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# A question of weightlessness

15 replies to this topic

### #1 GoldenPheonix

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Posted 06 January 2012 - 02:51 PM

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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### #2 Gen. Mike

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Posted 06 January 2012 - 04:35 PM

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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### #3 Peekay

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Posted 06 January 2012 - 04:51 PM

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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### #4 thoughtfulfellow

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Posted 06 January 2012 - 05:07 PM

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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### #5 phillip1882

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Posted 06 January 2012 - 05:35 PM

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, 06 January 2012 - 05:37 PM.

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### #6 jpf

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Posted 06 January 2012 - 05:36 PM

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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Posted 06 January 2012 - 07:10 PM

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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### #8 Moki

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Posted 06 January 2012 - 07:10 PM

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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### #9 phillip1882

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Posted 06 January 2012 - 08:32 PM

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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### #10 GoldenPheonix

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Posted 07 January 2012 - 06:24 AM

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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