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# A question of weightlessness

## Question

Suppose you are an astronaut.You are fired from the Earth's surface(assume Earth to have no atmosphere) from a cannon towards the moon in a cylindrical projectile(shape won't make much difference in our case).The projectile has no rockets or any other propulsion system.So basically the initial impulse from the cannon is driving you towards the moon.Assume the cannon force to be enough to carry you away so that you don't fall back onto the Earth.

Now,you are in space and the only forces(significant forces) acting on the rocket are the gravitational pulls of the Earth and the moon.The question is will you have any weight in the rocket?

Note-weight here refers to the normal reaction one receives from the floor.

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I would say No since weight "refers to the normal reaction one receives from the floor." The floor being the cylindrical projectile you are housed in. The floor would be moved (pulled) by gravity at the same rate as myself, thus movement from gravitational pull would be equal between myself and the floor. I therefore, would not have a reaction from the floor unless another force is applied to me individually from the floor, such as jumping off the ceiling towards the floor.

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I agree. We have to differentiate between the weight (non-contact force due to gravitational attraction) and the feeling of weightlessness (contact force from normal reaction). So, while the person will have a weight he will feel weightless similar to when you have a free fall in an amusement park ride.

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If the question refers to gravitational force pulling you toward an "floor" of the rocket, the answer is no. You will still have mass which, by Einsteins famous equation E=MC^2, will actually be slightly greater than it was before the launch. Furthermore, when you reach the Moon (or Sun should the shot miss) you will have weight once again. If you make it to the moon in one piece, you will have a weight of approximately 1/6 of Earth weight.

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i tend to agree, you'll be wieghtless the moment you stop accelerating and your velocity is enough to place you in orbit around the earth. if you're traveling at twice the rate the moon is orbiting around the earth, you'd be wieghtless at roughly r/2^2 where r is the distance between the earth and moon.

Edited by phillip1882
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The actual weight of a person is determined by his mass and the acceleration of gravity (Weight = Mass x Acceleration of Gravity.) One's "perceived weight" or "effective weight" comes from the fact that one is supported by floor, chair, etc. If all support is removed suddenly (as per your scenario) and the person begins to fall freely, the person feels suddenly "weightless". Weightlessness refers to a state of being in free fall in which there is no perceived support. Hence, under your definition of weight, the answer is no.

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Technically, since you are moving away from the Earth and toward the Moon, you will have perceived weight toward the Earth until you are at a point where the Moon's gravity overcomes that of Earth's. The reason why an astronaut feels weightless while in low Earth orbit (LEO) is due to the constant motion of the spacecraft. If the shuttle or ISS were to slow down such that they are 'motionless' over a point on the Earth while maintaining the same LEO, then the astronauts would experience their weight toward the Earth. If Gravity were so weak that you wouldn't feel it while moving directly away from the Earth then the Moon would not still be in orbit. The key here is that you are moving directly away from the Earth and not experiencing free-fall due to orbital motion.

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Now, weight is mass with gravity, yes? And gravity is equivalent to acceleration. Now, since you're probably experiencing acceleration as you get closer to the Moon, I would think, wouldn't that mean you technically had weight, though very little? Not sure on the physics, but I think that'd be true..

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if your rotating around the earth though at the same or higher velocity than that needed to achieve orbit, i belive you are wieghtless. that is, the effect gravity exerts on you is counter-balanced by your fall around the earth.

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Well done!

No is the right answer.As the astronaut and the spacecraft have the same acceleration/deceleration,the astronaut will be weightless.

This question came to my mind when I was reading Jules Verne's From The Earth to The Moon in which he said that the astronauts will have weight(as in under my definition) .He realized the same acceleration thing but still believed the astronauts to have weight I don't know why.

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Well done!

No is the right answer.As the astronaut and the spacecraft have the same acceleration/deceleration,the astronaut will be weightless.

This question came to my mind when I was reading Jules Verne's From The Earth to The Moon in which he said that the astronauts will have weight(as in under my definition) .He realized the same acceleration thing but still believed the astronauts to have weight I don't know why.

Jules Verne also did not calculate the G-force necessary while in the cannon barrel. The equired escape velocity is about 25000 mph and the rocket must achieve it before leaving the cannon. To simplyfy the calculation, if we assume that the barrel is long enough to require 1 second duration in the barrel and acceleration is uniform, this would require an acceleration of approximately 4,100,000 times the force of gravity. Not only would this ruin ones internal organs, it is doubtful that a hollow rocket could be built to withstand such force.
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Well done!

No is the right answer.As the astronaut and the spacecraft have the same acceleration/deceleration,the astronaut will be weightless.

This question came to my mind when I was reading Jules Verne's From The Earth to The Moon in which he said that the astronauts will have weight(as in under my definition) .He realized the same acceleration thing but still believed the astronauts to have weight I don't know why.

I don't believe you correctly worded your original post as you specifically pointed out that there were only two forces currently acting on the capsule: the gravitational pulls of the Earth and the Moon. Also, you indicated that the capsule would be launched such that it would make it to the Moon and not fall back onto the Earth. There would be only one point at which the astronaut would feel weightless and that point is the exact moment that the Moon's gravity exactly cancels out Earth's gravity. Now, this moment might last a decent amount of time depending on how close to zero the velocity of the capsule is as it approaches this point. Up to this point, the astronaut would experience Earth's gravity and its dampening as the capsule nears the Lagrangian point. Once the capsule passes through this point, the astronaut will begin to experience the Moon's gravity strengthening as it pulls the astronaut toward what could have before been described as the ceiling.

The astronaut's body begins at rest and is pushed toward the Moon by the 'floor' of the capsule; this continues until the capsule's velocity is reduced to zero or gravity from another celestial body overcomes the force of the capsule's movement. Since there is always (except in a Lagrangian point) an external gravitational force and the capsule is decelerating smoothly, the astronaut would neither be flung to the opposite side of the capsule nor feel weightless.

As an exercise of this principle, go and jump on a trampoline. You are now the capsule and the contents of your stomach are the astronaut. When you propel yourself into the air your stomach's contents are pressed against the bottom of your stomach until you reach the apex of your flight. At this point, you are not technically at a Lagrangian point, but you experience the same sort of cancellation of gravity, albeit much quicker than the capsule in the original post. The contents of your stomach are no longer forced against the bottom of your stomach, but they also are not propelled to the top of your stomach. If you were to grab onto a trapeze at the exact apex of your flight, then your stomach's contents would stay at the bottom and would not experience weightlessness, this only truly happens when you begin free-fall.

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I don't believe you correctly worded your original post as you specifically pointed out that there were only two forces currently acting on the capsule: the gravitational pulls of the Earth and the Moon. Also, you indicated that the capsule would be launched such that it would make it to the Moon and not fall back onto the Earth. There would be only one point at which the astronaut would feel weightless and that point is the exact moment that the Moon's gravity exactly cancels out Earth's gravity. Now, this moment might last a decent amount of time depending on how close to zero the velocity of the capsule is as it approaches this point. Up to this point, the astronaut would experience Earth's gravity and its dampening as the capsule nears the Lagrangian point. Once the capsule passes through this point, the astronaut will begin to experience the Moon's gravity strengthening as it pulls the astronaut toward what could have before been described as the ceiling.

The astronaut's body begins at rest and is pushed toward the Moon by the 'floor' of the capsule; this continues until the capsule's velocity is reduced to zero or gravity from another celestial body overcomes the force of the capsule's movement. Since there is always (except in a Lagrangian point) an external gravitational force and the capsule is decelerating smoothly, the astronaut would neither be flung to the opposite side of the capsule nor feel weightless.

As an exercise of this principle, go and jump on a trampoline. You are now the capsule and the contents of your stomach are the astronaut. When you propel yourself into the air your stomach's contents are pressed against the bottom of your stomach until you reach the apex of your flight. At this point, you are not technically at a Lagrangian point, but you experience the same sort of cancellation of gravity, albeit much quicker than the capsule in the original post. The contents of your stomach are no longer forced against the bottom of your stomach, but they also are not propelled to the top of your stomach. If you were to grab onto a trapeze at the exact apex of your flight, then your stomach's contents would stay at the bottom and would not experience weightlessness, this only truly happens when you begin free-fall.

If you were correct, then those videos we have from the space station must be faked for the space station is being held in orbit by the earths gravitational pull and yet we see that the astronauts are floating weightless in the station. Your trampoline example is also false for your stomach contents for it is not gravity that keeps stomach contents at the bottom of your stomach, being weightless after intial accelaration, only stay at the bottom of the stomach because there is no force to move them in another direction in realtion to your body.
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If you were correct, then those videos we have from the space station must be faked for the space station is being held in orbit by the earths gravitational pull and yet we see that the astronauts are floating weightless in the station.

The astronauts in the ISS feel weightless because they are in a constant state of free-fall. They are actually falling around the Earth while travelling around 17,000 miles per hour. This is quite different than travelling directly away from the Earth as in the original post.

Your trampoline example is also false for your stomach contents for it is not gravity that keeps stomach contents at the bottom of your stomach, being weightless after intial accelaration, only stay at the bottom of the stomach because there is no force to move them in another direction in realtion to your body.

I'm having difficulty parsing your argument here, but I'll try to provide some clarification. When you are standing still, the contents of your stomach are 'at rest' as your body is supporting the weight of the contents against Earth's gravity. Once you accelerate upward, those same contents will resist the upward thrust and this will be felt as if the contents suddenly became heavier. Once you begin decelerating, the contents do so as well since there is no longer a stronger force moving them upward while gravity continues to pull them downward; this also means the contents feel lighter than before. Once you reach zero velocity, your stomach's contents rapidly approach a velocity of zero as they lag only a fraction of a second (possibly more depending on bodily structure) behind. However, they will not remain this way and will also not 'slam' into the top of your stomach (depending on fullness). Once you begin free-fall, the contents will fall at the same rate as you are and will therefore not work their way to the top of your stomach, up the esophagus and out of your mouth. Otherwise, you'd see many sky-divers puking all over the place.

I have personally experienced extended periods of weightlessness during a once-in-a-lifetime ride on NASA's KC-135A Weightless Wonder. When their maneuver is properly executed, you experience 2.5 times Earth's gravity during ascent followed by a rapid decrease in apparent gravity as you top the parabola (sometimes into the negative when they nose-down too quick), then approximately 30 seconds of weightlessness as the plane falls from 36,000' down to about 25,000' followed this time by a rapid increase in apparent gravity as you prepare for the ascent again.

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I don't believe you correctly worded your original post as you specifically pointed out that there were only two forces currently acting on the capsule: the gravitational pulls of the Earth and the Moon. Also, you indicated that the capsule would be launched such that it would make it to the Moon and not fall back onto the Earth. There would be only one point at which the astronaut would feel weightless and that point is the exact moment that the Moon's gravity exactly cancels out Earth's gravity. Now, this moment might last a decent amount of time depending on how close to zero the velocity of the capsule is as it approaches this point. Up to this point, the astronaut would experience Earth's gravity and its dampening as the capsule nears the Lagrangian point. Once the capsule passes through this point, the astronaut will begin to experience the Moon's gravity strengthening as it pulls the astronaut toward what could have before been described as the ceiling.

The astronaut's body begins at rest and is pushed toward the Moon by the 'floor' of the capsule; this continues until the capsule's velocity is reduced to zero or gravity from another celestial body overcomes the force of the capsule's movement. Since there is

always (except in a Lagrangian point) an external gravitational force and the capsule is decelerating smoothly, the astronaut would neither be flung to the opposite side of the capsule nor feel weightless.

As an exercise of this principle, go and jump on a trampoline. You are now the capsule and the contents of your stomach are the astronaut. When you propel yourself into the air your stomach's contents are pressed against the bottom of your stomach until you reach the apex of your flight. At this point, you are not technically at a Lagrangian point, but you experience the same sort of cancellation of gravity, albeit much quicker than the capsule in the original post. The contents of your stomach are no longer forced against the bottom of your stomach, but they also are not propelled to the top of your stomach. If you were to grab onto a trapeze at the exact apex of your flight, then your stomach's contents would stay at the bottom and would not experience weightlessness, this only truly happens when you begin free-fall.

You are absolutely right about the two forces and the weightlessness at the point where the Earth's and Moon's gravitational forces cancel.However, I had defined what I meant by weight in this case in my original post and according to that definition, astronauts will be weightless.

Now for the trampoline example.

You will feel the food pushing down on your stomach but if the jump would have lasted for a sufficient amount of time for the food and the stomach to attain the same velocity, food would have become weightless.

In LEO's the spacecrafts fall towards the Earth however due to their forward motion they keep missing the Earth.Now since the spacecraft and astronauts fall at same rates,astronauts are weightless(again weightless means weightlessness as defined in original post).

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Hi!

Good Question!

referring to the laws of physics!

i'll come up with an advanced version of this problem!

so wait for it!

Regards,

Sina

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