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Homework Help: Newtons 3rd law+Horse Cart System+Action/Reactonhelp needed

  1. Feb 9, 2006 #1
    Newtons 3rd law+Horse Cart System+Action/Reacton..help needed ASAP!!!!!!!

    im a sophmore in the ib program and i'm in conceptual physics

    i have a test tommorow on newtons third law and all the stuff that goes along with it.. so i need an answer on this pronto!

    first of all.. if you are standing still.. is the considered an action/reaction force? i thought it was.. because the earth exerts a force up on you and you exert a force down on the earth..? but for some reason, my friend said it wasn't.. and i dont know..?

    then i have a question on the horse-cart system.. idk if you know of this but its an example found in alot of textbooks.. and it shows a horse attatched to a cart and the horse says 'I cannot accelerate this cart because the forces are equal and opposite of me and the cart and will cancel to zero'.. well i know thats not true.. but why.. and i also need to know..
    a)what makes the cart accelerate
    b)what makes the horse accelerate
    c)what makes the horse-cart system accelerate

    i need someone who KNOWS for SURE about this.. please do not give me a bogus answer and make me fail my test tommorow!
  2. jcsd
  3. Feb 9, 2006 #2
    well the standing still question is an action reaction pair and the other big question i think i know but, im not very good at describing stuff so sorry
  4. Feb 9, 2006 #3


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    Strictly speaking, there are two action/reaction couples in the standing still scenario:
    1. Gravity: Earth pulls you down, you pull the Earth up. Lots of motion here if it weren't for:
    2. Normal force couple: The ground resists your hurtling down to the Earth's C.M, pushing you up, while you push the ground down.

    Net result: both you and Earth at rest, with some stress contained in both of you.

    a) Cart accelerates due to the horse's action upon it.
    b) Horse accelerates due to the ground's action upon it.
    c) Horse+Cart accelerates due to the ground's action upon them.
    Last edited: Feb 9, 2006
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