Earth and the distance away from the center of the Earth
another object is. Now, you know that if you drop something it
falls. But did you know that as it falls it accelerates? That
means that as it falls it falls faster and faster and faster
until it hits the ground. On earth at sea level objects that
fall speed up at a rate of 9.8 m/s every second. So, for every
second something falls it falls 9.8 m/s faster with each passing
second. You can see that at this rate you are going pretty fast
real soon. (9.8 m/s is a little more than 20 miles per hour, so
in three seconds you are already going 60 mph!)
If you go way high up a mountain the acceleration due to
gravity is a little less than 9.8 m/s. If you go higher still
the acceleration is even less and so on. You can tell how high
you are by how fast objects that fall accelerate.
Now, here is the really neat part: all objects accelerate to
the Earth at exactly the same rate given the same distance from
the Earths center. In other words, even the Moon is accelerating
towards the Earth at a rate that is appropriate for its distance
from the Earth's center! If another object, say with the mass of
this desktop computer, were to be placed in the moon's orbit it
would circle the Earth at exactly the same speed as the moon.
Why? Because both objects are accelerating towards the Earth and
all objects, regardless of their mass, accelerate towards the
Earth at exactly the same rate.
So here is where it all comes together: If you know how far
an object is from the center of a planet or a sun and if you
know the rate of acceleration that that object is accelerating
towards that planet or that sun you can easily calculate the
mass of that planet or sun because only an object of that
planet's or sun's mass can accelerate an object towards it at
that rate at that distance.
24. Does a
car use the same amount of gas traveling a given distance at 60
mph than it does traveling the same distance at 10 mph (or any
other speed)? And if so, does a person burn the same amount of
calories walking a given distance as running it?
With respect to automobiles, there are several
factors affecting mileage. Transmission gearing is one. Lower
gears require the engine to turn more times than higher gears
for the same distance traveled, consuming more fuel per mile. At
higher speeds, air drag supplies a force that must be
counteracted by the engine, again consuming more fuel. (Drag, in
fact, increases with the SQUARE of speed so that resistance
increases four times every time speed doubles.) Engine design
can also determine the most efficient RPM for fuel consumption.
Given all of that, yes, speed DOES impact your car's fuel
efficiency. (Remember the original reason for a nationwide speed
limit of 55 mph--conserving gasoline?) Optimum fuel efficiency
should be experienced somewhere just above the speed at which
the highest gear becomes useable.
The human body, on the other hand, is not affected by the same
factors. Legs have only one 'gear', and only very fast runners
should experience air drag of any significance. Except for the
different mechanics used in walking vs. running, therefore, and
how that might affect muscle efficiency, I would not expect
speed to significantly impact the number of calories burned per
is it that when you put your finger over the tip of a hose the
water shoots out farther?
centers on the idea of matter conservation. If a certain amount
of water flows into the hose from the spigot, that water must
either flow out of the hose at the other end, or accumulate
inside the hose itself. Since a garden hose is made of stiff
plastic (generally), it won't expand to let more water
accumulate, so the water must eventually flow out of the end of
the hose. When you put your finger over the tip of the hose, you
decrease the amount of space the water has to flow through.
Since the same amount of water has to flow out of the hose
before and after you place you finger over the end, the water
must shoot out faster (to keep the amount of water flowing out a
constant). And, thus, since the water shoots out faster, it
travels farther. To find further information about this, I would
suggest any basic physics textbook, or, more in-depth, a
textbook on fluid mechanics.
When the hose is off, water is being
pushed against the valve at a constant pressure (measured in
force per unit area, like psi or Newtons per square meter).
Therefore force on the water is equal to pressure
x area. When the hose is turned on, water will be
released, and will go a distance depending on this pressure
and the opening area of the hose. If you put your finger
partway over the hose, there is less area for the water to
come out, and therefore the force is greater.
26. Atmospheric pressure is supposed to be
very large. Then why is it that we, along with everything else
on the earth, do not get crushed under this tremendous pressure?
You are correct about the fact that
atmospheric pressure is very large. Indeed, some would say
that it is very, very, very large! I guess you know that at
sea level that pressure is 14 pounds for every square inch
of our body surface area. I don't know about you, but, for
me that is a very, very large area!
We are not crushed by this tremendous pressure because we
grew up in it. I mean from the very beginning of life on
Earth, life grew up in this huge pressure. If we are being
pressed inward by this very large pressure we must be
pushing out with an equal pressure. This is how all life on
earth evolved. In fact, we push out all the time. I am sure
you have seen movies in which some characters are pushed out
into space. If the movie is done correctly you will see the
persons body continue to push outward without anything
pushing back. Thus the person explodes! The movie 'Total
Recall' has some especially good shots of this happening on
Mars, where the pressure of the atmosphere is much less than
Now, if you want to see something get crushed under the
pressure of a fluid you can go deep into the ocean. Water
pressure is much greater than air pressure. Humans can go
only a couple of hundred feet, with training, deep before
the pressure of the water is more than our bodies can bear.
But, you know that there are creatures that can live at
great depths without getting crushed; and now you know the
an actor playing the role of a balloonist and I want to be able
to explain in scientific terms why helium and hydrogen make
is your man for the answer to this question. He is the Greek
fellow who figured out that the weight of a fluid displaced by
an object in that fluid is equal to the buoyant force placed on
that object by the fluid. In other words there is a force,
called the buoyant force, that pushes in the direction opposite
gravity. the magnitude of that force is exactly equal to the
weight of the fluid the object displaces. So, you and I displace
about 100 Liters of air. Since we weigh a lot more that 100
liters of air the force of gravity is stronger that the buoyant
force of air. But, for a helium balloon this is not the case.
The amount of air this balloon displaces weighs more than the
balloon and so up it goes. The buoyant force of the air is
greater than the force of gravity.
Why is the weight of a helium balloon less than the weight of
the air it displaces? Well, because helium is a very light gas.
Air is mostly nitrogen which has an atomic weight of 14 while
helium has an atomic weight of only 4. Hydrogen, which is not
used for balloon because it is too dangerous, is even less
massive with an atomic weight of only 1.
Now, if you are a balloonist chances are you are not using these
gases in your balloon. A balloon used by balloonists today works
on hot air. The principle is the same for helium balloons but on
a much larger scale. For a hot air balloon to float in air it,
too, must displace an amount of air that weighs more than the
balloon. Yes, that is a LOT of air! This can be done because hot
air is less dense than cold air, thus less of it takes up more
space than an equal amount of cold air. So, the hotter the air
the less dense it is and the more space a little if it takes up
so that on balance the amount of hot air in the balloon is a
whole lot less than what it would take cooler air to fill the
same space. Thus the total weight of the balloon, the air
inside, the gondola, the fuel tanks (needed to heat the air
inside the balloon), and the passengers is less than the weight
of the air the balloon displaces.
28. The tires of airplanes
(at least the big ones) are inflated by nitrogen (instead of
air). Why is this done?
Air has a certain moisture content and it is generally very hard
to remove this moisture. If an airplane tires were filled with
air, at the flight altitude ice would form inside the tires
since the temp up there is about -30 degrees F. Landing with a
chunk of ice in the tire would make it out of balance and change
the tire pressure. Tires would probably burst.
On the other hand, nitrogen doesn't form a liquid till -173
0C and pure nitrogen has almost no moisture.
In addition, consider if the brakes overheat and cause a fire.
The nitrogen will not burn, but air has oxygen which will feed
the fire. Jet airline tires are fused. When the fuse is heated
it deflates the tire so they don't explode.
is heavier, wet or dry sand?
Which would you think is heavier? If wet sand is
simply dry sand with water in it than clearly wet sand is
heavier. But wait! We have to define our terms here. This is why
this is such a good question! I am impressed that you should
wonder about something so obvious!
When doing science one has to be very careful how they speak, or
write. Your question sounds like one I used to trick my elders
with when I was a child. 'Which,' I would ask, 'is heavier: A
ton of bricks or a ton of feathers?' The unthinking adult would
answer, to my delight, that the ton of bricks is heavier. I
realize your question is not meant to be a trick but it does
illustrate how careful one must be.
If you were to ask 'Which takes up more space, a pound of wet
sand or a pound of dry sand?' then the answer would be that a
pound of dry sand takes up more space since it takes more dry
sand to equal a pound. But, you asked which is heavier. Before
answering such a question one would have to ask how much space
that sand is taking up. In other words you could have asked:
Which is heavier, a bucket full of wet sand or a bucket full of
dry sand? The answer to that question would be the bucket of wet
sand since that bucket is full of sand and water.
At this point your question really gets good. Why? Because the
next question would be something like this: How could the bucket
of wet sand be heavier? After all, water is not a heavy (or
dense) as sand and if the sand in the bucket has to make room
for the water it would have to hold less sand and therefore the
sand and water combination should weigh less than the sand
Now, there is a good question! I can see why you, or anyone who
is a thinking person, would think, logically, that if you have
equal volumes of wet and dry sand the wet sand would weigh less.
this would have to be true since the water takes up the space
the sand would have taken if the water were not there and since
water is less dense than sand the combination of water and sand
to equal a volume of dry sand would have to weigh less.
There is just one thing: There are spaces between the sand, not
big spaces but spaces nevertheless; spaces big enough for water
to get into without moving any sand out of the way. In other
words, you could take your bucket of dry sand and fill it with
water and move not one single grain of sand out of the bucket.
So equal volumes of wet and dry sand would not weigh the same;
the wet sand would weigh more because it has more mass, the mass
of the water in-between the sand and the mass of the sand
itself. The dry sand has only the mass of the sand and the air
between the grains of sand.
But, you can see how wonderfully this question shows how careful
scientists must be when they ask a question. You know, the art
of asking a question is a much more valuable skill than the
ability to answer them. You are the better scientist for asking
the question than I am for answering it! Keep asking questions!
30. I am
11 yrs old and soon to be going to Magic Mountain to ride a
roller coaster for the first time. My Dad says that I really
don't need a safety harness when the roller coaster goes
upside-down because centrifugal force will hold me in. Is
Yes, this statement is
absolutely correct but a centrifugal force is a pseudo or
fictitious force which acts on the body due to its inertial
properties. The motion of a roller coaster behaves as a
vertical circular motion due to which centripetal as well as
centrifugal forces come into play during the entire motion
of roller coaster. When the roller coaster goes upside-down
then the tension T will be minimum and the resultant of the
tension and total weight of roller coaster 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, roller coaster along with its passengers does
not fall & he don't need a safety harness when the roller
coaster goes upside-down.
31. Is our moon's gravitational field strong enough to retain a
gaseous envelope and is there a gas with a sufficiently high
molecular weight that could be used to create an 'atmosphere'
for our moon?
Our moon has what
is called a transient atmosphere. There is gas on
the moon but it is in such a small quantity that it
really doesn't even count as an atmosphere. As to
your question, their is no gas that can provide a
permanent atmosphere to the moon.
Imagine the moon had an atmosphere similar to
Earth's. The sun would heat up the air molecules
giving them more kinetic energy This would cause
them to move faster, and faster until eventually
they would reach escape velocity and leave the
planet. The moon's escape velocity is very small,
and thus even a small amount of energy will make gas
on the moon escape from its gravity.
Even the earth is constantly bleeding gas out into
space, we just replenish our air supply through
biological processes, and the occasional asteroid
impact that brings in new gases.
32. Do you get wetter if
you run or walk in the rain?
close friend of
mine, and I actually performed a test on this. We put a
cardboard sheet over our head and one in front of us. We
walked over a 10ft space, it was a light drizzle so we
would be able to count the number of drops that hit the
sheets. The amount of rain drops that you will get hit
by (in idle conditions, the area each person walks is
equally dense as far as rain drops) is exactly the same
whether you run or if you walk, as long as the time you
are in the rain is the same.
The reason for the fact that you are hit by
approximately the same amount, is because when you run,
you are hit from above with less rain drops, however,
with your forward velocity increasing, you hit more and
more rain drops head on. As long as the time in the rain
for both people is the same, you should get a close to
equal number of rain drops. However, a person who
sprints 100yds as opposed to a person who walks the
100yds will get hit by much less rain, because of the
time it takes to travel the same distance.
So as a straight answer to your question, yes you will
get hit with less rain drops if you run (bearing in mind
there must be a significant difference in speed or
difference (to change time) than if you walked.
33. If space is a vacuum, then
how do space capsule's retrorockets steer?
The fact that space
is a vacuum nicely isolates this problem as a
demonstration of Newton's third Law, commonly phrased as
'every action has an equal and opposite reaction.'
It is not necessary for the rocket exhaust to push
against anything EXCEPT THE SHIP ITSELF. You see, when
the combustion of fuel takes place inside the rocket
(think of a long vertical cylinder with the 'bottom'
open) the exhaust gasses produced expand quickly in all
directions. The molecules slam into anything in their
path exerting a small force each. Thus some molecules
push against the 'right' side and some push against the
'left' side, and all these sideways forces cancel each
other out. Some molecules slam against the 'top' of the
cylinder, but since there is no bottom of the cylinder,
there is no force to cancel this out! Therefore the net
force will be in the 'up' direction.
Another way to think about the situation is as a
conservation of momentum problem. Any isolated system
will conserve total momentum. Thus if you imagine a
stationary ship, plus the fuel and oxygen molecules on
the ship, p=0. If a few trillion atoms of exhaust are
shot in one direction into the vacuum of space, the rest
of the ship must move in the OPPOSITE direction with the
same momentum (mass x velocity)
You can simulate this situation by throwing some bean
bags (or bowling balls, or any other objects you have
lying around) in one direction as you sit in a chair
with wheels or stand on roller blades or ice skates.
Whatever direction you push on the object, the object
will push back on your hand with the same force, but in
the opposite direction. Therefore you and the object you
throw will move in opposite directions.
both friction and air resistance were eliminated
from acting on the swinging pendulum, would gravity
act on the pendulum to slow it down and eventually
No, it wouldn't. If
all friction and air resistance was eliminated (plus,
the losses due to deformation of the string and the
like) the pendulum, under ONLY the effect of gravity,
would keep swinging indefinitely.
That's because gravitation is a 'conservative' force, it
does not drain any energy from the object moving under
it effect, it just converts the energy from one form to
another. When the pendulum reaches either end at its
highest point, all the energy is potential energy, and
the kinetic energy is zero. At the bottom of the swing,
the kinetic energy is maximum, while potential energy is
minimized. No energy is transferred out of the system,
so it must keep moving. In the presence of friction
however, energy is removed from the system in the form
of heat. (The air heats up a little, the contact point
of the string heats up a little... etc).
35. If there is anti-matter,
couldn't there be anti-energy?
When people speak of
anti-matter, it's important to recognize that
anti-matter isn't some sort of 'negative' matter. An
anti-particle is just as real as any other particle. If
you use Einstein's famous E=mc2
relation you find that an anti-particle has positive
energy, since it has a mass which is identical to the
mass of its partner 'particle.' The 'anti-' part of the
name just signifies the fact that the quantum numbers
that the particle has are opposite of those of the
corresponding particle. Most people have heard that
anti-matter annihilates with matter and gives rise to
energy. This is a result of the two particles having
exactly opposite quantum numbers -- but 'particles' in
our everyday world annihilate into new particles all the
Energy, by contrast, is a relative concept. The
important physical quantity is the difference in energy
between any two states of a system. We thus always speak
of positive energy values and typically eliminate
solutions which give rise to negative energies as
unphysical. In short, then, the existence of anti-matter
does not imply the existence of some sort of
36. Why the law of momentum
conservation is not violated when a ball rolls down
a hill and gains momentum?
Momentum is conserved
only for a closed system. When a system is affected by
external forces, the momentum definitely varies. When
the ball rolls down, it is experiencing force from the
However if that body and the earth is considered to
constitute a system, they attract each other with a
force “F”, which is equal to the gravitational force
between them. If the body is accelerating downwards, the
earth is also accelerating upwards, however not seen to
the eyes of the observer. When the body drops from the
height, it gains momentum down, while the earth gains
the same momentum up. The total momentum will be zero,
as was previously before the body started to roll. Since
the earth is very massive, you can not observe its
motion in reaction.
How does a
hydraulic jack work?
Hydraulic jacks and many other technological
advancements such as automobile brakes and dental chairs
work on the basis of Pascal's Principle, named for
Blaise Pascal, who lived in the seventeenth century.
Basically, the principle states that the pressure in a
closed container is the same at all points. Pressure is
described mathematically by a Force divided by Area.
Therefore if you have two cylinders connected together,
a small one and a large one, and apply a small Force to
the small cylinder, this would result in a given
pressure. By Pascal's Principle, this pressure would be
the same in the larger cylinder, but since the larger
cylinder has more area, the force emitted by the second
cylinder would be greater. This is represented by
rearranging the pressure formula P = F/A, to F = PA. The
pressure stayed the same in the second cylinder, but
Area was increased, resulting in a larger Force. The
greater the differences in the areas of the cylinders,
the greater the potential force output of the big
cylinder. A hydraulic jack is simply two cylinders
connected as described above.
If I am
standing on a mountain, Am I going faster than
someone at sea level? If so, is my day shorter or
will the higher vantage point of the horizon allow
my day to seem longer?
The further away you
are from the axis connecting the North and South Poles,
the faster you will be moving as a result of Earth's
rotation. So unless you are at either pole, climbing to
a mountain top WILL cause you to move faster than at the
same latitude at sea level. Your speed alone, however,
has no effect on the relative length of night and day.
One rotation still takes 24 hours...you are travelling
faster because your path around the Earth is longer.
At higher altitudes you will see more sunlight (and more
daylight time), though, because the higher vantage point
allows you to see the Sun 'over the horizon' more than
at sea level. Imagine being many thousands of miles
above sea level, where the apparently smaller Earth is
able to block out even less sunlight. At an infinite
'altitude', in interplanetary space, daylight is
I use many
fans to cool my airless, New York apartment. One of
my fans has two blades, one has three blades and two
others have five blades. What considerations
determine how many blades a fan has? How does the
number of blades a fan has affect the amount and
velocity of the air it moves? And, as an aside, are
the underwater propellers that are used for moving
boats and ships subject to the same considerations?
The number of blades in a fan depends upon three
primary considerations: how much power is available
to turn them, the volume and velocity of air
required, and the restrictions imposed by noise.
Having more blades is less efficient than having fewer blades, but a fan
with fewer blades needs to turn faster to move the
same volume and will consequently have a higher tip
speed and be noisier.
Basically, assuming that there are no power limits, fewer blades on your
fan mean more revolutions per minute, faster moving
air, higher noise levels and maximum efficiency.
Many blades mean fewer revolutions per minute, a
higher volume of air moved, lower noise and lower
the Earth rotate?
The Earth rotates simply because it has not yet stopped
moving. The Solar System, and indeed the Galaxy, were formed by
the condensation of a rotating mass of gas. Conservation of
angular momentum meant that any bodies formed from the gas would
themselves be rotating. As frictional and other forces in space
are very small, rotating bodies, including the Earth, slow only
very gradually. The Moon, a much smaller and lighter body, has
effectively already stopped rotating because of the
gravitational drag exerted by the Earth, and now always keeps
the same face turned towards us.