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Homework Help: Newton third law and finding the reverse distance

  1. Aug 8, 2010 #1
    hi am new here ..... and sorry for my weak English
    i have a question and a problem in physics that i need to know it's solution
    i will give you the main idea without numbers and you can assume any numbers
    that you find it suitable for the problem...

    the problem say: if a small ball is moving toward an air source that pump the air toward the ball .....the ball will not be affected by the power of the air source until it reach a certain point and then it will reverse it's direction ......
    i need to know the distance that the ball move backward based upon newton third law
  2. jcsd
  3. Aug 8, 2010 #2
    The ball will always be affected by the air source, it will move backward indefinitely as time approaches infinity. The only distance you could calculate precisely is the point at which it reverses direction.
  4. Aug 9, 2010 #3
    thanks for ur replay....look at the pic....if the ball is far enogh from the air source......and moveing toward it ....the ball will resist the power of the air source for a while or for a distance until the air source powr become stronge enoght to prevent the ball from keep going toward and make it reverse it's movment......then we have reaction....i need to know the distance that the ball will move in the opposite direction.

    Attached Files:

    Last edited: Aug 9, 2010
  5. Aug 9, 2010 #4
    first answer me something, if i push a ball on a surface (assumed to be ideal without friction), then can u determine the distance it will travel before it stops?
    u definitely cant, so is the answer for ur qs!! the ball travels backwards to infinity

    there might be some additional info that u might have missed in the question!
  6. Feb 9, 2011 #5
    sorry for late replay but my internet connection was disconnect

    could u plz show me the calculation ?
    or any other member could do that and thanks in advance

    thanks for u .....may be because am not good in physics so i didnt put the question and the case in good words.....but that is all informations that i have........
    Last edited: Feb 9, 2011
  7. Feb 9, 2011 #6
    There is no need for a calculation. You can deduce it from Newton's first if you want to. The air pump will decelerate the ball going to the right and then accelerate it to go to the left. As long as there is nothing to stop it, e.g. another external force it will continue to go forever.
    Note: In this problem friction plays a role. But the pressure from the air pump will keep the ball in motion.
  8. Feb 9, 2011 #7
    hi......thanks for ur reply betel
    could u plz give me a hypothetical example with a numbers
    am in a need for numerical example if u could help me
  9. Feb 9, 2011 #8
    If you understood what we said you should be able to find some numbers yourself.

    If you need explicit numbers for your homework we will not do them for you here. We can help you understand the physics behind it but we will not present you with the solution.
  10. Feb 9, 2011 #9
    :smile: :smile: :smile:
    hi and thanks.......it's not a homework it's just a practical case that am thinking about.....bay the way i finished study and forgut about homeworks since 2001, untill u remind me about that days :smile:

    if any member would like to help me i will appreciate
  11. Feb 9, 2011 #10
    So what do you actually want to know? If you want exact numbers you need to give an exact setup.
  12. Feb 9, 2011 #11
    my problem is that am a practical in thinking or in acting
    i dont like theories or just a word of description so i like a numerical example that help me and show the case in numbers
    that all
  13. Feb 9, 2011 #12
    For the question as you stated it in this setup the answer is infinity regardless of any other numbers.
  14. Feb 9, 2011 #13
    ok thanks betel.........
    i will think agine about my setup and try to change it and post agine
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