From HOW THINGS WORK http://landau1.phys.virginia.edu/Education/Teaching/HowThingsWork/

 

 

How does an astronaut get prepared for the long period of antigravity

that he is going to be put on? -- ASB, Chiapas, Mexico

 

When an astronaut is orbiting the earth, he isn't really weightless. The earth's gravity is still pulling him toward the center of the earth and his weight is almost as large as it would be on the earth's surface. What makes him feel weightless is that fact that he is in free fall all the time! He is falling just as he would be if he had jumped off a diving board or a cliff. If it weren't for the astronaut's enormous sideways velocity, he would plunge toward the earth faster and faster and soon crash into the earth's surface. But his sideways velocity carries him past the horizon so fast that he keeps missing the earth as he falls. Instead of crashing into the earth, he orbits it.

 

During his orbit, the astronaut feels weightless because all of his "pieces" are falling together. Those pieces don't need to push on one another to keep their relative positions as they fall, so he feels none of the internal forces that he interprets as weight when he stands on the ground. A falling astronaut can't feel his weight.

 

To prepare for this weightless feeling, the astronaut needs to fall. Jumping off a diving board or riding a roller coaster will help, but the classic training technique is a ride on the "Vomit Comet"--an airplane that follows a parabolic arc through the air that allows everything inside it to fall freely. The airplane's arc is just that of a freely falling object and everything inside it floats around in free fall, too--including the upward. It slows its rise until it reaches a peak height and then continues arcing downward faster and faster. The whole trip lasts at most 20 seconds, during which everyone inside the plane feels weightless.

 

Why does a roller coaster end on a lower level than where it starts? --

L, Staten Island, New York

 

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.

 

What are positive and negative g's?

 

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.

 

What role do gravity and inertia play in making a roller coaster work?

 

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.

 

What is the difference between apparent weight and true weight?

 

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.

 

How does a roller coaster work?

 

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.

 

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

 

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.

 

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

 

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.

 

What do you feel g-forces when you ride on a roller coaster? - F

 

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.

 

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

 

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.

 

Does water drain in the opposite direction in the southern

hemisphere? - TL

 

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 wash basin. 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.

 

How can one prove to students that the earth rotates. Any instructions

on how to build a pendulum to show rotation or some other way? -

KC

 

There are many indirect indications that the earth rotates,

including the motions of celestial objects overhead, the

earth's winds--particularly the counter-clockwise rotation of

surface winds in northern hemisphere hurricanes, and the

outward bulge of the earth around its equator. But for a more

direct indication, a Foucault pendulum is a good choice.

 

Unfortunately, a Foucault pendulum isn't easy to interpret or

build. It would be easiest to interpret if it were at the north

pole, where it would swing back and forth in a fixed plane as

the earth turned beneath it. To a person watching the

pendulum from the ground, the pendulum's swinging arc

would appear to complete one full turn each day. However,

elsewhere in the northern hemisphere, the plane of the

pendulum does change and the pendulum's swinging arc will

appear to complete less than one full turn each day.

Nonetheless, the fact that the arc shifts at all is an indication

that the ground is accelerating and that the earth is turning.

 

The problem with building a Foucault pendulum is that it

must retain its swinging energy for hours or even days and

that it must not be perturbed by activities around it. It must

have a very dense, massive pendulum bob supported on a

strong, thin cable and that cable must be attached to a rigid

support overhead. The longer the cable is, the longer it will

take the bob to complete each swing and the more slowly the

pendulum will move. Slow movements are important to

minimize air resistance. If I were building a Foucault

pendulum, I'd find a tall empty shaft somewhere, away from

any moving air, and I'd attached a lead-filled metal ball

(weighing at least 100 pounds but probably more) to the top

of the shaft with a thin steel cable. I'd make sure that nothing

rubbed and that the top of the cable never moved. (Over the

long haul, there is the issue of damage to the top of the cable

because of flexure...it will eventually break here. Wrapping

the cable around a drum so that there is no specific bending

point helps.) Then I'd pull the pendulum away from its

equilibrium position and let it start swinging slowly back and

forth. Over the course of several hours, its swing would

decrease, but not before we would notice that its arc had

turned significantly away from the original arc because of the

earth's rotation.

 

What is the "optimal" weight distribution for a pinewood derby car --

in front/behind, above/below the center of gravity? - BP

 

I'll assume that the car starts on a slope and coasts downhill

to a level finish. If that's the case, then you want to put the

car's center of gravity as far back in the car as you can get it.

That way, the center of gravity will start as high as possible

in the tilted car and will finish as low as possible in the level

car. During a race, the car obtains its kinetic energy (its

energy of motion) from its gravitational potential energy. The

farther the car's center of gravity descends during the race,

the more gravitational potential energy will be converted to

kinetic energy and the faster the car will go.

 

What is the "optimal" shape for a pinewood derby car -- I'm guessing

some sort of short, flat, thin rectangle. - BP

 

The car's biggest obstacle is air resistance, which in this case

is a force known as "pressure drag." The pressure drag force

is proportional to the size of the turbulent wake the car

creates in the air as it passes through the air. Streamlining is

important to minimizing this wake. The thinner and shorter

you can make the car, the smaller its wake will be. The ideal

shape would be an airfoil, like those used in airplane wings

and bodies. These carefully tapered shapes barely disturb the

air at all and experience very little pressure drag. If you

design your car to resemble a wingless commercial jet

airliner, you will be doing pretty well.

 

Why is the outward force in a loop-the-loop a "fictitious" force? Why

isn't it a "real" force?

 

A real force causes acceleration. If the outward "fictitious"

force on a circling object were "real," that object wouldn't

circle. It would accelerate outward. When you swing an

object around on a string, you feel the object pulling outward

on the string. But it isn't itself being pulled outward by

anything. What you're feeling is the object's inertia trying to

make it travel straight. The inward force you're exerting on it

isn't opposing some real force, it's causing the object to

accelerate inward.

 

When you spin an object around a fixed point, a sling for example,

does the object at the end build up energy that causes it to shoot out

quickly when released?

 

Yes. As you whip the object around on a string, you are

doing work on it. You do this by making subtle movements

with your hand, exerting forces that aren't exactly toward the

center of the circle. When you do this, the object begins to

travel faster and faster, so its kinetic energy increases.

Traveling in a circle doesn't change this kinetic energy

because kinetic energy is proportional to speed squared, and

doesn't depend on direction. Finally, when you let go of the

string, the object stops circling and begins to travel in a

straight line. It carries with it all the kinetic energy you gave

it by whipping it about.

 

When a ball swings in a horizontal circle at the end of a string, what's

the force on the ball pulling it straight? What's the force pulling it out?

 

Let's neglect gravity, which isn't important in this horizontal

motion problem. When a ball swings in a circle at the end of

a string, there is only one force on it and that force is inward

(toward the center of the circle). We call such a force a

centripetal force, meaning toward the center. There are many

kinds of centripetal forces and the string's force is one of

them. As for the ball's tendency to travel in a straight line,

that's just the ball's inertia. With no forces acting on it, it will

obey Newton's first law and travel in a straight line. There is

no real force pulling the ball outward. But a person riding on

the ball will feel pulled outward. We call this feeling a

fictitious force. Fictitious forces always appear in the

direction opposite an acceleration. In this case (an object

traveling in a circle) the outward fictitious force is called

centrifugal "force." But remember that it's not a real force;

it's just the object's inertia trying to make it go in a straight

line.

 

If you feel fictitious force upward on a loop the loop, how can that

fictitious force make objects fall upward? Is fictitious force fictional

or real?

 

As you travel over the top of the loop the loop, you observe

the world from an inverted perspective. The sky is below

you and the ground is above you. If you were to take a coin

out of your pocket and release it, you would see it fall toward

your seat. From that observation, and the feeling of being

pressed into your seat, you might think that gravity is

suddenly pulling you toward the sky. It isn't. Gravity is still

pulling you toward the ground, but you are in a car that is

accelerating rapidly toward the ground. As a result, the car is

having to push you toward the ground with a force on the

seat of your pants. You feel pressed into your seat because

the car is pushing you downward hard. When you release the

coin, it seems to fall toward the sky, but it's really just falling

more slowly than you are. With the car pushing you

downward, you're accelerating toward the ground faster than

the coin and you overtake it on the way down. It drifts

toward the seat of the car because the car seat accelerates

toward it. As you can see, the only forces around are the

force of gravity and support forces from the car. There is no

outward or upward force here. The fictitious force is truly

fictional; a way of talking about the strange pull you feel

toward the outside of the loop.

 

If the fictitious force you experience on a loop-the-loop isn't greater

than your weight, will you fall?

 

Yes. If you go over a loop-the-loop too slowly, so that you

don't accelerate downward quickly enough, you will leave

the track and fall. That's why some roller coasters strap you

in carefully before taking you upside-down slowly. Without

the supports, you would fall out of the car.

 

If all the kids on the merry-go-round are clustered around its center

while it is spinning at a constant angular velocity, then if all the kids

were to "cautiously" move away from its pivot to the outer edges

(while still spinning), would that cause the merry-go-round to slow

down faster than if they had remained in the center?

 

Yes. When the kids move away from the center, the

merry-go-round will slow down. If they then return to the

center, the merry-go-round will speed up!

 

Can you explain the term centripetal?

 

Centripetal means "directed toward a center." A centripetal

force is a force that's directed toward a center. For example,

a ball swinging around in a circle at the end of a string is

experiencing a force toward the center of the circle--a

centripetal force. Because the ball accelerates in the direction

of the force, it accelerates centripetally. And because it

experiences a fictitious force in the direction opposite its

acceleration, it experiences an outward fictitious force away

from the center of the circle. That fictitious force is called

centrifugal "force." However, you should always recognize

that this outward "force" is not a force at all, but an effect

caused by the ball's inertia--its tendency to travel in a straight

line.

 

 

Last Updated Wednesday, November 24, 1999 at 8:46:37

Copyright 1997-1999 © Louis A. Bloomfield, All Rights Reserved