The dumb questions thread

[bilby]

Here is another one. I recently watched Space Odyssey. In a scene Dave is running inside the rotating cylinder. If he gets both feet off the surface what happens? I'd think he is in zero g with momentum from the rotation when he was in contact. Is it possible to run as depicted in the movie?

If he jumps up his momentum carries him forward and at some point he hits the surface.

That makes sense.

If Dave jumps "straight up", he will be carried on a vector that is the sum of a tangent to the cylinder and a vector that lies on the diameter of the cylinder. This vector intersects the cylinder somewhere "ahead" of Dave. The higher he jumps, the greater the arc he traverses.

With each step, Dave is propelling himself along a tiny chord. It would feel considerably different (and require different biomechanics) than normal running since he is only accelerating while his feet are touching the ground, whereas one is always accelerating (and and down) while running in gravity.

So while one probably wouldn't run using Dave's technique, one would still be able to run in some fashion.

Not only is it possible; it's been done - on Skylab.

[YOUTUBE]http://www.youtube.com/watch?v=S_p7LiyOUx0[/YOUTUBE]

The cylinder in this clip isn't rotating, but the astronaut provides his own angular momentum by running - the cylinder need not rotate (although if he keeps running, Skylab will rotate the other way in accordance with the law that every action causes an equal and opposite reaction. The space station has a lot more inertia than the astronaut, so the reaction is only a very slow rotation of the station, and angular momentum is conserved, so when he stops rotating, he can only do so by halting the spin he imparted to Skylab).

 

[Bomb#20]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Half the earth's escape velocity at what distance from the earth? Escape velocity is a radius-dependent phenomenon.

If you mean half of 25,000 mph, escape velocity at the surface, then probably so. I can think of some ways it might be done. A space elevator dangling from the moon down to low earth orbit should do the trick. If you mean half of escape velocity at the craft's actual distance from the earth at each point on its trajectory, then no, it's impossible regardless of the path. When it's sitting still, landed on the moon, it's already going faster than half of the earth's escape velocity at that distance.

 

[humbleman]

Here is another one. I recently watched Space Odyssey. In a scene Dave is running inside the rotating cylinder. If he gets both feet off the surface what happens? I'd think he is in zero g with momentum from the rotation when he was in contact. Is it possible to run as depicted in the movie?

If he jumps up his momentum carries him forward and at some point he hits the surface.

That makes sense.

If Dave jumps "straight up", he will be carried on a vector that is the sum of a tangent to the cylinder and a vector that lies on the diameter of the cylinder. This vector intersects the cylinder somewhere "ahead" of Dave. The higher he jumps, the greater the arc he traverses.

With each step, Dave is propelling himself along a tiny chord. It would feel considerably different (and require different biomechanics) than normal running since he is only accelerating while his feet are touching the ground, whereas one is always accelerating (and and down) while running in gravity.

So while one probably wouldn't run using Dave's technique, one would still be able to run in some fashion.

Not only is it possible; it's been done - on Skylab.

[YOUTUBE]http://www.youtube.com/watch?v=S_p7LiyOUx0[/YOUTUBE]

The cylinder in this clip isn't rotating, but the astronaut provides his own angular momentum by running - the cylinder need not rotate (although if he keeps running, Skylab will rotate the other way in accordance with the law that every action causes an equal and opposite reaction. The space station has a lot more inertia than the astronaut, so the reaction is only a very slow rotation of the station, and angular momentum is conserved, so when he stops rotating, he can only do so by halting the spin he imparted to Skylab).

In Space Odyssey movie the micro-gravity inside the spaceships is ignored, as well it is with a different gravity when walking on the Moon

 

[fast]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Half the earth's escape velocity at what distance from the earth? Escape velocity is a radius-dependent phenomenon.

If you mean half of 25,000 mph, escape velocity at the surface, then probably so. I can think of some ways it might be done. A space elevator dangling from the moon down to low earth orbit should do the trick. If you mean half of escape velocity at the craft's actual distance from the earth at each point on its trajectory, then no, it's impossible regardless of the path. When it's sitting still, landed on the moon, it's already going faster than half of the earth's escape velocity at that distance.

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

 

[bilby]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Half the earth's escape velocity at what distance from the earth? Escape velocity is a radius-dependent phenomenon.

If you mean half of 25,000 mph, escape velocity at the surface, then probably so. I can think of some ways it might be done. A space elevator dangling from the moon down to low earth orbit should do the trick. If you mean half of escape velocity at the craft's actual distance from the earth at each point on its trajectory, then no, it's impossible regardless of the path. When it's sitting still, landed on the moon, it's already going faster than half of the earth's escape velocity at that distance.

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

You are going to have some serious problems with your landing if you never exceed 1,000 mph relative to the Earth, because the moon is moving slightly more than twice as fast as that (approx 2,288 mph)

If I recall correctly, your idea formed the basis of the Lunar exploration program of the Duchy of Grand Fenwick back in the 1960s, and is detailed in the documentary movie about that program.

 

[Elixir]

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

You are going to have some serious problems with your landing if you never exceed 1,000 mph relative to the Earth, because the moon is moving slightly more than twice as fast as that (approx 2,288 mph)

All those speeds are relative to a point on earth's surface, which is a silly idea from the start. The point on the earth's surface, unless near one of the poles, will provide variable relative "speeds" as the earth rotates and the point in question recedes on the eastern horizon at 1700 mph (if on the equator) then approaches at 1700 mph 12 hours later...

 

[bilby]

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

You are going to have some serious problems with your landing if you never exceed 1,000 mph relative to the Earth, because the moon is moving slightly more than twice as fast as that (approx 2,288 mph)

All those speeds are relative to a point on earth's surface, which is a silly idea from the start. The point on the earth's surface, unless near one of the poles, will provide variable relative "speeds" as the earth rotates and the point in question recedes on the eastern horizon at 1700 mph (if on the equator) then approaches at 1700 mph 12 hours later...

Actually they are relative to the Earth's centre of mass.

As you point out, locations on the Earth's surface are moving at various speeds relative to that benchmark, so you can use this to get a head start - that's why the actual moon shots were launched in an easterly direction, from a point as close to the equator as practical, and with a nice expanse of unpopulated ocean to crash into immediately to the east. Cape Canaveral wasn't chosen as the launch site solely because the NASA executives wanted an excuse to be close to Disneyworld.

 

[Loren Pechtel]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Not with current technology. Lets take the minimum approach to doing this:

Accelerate to 12,500 mph, then continue to boost sufficient to counter gravity to maintain this velocity until you are 1,641 miles up. Cut your engines and do the lunar part of the mission Apollo style. Upon return you must light your booster again when you're 1,641 miles up but you can cut it as you enter the atmosphere, drag will suffice at that point.

I don't know the equations to figure those nasty burns near Earth. I could brute force it but there's a simple back-of-the-envelope calculation that's good enough. In that fall from 1,641 miles to the atmosphere the Apollo craft picked up about 12,500 mph. Thus at a minimum our booster must be able to supply another 25,000 mph. This is approximately what the Saturn V could do--and note that the payload pushed by the Saturn V was only 1% of the total launch weight. Thus we need a rocket 100x as big as the Saturn V to launch this. Square-cube law is going to be very nasty at this point, I don't believe it could be built. (Note that the actual answer is considerably worse--because you're going slower gravity has more time to act, you need even more fuel than this. Given the exponential nature of the rocket equation even a small change in velocity at this point costs vast amounts of fuel.)

 

[Loren Pechtel]

Here is another one. I recently watched Space Odyssey. In a scene Dave is running inside the rotating cylinder. If he gets both feet off the surface what happens? I'd think he is in zero g with momentum from the rotation when he was in contact. Is it possible to run as depicted in the movie?

If he jumps up his momentum carries him forward and at some point he hits the surface.

That makes sense.

If Dave jumps "straight up", he will be carried on a vector that is the sum of a tangent to the cylinder and a vector that lies on the diameter of the cylinder. This vector intersects the cylinder somewhere "ahead" of Dave. The higher he jumps, the greater the arc he traverses.

With each step, Dave is propelling himself along a tiny chord. It would feel considerably different (and require different biomechanics) than normal running since he is only accelerating while his feet are touching the ground, whereas one is always accelerating (and and down) while running in gravity.

So while one probably wouldn't run using Dave's technique, one would still be able to run in some fashion.

No--you don't feel acceleration due to gravity. When both feet leave the ground you're in free fall. It doesn't matter if you're in a space station or sitting on Earth.

What we typically think of as feeling the acceleration of gravity is actually the opposite--we feel the resistance to that acceleration. When you stand there you feel gravity--but you're not moving, there's no acceleration. If you're on the vomit comet you accelerate but you don't feel it--you float around.

 

[Wiploc]

Square-cube law is going to be very nasty at this point, I don't believe it could be built.

In-flight fueling, perhaps. The question isn't about practicality or safety, just possibility.

 

[Loren Pechtel]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Half the earth's escape velocity at what distance from the earth? Escape velocity is a radius-dependent phenomenon.

If you mean half of 25,000 mph, escape velocity at the surface, then probably so. I can think of some ways it might be done. A space elevator dangling from the moon down to low earth orbit should do the trick.

A space elevator extending outside the Hill sphere? Sounds like a recipe for trouble!

If you mean half of escape velocity at the craft's actual distance from the earth at each point on its trajectory, then no, it's impossible regardless of the path. When it's sitting still, landed on the moon, it's already going faster than half of the earth's escape velocity at that distance.

Ouch, missed that little detail. I was thinking only of the traditional 25,000 mph answer.

 

[fast]

The point of my dumb question was to weed out my possible misinterpretation of escape velocity. Given a sufficiently advanced technology to overcome the volume and weight of fuel, there seems to be no reason why we couldn't eventually stray far from earth without ever reaching such speeds.

For instance, with the right propulsion system capable of enormous light weight regenerating power, we should be able to travel towards the sky and take a break and just hover after every mile if we chose. We could travel at 20MPH and eventuallly escape the gravitational pull of earth. It might take a long while, but the point is that escape velocity has a ring to it that seems to suggest 25,000 mph or so is necessary to go to the moon. Maybe to get into a very slow decaying orbit perhaps.

 

[bilby]

The point of my dumb question was to weed out my possible misinterpretation of escape velocity. Given a sufficiently advanced technology to overcome the volume and weight of fuel, there seems to be no reason why we couldn't eventually stray far from earth without ever reaching such speeds.

For instance, with the right propulsion system capable of enormous light weight regenerating power, we should be able to travel towards the sky and take a break and just hover after every mile if we chose. We could travel at 20MPH and eventuallly escape the gravitational pull of earth. It might take a long while, but the point is that escape velocity has a ring to it that seems to suggest 25,000 mph or so is necessary to go to the moon. Maybe to get into a very slow decaying orbit perhaps.

The problem with going to space isn't that it's far away - It isn't.

There's no reason at all why you can't get the short distance to space at a constant low speed.

But if you want to STAY there, you need to go fast.

https://what-if.xkcd.com/58/

Space is about 100 kilometers away. That's far away—I wouldn't want to climb a ladder to get there—but it isn't that far away. If you're in Sacramento, Seattle, Canberra, Kolkata, Hyderabad, Phnom Penh, Cairo, Beijing, central Japan, central Sri Lanka, or Portland, space is closer than the sea.

Getting to space is easy. It's not, like, something you could do in your car, but it's not a huge challenge. You could get a person to space with a small sounding rocket the size of a telephone pole. The X-15 aircraft reached space just by going fast and then steering up.

But getting to space is easy. The problem is staying there

 

[fast]

The point of my dumb question was to weed out my possible misinterpretation of escape velocity. Given a sufficiently advanced technology to overcome the volume and weight of fuel, there seems to be no reason why we couldn't eventually stray far from earth without ever reaching such speeds.

For instance, with the right propulsion system capable of enormous light weight regenerating power, we should be able to travel towards the sky and take a break and just hover after every mile if we chose. We could travel at 20MPH and eventuallly escape the gravitational pull of earth. It might take a long while, but the point is that escape velocity has a ring to it that seems to suggest 25,000 mph or so is necessary to go to the moon. Maybe to get into a very slow decaying orbit perhaps.

The problem with going to space isn't that it's far away - It isn't.

There's no reason at all why you can't get the short distance to space at a constant low speed.

But if you want to STAY there, you need to go fast.

https://what-if.xkcd.com/58/

Space is about 100 kilometers away. That's far away—I wouldn't want to climb a ladder to get there—but it isn't that far away. If you're in Sacramento, Seattle, Canberra, Kolkata, Hyderabad, Phnom Penh, Cairo, Beijing, central Japan, central Sri Lanka, or Portland, space is closer than the sea.

Getting to space is easy. It's not, like, something you could do in your car, but it's not a huge challenge. You could get a person to space with a small sounding rocket the size of a telephone pole. The X-15 aircraft reached space just by going fast and then steering up.

But getting to space is easy. The problem is staying there

So, escape velocity is the orbital speed necessary to escape the pull of gravity so that we can stay in space and not be pulled back to earth. With unlimited thrust capability and no concerns for fuel, however, reaching an orbital trajectory with sufficient speed to maintain a safe stay is unnecessary.

We just need to constantly produce thrust to maintain a position. Reaching orbit is just the practical route to take given our technological limitations.

 

[bilby]

The problem with going to space isn't that it's far away - It isn't.

There's no reason at all why you can't get the short distance to space at a constant low speed.

But if you want to STAY there, you need to go fast.

https://what-if.xkcd.com/58/

So, escape velocity is the orbital speed necessary to escape the pull of gravity so that we can stay in space and not be pulled back to earth. With unlimited thrust capability and no concerns for fuel, however, reaching an orbital trajectory with sufficient speed to maintain a safe stay is unnecessary.

We just need to constantly produce thrust to maintain a position. Reaching orbit is just the practical route to take given our technological limitations.

Pretty much.

Escape velocity is the velocity at which you must go if you don't want to spend fuel, but you do want to stay up. Lower velocities mean falling back to Earth (unless you expend energy to prevent that happening).

 

[beero1000]

The problem with going to space isn't that it's far away - It isn't.

There's no reason at all why you can't get the short distance to space at a constant low speed.

But if you want to STAY there, you need to go fast.

https://what-if.xkcd.com/58/

So, escape velocity is the orbital speed necessary to escape the pull of gravity so that we can stay in space and not be pulled back to earth. With unlimited thrust capability and no concerns for fuel, however, reaching an orbital trajectory with sufficient speed to maintain a safe stay is unnecessary.

We just need to constantly produce thrust to maintain a position. Reaching orbit is just the practical route to take given our technological limitations.

Yes, the important thing is that you overcome the gravitational potential energy. It doesn't matter if you do it with one burst of kinetic energy (escape velocity) or with a small continuous source of power. In theory, you could climb a ladder to the moon, assuming you could find one that reached and was strong enough.

 

[Jokodo]

The problem with going to space isn't that it's far away - It isn't.

There's no reason at all why you can't get the short distance to space at a constant low speed.

But if you want to STAY there, you need to go fast.

https://what-if.xkcd.com/58/

So, escape velocity is the orbital speed necessary to escape the pull of gravity so that we can stay in space and not be pulled back to earth. With unlimited thrust capability and no concerns for fuel, however, reaching an orbital trajectory with sufficient speed to maintain a safe stay is unnecessary.

We just need to constantly produce thrust to maintain a position. Reaching orbit is just the practical route to take given our technological limitations.

Yes, the important thing is that you overcome the gravitational potential energy. It doesn't matter if you do it with one burst of kinetic energy (escape velocity) or with a small continuous source of power. In theory, you could climb a ladder to the moon, assuming you could find one that reached and was strong enough.

Good luck with safely landing on the moon after your jump at the relative velocity a ladder stationary relative to the surface of the earth would give you, though!

 

[beero1000]

Yes, the important thing is that you overcome the gravitational potential energy. It doesn't matter if you do it with one burst of kinetic energy (escape velocity) or with a small continuous source of power. In theory, you could climb a ladder to the moon, assuming you could find one that reached and was strong enough.

Good luck with safely landing on the moon after your jump at the relative velocity a ladder stationary relative to the surface of the earth would give you, though!

Eh, if they've made the 385,000km climb, I'm willing to extend the benefit of the doubt on their ability to tuck and roll.

 

[bigfield]

No--you don't feel acceleration due to gravity. When both feet leave the ground you're in free fall. It doesn't matter if you're in a space station or sitting on Earth.

Free fall means that you are accelerating due to gravity. As soon as your feet leave the ground on Earth, your entire body accelerates towards the ground at 9.8ms^-2, due to gravity.

Everyone in a spaceship is also in free fall. The astronauts are accelerating due to the gravity of the Earth, and Dave in Discovery One is accelerating due to the gravity of the sun, and to an infinitesimal degree, the gravity of the planets. However, these astronauts experience no acceleration relative to the vessels they inhabit, which are in free fall as well.

 

[steve_bank]

Some may confuse orbital velocity with gravitational escape.

If you launch straight up it does not matter how fast you go, as long as thrust is greater than gravity, which diminishes with distance. At some point Earth's gravity is negligible and you have escaped.

Achieving orbit and staying in orbit balistically requires a velocity,. I expect a kinetic energy 0.5mv^2 of less than the gravitational energy at the desired orbit. Which I assume is why orbits decay over time. In orbit you are always falling back to Earth.