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Homework Help: Second Law Pairs, Third Law Pairs, or Neither?

  1. Sep 27, 2010 #1
    1. The problem statement, all variables and given/known data
    All of the force pairs described below are equal in magnitude and opposite in direction. Which of the pairs are Newton's third-law pairs, which are second-law pairs, and which are neither?
    1. A baseball pushes on a window breaking it; the window pushes back on the baseball
    2. A team of dogs pulls a sled, moving it; the sled pulls backward on the dogs.
    3. A jet's engines thrust it forward; air friction pushes it back.
    4. The earth attracts a stone; the stone attracts the earth.
    5. A car exerts a forward force on a trailer; the trailer tugs backward on the car.
    6. Gravity tugs downward on a box sitting on the ground; the ground pushes up on the box.

    2. Relevant equations
    Newton's Second and Third Laws

    3. The attempt at a solution
    1. 3rd Law Pair because the action force is acting on the window, while the reaction force is acting on the ball.
    2. 3rd Law Pair because the action force is on the sled, reaction force is on the dogs.
    3. Neither because the forces are acting on one object (which eliminates 3rd law pair) and the object is still moving, so it can't be second law.
    4. 3rd Law Pair because the earth attracts the stone (action force) and the stone attracts the earth (reaction force).
    5. 3rd Law Pair because the action force acts on the trailer and the reaction force acts on the car.
    6. 2nd Law Pair because the forces are related to one object, and come from different sources.

    For which number(s) is my logic wrong? Am I close for most?
  2. jcsd
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