Make your own free website on

Google WWW



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 motion.

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.

Another answer

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 mile.

25. Why is it that when you put your finger over the tip of a hose the water shoots out farther?
The answer 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.

another answer
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 Earth's.

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 reason why.


27. I'm an actor playing the role of a balloonist and I want to be able to explain in scientific terms why helium and hydrogen make balloons rise.

Archimedes 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.


29. Which 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 alone!

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 this true?

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?

A 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.



34. If 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 stop?

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 time too.

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 anti-energy.



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 earth.

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.



37. 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.

38. 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 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 continuous.



39. 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 efficiency.


40. What makes 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.  



If you want certain questions answered, please write to