The 1950 science fiction movie "Rocketship X-M" has its share of science mistakes, but this
one caught my interest...

As the ship in the movie is under acceleration towards the Moon they begin to experience "zero-gravity" in that some objects (those that can be manipulated with wires - a jacket, harmonica, etc) begin to float while nothing else is affected. The leader of the expedition (who also designed the rocket) explains that "gravity falls off gradually as we get farther away from the Earth" which doesn't explain the selectivity. Anyway,

Let's imagine a similar space ship on the way to the Moon, that after attaining Earth orbit, accelerates to trans-lunar travel and is now traveling at a constant velocity, say 25,000 mph and is 50,000 miles from Earth. How would objects behave gravity-wise under those conditions? Is this just a straightforward gravity calculation?

Except for when the spaceship is firing its rockets, it is free fall the whole time and the crew and ship are completely weightless. The weight you feel comes from *resisting* gravity, not the gravity itself. When you're in free fall, you feel nothing (neglecting tidal forces which are miniscule here).

The spaceship would be under the influence of the gravity of both earth and moon and that free-fall trajectory is a straightfoward calculation. Well, actually calculating in what direction to fire so you end up at the same place and time the Moon will be, and with low enough relative velocity to enable you to insert yourself in orbit is a pretty involved calculation, but it's straightfoward.

During the Apollo trips to the Moon, the only time the astronauts felt weight (inertial forces) was when they were firing their rockets. That force was pretty large during the ride to orbit on the big Saturn V, but the 3rd stage TLI burn and the SM burn to put them in lunar orbit was pretty mild by comparison.

-Richard

The idea that gravity gradually diminishes as you fall away from the Earth is an old, and incorrect, idea. It was used in When Worlds Collide (1934 or so), and in countless space operas before and after. I don't know when it was first stated, or by whom, nor when the correct situation, as described by publius, was worked out.

Fred

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Fred,

The realization that one "feels no force of gravity" goes back to Einstein at least, and I'd imagine it was known before then. This is the basis for the Equivalence Principle actually (Einstein supposedly quipped that he saw a guy fall off the roof and get up with no injury, remarking that he felt no force of gravity -- I don't know for sure if he really said that or if it's just legend) which led to GR and its elegant view of this.

So, it was known to physics well before it was known in wider circles. Even today, it's a source of confusion for the general public.

-Richard

Jules Verne wrote that weightlessness occurred at the point in the lunar voyage where the Earth's and Moon's gravity canceled out, in From the Earth to the Moon, 1867. I don't know if he was first.

NASA: JULES VERNE SPACE BOOK GALLERY

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My dear Publius.

Thank you for your answer. I suppose it is only my complete intellectual density which compels me to ask you to clarify it.

Note, please, perhaps by rereading my question carefully, that I did not, and am not asking about the case of a space ship in orbit.

Note that I am not asking about a space ship under acceleration, hence the clever use of the term "constant velocity".

Also note that I am not asking about the trajectory of anything.

I am posing an artificial situation. Imagine a space ship which took off from the surface of the Earth, in Florida, we'll say, and going straight up. never going into orbit, attaining escape velocity and then turning OFF its engines. It is now 50,000 miles above the Earth, moving at a CONSTANT VELOCITY.

What would be the behavior of objects in the space craft then, and why?

Constant velocity? The stuff in the spacecraft would be drawn to earth with somewhat less than the gravitational force on Earth's surface. You'd need some kind of thruster to prevent the spacecraft from accelerating towards earth too though.

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G'day Calculon

In the situation you describe, once the engines are switched off, all objects in the spacecraft become weightless, and will float around unless restrained. The reason they do so is that they're all travelling (accelerating) at the same speed.

However, you appear to have a couple of misconceptions, though they don't overly affect the answer to your question.

1. A constant velocity in the situation you describe is not possible. As soon as the engines are switched off, the spacecraft will begin to slow down with reference to the Earth, because of the Earth's gravity. Therefore, its speed will reduce from 25,000 mph. However, as all objects in the spacecraft are equally affected by the Earth's gravity, there is no different effect on the spacecraft and the objects within it. Therefore, everything will still float around unless restrained.

2. A constant velocity can only be maintained by an object in a perfectly circular orbit. As long as no engines get switched on, the effect is exactly the same as for the spacecraft travelling to the Moon.

The situation you describe is pretty much what happened on the Apollo missions to the Moon. The Saturn V rocket's engines fired to put the Apollo spacecraft into orbit around the Earth, and fired again to send the spacecraft to the Moon. During each time the rocket's engines fired, the crew and all objects in the spacecraft experienced gravity due to the accleration. But while they were oribiting the Earth, and while they were travelling to the Moon, they were weightless. And referring back to point 1 above, when the Apollo spacecraft first left Earth orbit, it was travelling at about 25,000 mph. But after a couple of days, the pull of the Earth's gravity had slowed it down to only about 5,000 mph. Nevertheless, throughout this time the astronauts were weightless.

I hope this helps, but if you have any further questions, please ask us.

I'm pretty sure Newton understood it, although I didn't find anything on the internet to support that. I'll jsut keep looking

Newton made made a law of universal gravitation. One of the things that the law states is that the force of gravity between two objects fades away as they drift apart.

F=Gmm/r^2

Well, to be somewhat pedantic, a constant velocity is possible, you simply must use your engines to maintain it. Also, a circular orbit is not a constant velocity, as velocity is a vector, and even though a circular orbit has a constant speed, the direction vector is constantly changing.

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without firing thrusters, constant velocity of a rocket can be maintained at a location between the earth and the moon, where the force exerted by the moon on the rocket is equal to the force exerted by the earth on the rocket

Throttle on, they fall to the rear of whatever compartment they're in, just as they would if the rocket were standing tall on the launch pad.

Throttle off, they and the astronauts would float around, unless they were secured in place (which is what happens most of the time.

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Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.

It's a very simple answer. Engines OFF: things float around/you feel weightless. Engines ON: the objects move toward the back of the ship (from where the engines are firing). That's really all you have to know. It really doesn't matter where you are.

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First, it won't be moving a constant velocity - it will be slowing down, it simply will never slow down to a complete stop. Second, it is in orbit, it simply is in an extremely elongated open orbit with an eccentricity well over 1.

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