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Action-Reaction Forces (Simple Problems)

  1. Feb 1, 2007 #1
    1. The problem statement, all variables and given/known data

    I promise - my last set of questions.

    1. A hockey stick hits a puck on the ice. Identify an action-reaction pair, and compare the forces exerted by each object.

    a. The stick exerts a force on the puck; the puck exerts a force on the stick.
    b. The stick exerts a force on the puck; the puck exerts a force on the ice.
    c. The puck exerts a force on the stick; the stick exerts a force on the ice.
    d. The stick exerts a force on the ice; the ice exerts a force on the puck.

    2. A leaf falls from a tree and lands on the sidewalk. Identify an action-reaction pair, and compare the forces exerted by each object.

    a. The tree exerts a force on the leaf; the sidewalk exerts a force on the leaf.
    b. The leaf exerts a force on the sidewalk; the sidewalk exerts a force on the leaf.
    c. The leaf exerts a force on the tree; the sidewalk exerts a force on the leaf.
    d. The leaf exerts a force on the sidewalk; the tree exerts a force on the leaf.

    3. As a basketball player starts to jump for a rebound, the player begins to move upward faster and faster until his shoes leave the floor. During the time that the player is in contact with the floor, the force of the floor on the shoes is:

    a. greater than the player's weight.
    b. equal in magnitude and opposite in direction to the player's weight.
    c. less than the player's weight.
    d. zero

    2. Relevant equations

    None

    3. The attempt at a solution

    As you can see, I have trouble understanding these type of problems.

    1. Would it be B, for isn't it like a chain reaction?
    2. Would it be c, because the leaf will fall, and the sidewalk's force will make it stay for the time.
    3. I think it would be either c or d, because if it was greater than the player's weight, he wouldn't jump, right?

    I may be completely off, but I really need help with this! Please. :eek:
     
  2. jcsd
  3. Feb 1, 2007 #2
    Does no one know how to do these? I really need to know how soon! Please kindly help me.
     
  4. Feb 1, 2007 #3

    PhanthomJay

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    You're way off. For action-reaction pairs, Newton's 3rd law states that if an object A exerts a force on object B, then object B must exert a force on object A, that is equal in magnitude, but opposite in direction. For your last question, if the jumper moves faster and faster during the time he is in contact with the floor, he must be accelearting during that period. What does Newton 2 tell you about or bodies that are accelerating??
     
  5. Feb 1, 2007 #4
    So would it be greater than the player's weight, as it is inversely proportional?? I think I have this all mixed up.
     
  6. Feb 1, 2007 #5

    PhanthomJay

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    Well, it will be greater, on account of newton 2: there must be a net force acting on the jumper in order for her to accelerate in the direction of the net force. Since her weight always acts down , the floor must push up with a force greater than her weight, in order for there to be a net force acting up to accelearte her off the floor.
     
  7. Feb 1, 2007 #6
    I see. Can you help me with the other ones please?
     
  8. Feb 1, 2007 #7

    PhanthomJay

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    Gee, I thought i did. If a stick exerts a force on a puck, the puck exerts an equal force on the stick, in the opposite direction. These are force pairs per newton 3. Sure there are forces of the ice on the puck, or the stick on the ice, but the problem is not asking about these. It asks for the action-reaction pairs to be chosen from the given choices.
     
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