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Third-law Argument

  1. Nov 1, 2006 #1
    ok, got into an argument with a kid in my physics class about this one...

    Which of the following are third-law partners?

    a. A thrust force from its propeller pulls a plane forward; a drag force pushes it backward.
    b. A car exerts a forward force on a trailer; the trailer tugs backward on the car.
    c. A motorboat propeller pushes backward on the water; the water pushes forward on the propeller.
    d. Gravity pulls down on a person sitting in a chair; the chair pushes back up on the person.

    I just said d would be...he was saying b,c,and d...but i'm thinking there is a third force acting on both number b and c which will not make them equal and opposite in magnitudes.
  2. jcsd
  3. Nov 1, 2006 #2


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    Neither you nor your friend are correct. Two are third law partners, and two are not. I'm afriad your choice is not one of them. See if you can figure out why d is not.
  4. Nov 1, 2006 #3
    hmm well if D isn't then gravity is only acting on one object then which is the chair. I didn't think it was A because the thrust force is going to overcome the drag force which will not make it equal in magnitude...I'm stuck between B and C then.
  5. Nov 1, 2006 #4


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    If A exerts a force on B, then B exerts an equal but opposite force on A. Seems like b and c fit that bill exactly. Now what say you about d?
  6. Nov 1, 2006 #5


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    a and d share a common problem that excudes them. b and c do not have this problem.
  7. Nov 1, 2006 #6
    For D, the 3rd Law pair of gravity on the chair is actually the gravitational force that is exerted by the chair on the Earth. Gravity acts both ways, as with any other force.
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