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What are some examples of Newton's third (3rd) law?

  1. Mar 21, 2012 #1
    If you want, could you give me at least 5 so that I can fully understand.
     
  2. jcsd
  3. Mar 21, 2012 #2
  4. Mar 21, 2012 #3
    http://en.wikipedia.org/wiki/Newton_third_law

    A ball hang by a string which attached to a ceiling.
    The ball pull the string down, the string pull the ball up.
    The string pull the ceiling down, the ceiling pull the string up
     
  5. Mar 21, 2012 #4
    Newtons third law states that if an object A applies a force on object B (called action)

    Object B applies an equal and oppsite force on object A (called reaction).

    REMEMBER: Action and Reaction forces always act on different object.

    IF ACTION ACTS ON B , then REACTION ACTS ON A.

    Eg 1

    The earth pulls you by force mg towards it, you pull the earth towards yourself with reaction force whose value is also mg.

    Eg 2:
    Cristiano Ronaldo applies a force on ball to kick it, the ball applies the same force on Ronaldo which is the reaction.

    Eg 3:

    When i pressed the key 3 on my keyboard with a force, the key pushed back on me with a reaction force.

    Eg 4: this is an imp one.

    a book rests on a table.
    The earth is pulling it down with force mg.
    However, the table is applying an opposing force F which balances mg to keep the book at rest.

    HOWEVER, THE FORCE F IS NOT THE REACTION FORCE TO mg.

    remember, action and reaction always act on different objects.

    Here both F and mg both act on A so they cant be action reaction pair.

    In fact, The reaction force of mg on book by earth, is actually acting on earth.

    The reaction of force F on book by table acts on table.

    In this particular case F=mg which is why the book is at rest, but they are not action reaction forces.


    Eg 5: Another imp one.

    While walking we apply a force on earth in backward direction with our feet.
    Its reaction acts on us and pushes us forward making us walk.


    Hope this helps.
     
    Last edited: Mar 21, 2012
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