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Magnet question

  1. Mar 16, 2009 #1
    I have a question concerning magnets.

    Lets say i have 2 magnets each wieghing 1 gram each sitting on a scale. The scale reads 2 grams. What if i was able to stack the magnets by lining up the same poles so they repel each other and the magnet on top was floating over the magnet on the bottom. Would the scale still read 2 grams or 1 gram since the other magnet is floating over the one thats still sitting on the scale?

    Thx :)

    BTW i know this is a noob question so thanks for bearing with me. :)
    Last edited: Mar 16, 2009
  2. jcsd
  3. Mar 16, 2009 #2
    It would still read 2 grams.

    The top magnet is floating because the bottom magnet is pushing up on it with a force equal and opposite it's weight. For every action, there is and equal and opposite reaction, so the bottom magnet is pushed down by the floating magnet.

    The bottom magnet is pushed into the scale with it's own weight (1g) combined with the top magnet's weight (1g).

    Good question! Does my answer make sense?
  4. Mar 16, 2009 #3
    Not a bad question at all. Sounds like something that would be fun to try..,

    *Goes in search of magnets and scales*
  5. Mar 16, 2009 #4
    Yes! Thank you for the reply. I thought it would be 2 grams and it kinda stinks because i had an idea if it the answer was one gram. Its still good to know however. If anyone else has anything to add id love to hear it. :)
  6. Mar 16, 2009 #5
    What was your idea? It's okay if you just want to keep it to yourself, but if you don't care about it tell us, it sounds like it might be interesting.
  7. Mar 16, 2009 #6
    If the answer was one gram, then why not make the floating magnet weigh 500 tons? The you could lift 500 tons just by lifting a gram.

    The lesson is that we can often check our answers in physics by looking at extreme cases.
  8. Mar 16, 2009 #7
    That really wasnt the question. If i use your analogy then i would be asking if the scale would wiegh 100k pounds and one gram.

    The idea i had had to do with friction to the ground. Basically if an object was suspended in space, even if it is a huge massive object, it would almost be effortless to move. That is all i will say of my idea.

    *sighs back to the drawing board* lol
  9. Mar 16, 2009 #8
    You actually have a very good idea here. You will still have an equal and opposite reaction, ie the force on the ground, but if you have stationary magnets and attach repelling magnets above, you would be correct--it would only take force to accelerate the object. Look at bullet trains. Many are now using magnetic propulsion--the magnets alternate positive/negative down the track, attracting and repulsing the train (several dozen tons) above.
  10. Mar 18, 2009 #9
    What good idea? I don't see it.

    Maglev technology is well known and established. However, as pointed-out, it requires a continuing pathway sub-structure physically attached to the ground.

    It is my hope that the OP does not assume that this can be done without that.

    That is, a simple wheeled rolling cart with a magnet on the cart and repulsing a magnetic mass above it WILL NOT move easier than a cart containing the whole mass without magnets.
  11. Mar 18, 2009 #10
    Yes im fully aware now thank you for the reply. I have another question and i didnt want to do another post so here it goes.

    Magnets are rated in strength by a value (pull force). Some of them are have a pull force of 550 lbs. what exactly does that mean? Im assuming it means that the magnet could suspend 550 lbs vertically?

    Also ive looked around to find out if the repel force and pull force would be equal on a given magnet(ex. 550 pull=550 repel) and ive gotten differing answers. Maybe one of you know?

    Finally lets say i have 2 magnets each with 225 lbs pull force. Would those values add? In other words would it be the same as having a single magnet that has 550 lbs pull force?

    Im not trying to reinvent the wheel here lol. But i plan on buying some magnets off the net and id rather know what im getting myself into before i order. :)

    Again thank you all for your posts!
  12. Mar 19, 2009 #11

    A magnet rated at 100/lbs "pull force" will hold in contact a 100/lb iron plate, and it will take approx. 100/lbs of force to "pull them apart"
    If the two were an inch apart, for example, the value would be less, and if 1-foot apart practically non-existent with respect to suspending the iron plate or pulling them apart.

    The repulsing force is quite similar, though involves a different arrangement.
    In any event, the "repulsive" force could be considered similar to that of a metal spring. A spring rated at 550/lbs of compression will compress approx. to the point where all the coils are touching each other. Not exactly touching necessarily, but you get the point.

    As such, 2 magnets in a repulsive arrangement has a maximum compressive load approx. equal to the load as if the two repulsive magnets were forced to come together, touching each other.
    In other words, if you have 2 NIB magnets rated at 100/lbs pull force, and you orient them such to repel, it would take approx. 200/lbs of pressure to cause a face(N/N or S/S) contact.

    I hope this makes any sense, as well as hoping I am correct.
  13. Mar 19, 2009 #12

    great post!!!!! i will go off of this thank you.:)

    i have another question and it kinda relates to this post..... well kinda.

    atmospheric pressure is caused by the wieght of the air in a column above us.(correct?) anyway when a plane or anything that flys through the air is suspended in the air does the atmospheric pressure go up due to the plane or whatever object is suspended in the air? even if the amount is almost immeasurable? or am i thinking of this incorrectly? thx :)
  14. Mar 20, 2009 #13
    In a sense you are most certainly correct.
    Any object introduced into a pressurized environment will change that local environment.
    To what extent it does change might be considered negligible or substantial, depending on many factors.

    Perhaps someone else more educated could respond on the specifics.
  15. Mar 25, 2009 #14


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    Just draw a force diagram and you will see what the scale will show
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