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Homework Help: A Question About Force

  1. May 20, 2005 #1
    I have been having problems with this question, I can't figure out at all how to do this, does anyone know?

    "Draw a sketch of yourself standing on a beach carrying a bag (perhaps containing some diving equipment) in your right hand.

    Identify the reaction force for each of the forces described below and show (by drawing arrows) the action and reaction pairs on the sketch that you made. You can draw separate sketches for clarity. Remember that action and reaction always act on different bodies! So don't forget to show the other body in the sketch.

    1. Your weight force
    2. The force exerted by the sand on you
    3. The force that your hand exerts on the bag
    4. The force that the bag exerts on the Earth "
  2. jcsd
  3. May 20, 2005 #2

    Chi Meson

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    Science Advisor
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    This is an exercise in Newton's third law. When ever you are finding the "reaction force" the first step is to state (say it aloud, or write it down) the "action force."

    Such as for the first one: Your weight is "the earth pulling on you."
    To get the reaction force, change the position of the nouns in the statement.

    It's as simple as that (and the arrow points inthe opposite direction).
  4. May 20, 2005 #3

    Doc Al

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    Staff: Mentor

    Just to add to Chi Meson's advice, recall that Newton's 3rd law tells us that all forces come in pairs. If A exerts a force on B, then B exerts an equal and opposite force on A. (If you call one force the "action", then the other force is the "reaction".) The trick is being able to identify the actors A and B.
    Last edited: May 20, 2005
  5. Jun 4, 2005 #4
    That is a very interesting problem to the beginning students of Physics concerning to the 3rd Newton´s Law.

    I think the solution is as follows:

    1. Your weight force:action by the earth on me.
    reaction:part of the force exerted by the sand on me (part of the #2 below)

    2. The force exerted by the sand on you:reaction of #1 force + #4 force

    3. The force that your hand exerts on the bag:action from me on the bag
    reaction:the downward force exerced by the bag on my hand

    4. The force that the bag exerts on the Earth:action that is sumed up to the reaction of the #1 force
    reaction:the force that the Earth exerts on the bag
    Last edited: Jun 4, 2005
  6. Jun 5, 2005 #5

    Doc Al

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    Staff: Mentor

    Note that the problem is merely to identify the action/reaction force pairs involved.

    With that in mind:

    You have correctly identified the actors for the action (the earth pulling you down), but neglect them in describing the reaction. The reaction is simply: the gravitational force of you pulling the earth up. (The force of the sand on you is not part of this action/reaction pair.)

    Again, just state the reaction force: what exerts a force on what?


    Right! The bag exerts a gravitational pull up on the earth; and the earth exerts a gravitational pull down on the bag.
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