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Centrifuges and Roller Coasters
Page 2 of 3 (23 Questions and Answers)

682. Does water drain in the opposite direction in the southern hemisphere? - TL
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In principle, yes, but in practice, no. To explain why, I'll begin with the origins of directional circulations on earth. Because the earth is turning, motions along its surface are complicated. The ground at the equator is actually heading eastward at more than 1000 miles per hour. The ground north or south of the equator is also heading eastward, but not as quickly. The ground's eastward speed gradually diminishes until, at the north and south poles, there is no eastward motion at all. As a result of this non-uniform eastward motion of the ground, objects that travel in straight lines because of their inertia end up drifting eastward or westward relative to the ground. For example, if you took an object at the equator and threw it directly northward, it would drift eastward relative to the more slowly moving ground. If someone else threw an object southward from the north pole, that object would drift westward relative to the more rapidly moving ground. In the northern hemisphere, objects approaching a center tend to deflect away from that center to form a counter-clockwise circle around it. This process is reversed in the southern hemisphere so that objects approaching a center there tend to form a clockwise circle around it. Thus hurricanes are counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.

When water drains from a basin in the northern hemisphere, it flows toward a center and should have a tendency to deflect into a counter-clockwise swirl. However, the effect is very weak in a small washbasin. The direction in which the water swirls as it drains is determined by other effects such as how the water was sloshing before you opened the drain or how symmetric the basin is. For this earth's rotation-driven swirling effect (the Coriolis effect) to dictate the direction of a circulation the objects involved must move long distances over the earth's surface. Even tornadoes don't always rotate in the expected direction; they're just not big enough to be spun consistently by the Coriolis effect.


753. In today's lecture, you stated that a person accelerating downward OR UPWARD does not feel the effects of gravity. How do you explain the g-forces felt by astronauts at escape velocity? - TH
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In the lecture, I said that a person who is falling does not feel the effects of gravity, even when they are traveling upward. But when they are falling, they are accelerating downward at a very specific rate—the acceleration due to gravity, which is 9.8 meters/second2 at the earth's surface. When an astronaut is accelerating upward during a launch, they are not falling and they do feel weight. In fact, because they are accelerating upward, they feel particularly heavy.

858. What do you feel g-forces when you ride on a roller coaster? - F
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Whenever you accelerate, you feel a gravity-like sensation "pulling" you in the direction opposite your acceleration. What you feel isn't really a force—it's really just your own inertia trying to keep you going in a straight line at a constant speed. In other words, your inertia is trying to keep you from accelerating. For example, whenever you turn left in a roller coaster, your inertia opposes your leftward acceleration and you feel "pulled" toward the right. This "pull" of inertia is sometimes called a "fictitious force" but you should remember that it isn't a force at all, no matter how "real" it feels. Perhaps the most striking effect of acceleration occurs during your trip around a vertical loop-the-loop. When you are arcing around the top of the loop-the-loop, you are accelerating downward so quickly that you feel an enormous "fictitious force" upward. This "fictitious force" has a stronger effect on you than the real force of gravity, so you feel as though you are being pulled upward. The result is that you feel pressed into your seat, even though your seat is actually upside-down.

881. If you lived on the moon, would it be easier to adapt to living with the moon's gravity, or to create an artificial environment with the gravity of earth? — MK, Orlando, FL
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Building an environment that made you feel what appeared to be the earth's gravity would be a substantial undertaking. The only way to simulate gravity is through acceleration and the only way to make a person experience acceleration continuously is to swing them around in a circle. So this environment will have to swing its occupants around in a circle. However, we are extremely sensitive to changes in orientation, so that we can tell the difference between true gravity and the experience of being swung around in a small circle. To avoid the dizzying feeling of having our orientations changed rapidly, the turning environment would have to be extremely large. It would have to be a huge rotating wheel, looking like a heavily banked circular racetrack that spun at a steady pace and completed something like one full turn per minute. The occupants would have to live on the long, thin surface of this turning racetrack. Building such a device on earth wouldn't be easy. Building it on the moon would be much harder. I wouldn't plan on trying to simulate the earth's gravity on the moon. So I vote for just putting up with the moon's weaker gravity.

935. Is it possible to greatly increase the speed of a roller coaster, while retaining some safety, by applying the same theory that is used in Bullet Trains? — JA, Henderson, NV
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While roller coasters could be made faster if they used the high performance tracks of bullet trains, smoothing out the tracks would only make the ride less jittery and wouldn't reduce the accelerations needed to complete the turns. The faster the train moves, the faster everything must accelerate as the track bends. Doubling the speed of the roller coaster would double the changes in velocity associated with each bend and would halve the time available to complete that change in velocity. As a result, doubling the roller coaster's speed would quadruple the accelerations it experiences on the same track and thus will quadruple the forces involved during the ride. A roller coaster ride already involves some pretty intense forces and accelerations. If those forces and accelerations were increased by a factor of 4, they would be more than most people could handle. Thus I wouldn't expect many riders on a double-speed bullet train roller coaster.

988. How does a roller coaster work?
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A roller coaster is essentially a gravity-powered train. When the chain pulls the train up the first hill, it transfers an enormous amount of energy to that train. This energy initially takes the form of gravitational potential energy—energy stored in the gravitational force between the train and the earth. But once the train begins to descend the first hill, that gravitational potential energy becomes kinetic energy—the energy of motion. The roller coaster reaches maximum speed at the bottom of the first hill, when all of its gravitational potential energy has been converted to kinetic energy. It then rushes up the second hill, slowing down and converting some of its kinetic energy back into gravitational potential energy. This conversion of energy back and forth between the two forms continues, but energy is gradually lost to friction and air resistance so that the ride becomes less and less intense until finally it comes to a stop.

1000. What is the difference between apparent weight and true weight?
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Your true weight is caused by gravity—it is the force exerted on you by gravity; usually the earth's gravity. Your apparent weight is the sum of your true weight and a fictitious force associated with your acceleration. Whenever you accelerate, you experience what feels like a gravitational force in the direction opposite your acceleration. Thus when you accelerate to the left, you feel a gravity-like experience toward your right. It is this effect that seems to throw you to the right whenever the car you are riding in turns toward the left. In fact, this effect is caused by your own inertia—your own tendency to travel in a straight line at a constant speed. Your apparent weight can be quite different from your true weight. Perhaps the most striking example occurs on the loop-the-loop of a roller coaster. While your true weight remain downward throughout the ride, as it always is, your apparent weight actually becomes upward as you pass around the top of the loop-the-loop. You are accelerating downward so rapidly at the top of the loop that the experience you have is one of a gravity-like force that is pulling you skyward. Since the car you are riding in is invert and above you, you feel pressed into your seat even though the ground is in the other direction.

1032. What role do gravity and inertia play in making a roller coaster work? — B
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Gravity provides the energy source for a roller coaster and inertia is what keeps the roller coaster moving when the track is level or uphill. Once the roller coaster is at the top of the first hill and detaches from the lifting chain, the only energy it has is gravitational potential energy (and a little kinetic energy—the energy of motion). But once it begins to roll down the hill, its gravitational potential energy diminishes and its kinetic energy increases. Since kinetic energy is related to speed, they both increase together.

At the bottom of the first hill, the roller coaster has very little gravitational potential energy left, but it does have lots of kinetic energy. The roller coaster also keeps moving, despite the absence of gravitational potential energy. You can view its continued forward motion as either the result of having lots of kinetic energy or a consequence of having inertia. Inertia is a feature of everything in our universe—a tendency of all objects to keep doing what they're doing. If an object is stationary, it tends to remain station. If an object was moving forward at a certain speed, it tends to keep moving forward at a certain speed. Inertia tends to keep the roller coaster moving forward along the track at a certain speed, even when nothing is pushing on the roller coaster. While the roller coaster will slow down as it rises up the next hill, its inertia keeps it moving forward.


1175. What are positive and negative g's?
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Let me start with the concept of inertia. Like all objects in this universe, we naturally tend to keep doing what we're doing—if we are stationary, we tend to remain stationary, and if we are moving, we tend to keep moving in a straight line at a steady pace. In fact, the only way that your speed and/or direction of travel (in short, your velocity) can change is if something pushes on you. When that happens, you accelerate (which is to say your velocity changes).

Whenever you accelerate, the various parts of your body can no longer follow their inertia; they must accelerate, too. This acceleration requires forces within your body and you can feel these forces. In fact, they make it feel as though a new type of gravity were acting on the parts of your body. You can't distinguish true gravity from the experience of acceleration because they feel exactly the same. The strength of this gravity-like experience depends on how fast you accelerate and it points in the direction opposite your acceleration. If you accelerate upward, as you do when an elevator first starts moving upward, this gravity-like sensation points downward and you feel extra heavy (the experience of "positive g's") If you accelerate downward, as you do when a rising elevator comes to a stop, this gravity-like sensation points upward and you feel unusually light (the experience of "negative g's") Since there is no fundamental limit to how rapidly one can accelerate, these positive and negative g's can become extremely strong and can easily feel stronger than the true force of gravity. However, when these gravity-like sensations become a few times stronger than gravity itself, they become difficult to tolerate. That's why elevators start and stop gradually and why the turns on roller coasters aren't too sharp.


1319. Why does a roller coaster end on a lower level than where it starts? — L, Staten Island, New York
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A roller coaster is a gravity-powered train. Since it has no engine or other means of propulsion, it relies on energy stored in the force of gravity to make it move. This energy, known as "gravitational potential energy," exists because separating the roller coaster from the earth requires work—they have to be pulled apart to separate them. Since energy is a conserved quantity, meaning that it can't be created or destroyed, energy invested in the roller coaster by pulling it away from the earth doesn't disappear. It becomes stored energy: gravitational potential energy. The higher the roller coaster is above the earth's surface, the more gravitational potential energy it has.

Since the top of the first hill is the highest point on the track, it's also the point at which the roller coaster's gravitational potential energy is greatest. Moreover, as the roller coaster passes over the top of the first hill, its total energy is greatest. Most of that total energy is gravitational potential energy but a small amount is kinetic energy, the energy of motion.

From that point on, the roller coaster does two things with its energy. First, it begins to transform that energy from one form to another—from gravitational potential energy to kinetic energy and from kinetic energy to gravitational potential energy, back and forth. Second, it begins to transfer some of its energy to its environment, mostly in the form of heat and sound. Each time the roller coaster goes downhill, its gravitational potential energy decreases and its kinetic energy increases. Each time the roller coaster goes uphill, its kinetic energy decreases and its gravitational potential energy increases. But each transfer of energy isn't complete because some of the energy is lost to heat and sound. Because of this lost energy, the roller coaster can't return to its original height after coasting down hill. That's why each successive hill must be lower than the previous hill. Eventually the roller coaster has lost so much of its original total energy that the ride must end. With so little total energy left, the roller coaster can't have much gravitational potential energy and must be much lower than the top of the first hill.

It's then time for the riders to get off, new riders to board, and for a motor-driven chain to drag the roller coaster back to the top of the hill to start the process again. The chain does work on the roller coaster, investing energy into it so that it can carry its riders along the track at break-neck speed again. Overall, energy enters the roller coaster by way of the chain and leaves the roller coaster as heat and sound. In the interim, it goes back and forth between gravitational potential energy and kinetic energy as the roller coaster goes up and down the hills.


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