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Objects in orbit are weightless?

  1. Jul 4, 2011 #1
    Hello PF.

    I learned, for some time ago, that objects in space in orbit are weightless and to this day it still blows my mind.

    Its rumored that Issac Newton sat under the apple tree and in the same view he saw that apple drop from the tree to the ground he also noticed the moon up in space and he asks him self; "Why is the apple falling when the moon is not?"

    The same gravitational force would be operating on them both. But the moon is falling.

    As far as I have learned if we take an example like an cannonball. If we shoot it from a hill with a little speed it will go not very far before it hits the ground. Shoot the cannonball a little faster it goes further before it hits the ground. So there must be a certain speed where you can launch the cannonball so it stays in orbit and never hits the ground.

    The cannonball is in free fall towards earth and therefore weightless. The amount that the cannonball is falling towards earth is the amount that earth have curved away from the cannonball so it stays in orbit and never hits the ground. Am I correct?

    Now back to my first question.

    I have done a little experiment with a bottle of water. I cut small holes from bottom to the top so the water will come out. If you take that bottle and drop it the instant it leaves your hand it is in free fall and therefore weightless which means the water does not slip out of the hole until it hits the ground.

    Is the object in space for instance the earth and the moon in free fall and therefore weightless? After the research I've done and the experiment all evidence show that its true but it still blows my mind. So I would be very glad if you can explain to me why that is and if I am wrong please also say.

  2. jcsd
  3. Jul 5, 2011 #2
    Yes , your conclusion that free fall=weightless is definitely correct. One has to carefully distinguish between two quantity namely: the mass and weight of an object. Mass is an inherent property of an object and is an invariant in classical mechanics. Now weight on the other hand is associated with the normal reaction a body provides when in contact with some other body. e.g. your weight on Earth is equal to the your reaction your body provides against the surface of the Earth. In absence on any other body to act upon, i.e. in free fall the idea of weight loses its importance.
    Another interesting idea is that even mass of an object depends upon the presence of all other objects in the universe, known as Mach's principle.
    Hope this helps :)
  4. Jul 5, 2011 #3
    Very helpful indeed. I have taken the difference between mass and weight for granted. I just looked it up after reading your reply and I got a bigger understanding.

    Mach's principle sounds very interesting I will check it out later.

    And just to make sure I understand mass - weight.

    Mass is an expression for the amount of matter in a physical body. Its a inherent quantity which means it is permanent contra weight which is, as you pointed out, equal to your reaction your body provides against the surface of the Earth. So as soon I loose contact with the Earth surface for x period of time I will be weightless until I make contact to the surface again but my mass will be the same as an inherent quantity?

    Thank you.

    Last edited: Jul 5, 2011
  5. Jul 5, 2011 #4
    Just to clarify this a little further we can define the weight of an object as being a measure of the pull of gravity on that object.Whereas the mass of an object remains constant when it is moved from one place to another,its weight can vary,for example it becomes heavier when moved to a place where the gravitational force is stronger.
    Objects in free fall,for example,astronauts in orbit will experience not "true weighlessness"(it is their weights that keep them in orbit) but "apparent weightlessness".They accelerate towards the earth at the same rate as their spacecraft and experience zero reaction forces this giving the illusion of weightlessness.
  6. Jul 5, 2011 #5


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    Staff: Mentor

    Concise answer: we don't feel our weight because gravity is pulling us down, we feel it because the earth is pushing us up.
  7. Jul 5, 2011 #6

    D H

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    This is but one of several definitions of 'weight'.

    Whether the weightlessness of astronauts in orbit is an "illusion" depends on what one means by the term 'weight'. Calling weight as measured by a scale "apparent weight" and further denigrating this by calling what astronauts feel on orbit as "the illusion of weightlessness" is, well, denigrating.

    I could use denigrating terms to describe what you are calling 'weight'. For example,
    • Tautological weight, because it is tautologically defined as the product of mass and acceleration due to gravity,
    • Fictional weight, because gravity is a fictitious force in general relativity,
    • Unmeasurable weight, because there is no way to measure that quantity.

    It's best to avoid those inflaming adjectives, replacing them with value-neutral ones. So, what you call weight is better called "gravitational weight" while what you call "apparent weight" is better called "scale weight".

    Regarding that the feeling of weightlessness is an illusion: The equivalence principle says otherwise.
  8. Jul 5, 2011 #7


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    Staff: Mentor

    I guess you meant PHYSICAL body?
  9. Jul 5, 2011 #8
    DH,from whateverworks opening post I judged his present level of knowledge on this particular topic to be between GCSE and S level(UK exam system) and I tried to pitch,what I hoped to be helpful comments,at the right level.I feel that your comments here have been unhelpful.
    Firstly the definition of weight that I used is one that's acceptable for students up to A level standard in the UK and after a search I found (what I expected to be the case) that it is a definition in wide use including by certain mentors and advisors on PF.I even searched the NIST guide to the SI (4:ISO 80000-4) where,in summary,it is stated "and thus the weight of a body is generally the local force of gravity on the body in a vacuum".
    Secondly,after searching,I found several sources backing up the S level /A level summary of apparent weightlessness I described.See,for example: Hyperphysics..."Feeling Weightless When You Go Over The Hump.
    Thirdly you advised me that I should not use the term apparent weight impying that this was denigrating and yet my little search confirmed what I already knew which is that "apparent weight" is an accepted term and one in common usage including by yourself.Just search these forums to see.
  10. Jul 7, 2011 #9
    It would be very helpful if you can confirm if that is correct. It's sounds true after the research I've done.
  11. Jul 7, 2011 #10


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    It isn't realistic, but more or less correct.
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