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What are the equal-and-opposite forces of friction forces?

  1. Sep 29, 2015 #1
    1. The problem statement, all variables and given/known data

    Yl6iOSw.png

    A block rests on a table, and a second block, connected to the first with a rope, hangs over the side of the table. See the illustration. (Assume the pulley is massless and frictionless.) All objects pictured are stationary.

    Identify every force involved in the pictured situation.

    2. Relevant equations

    Newton II and III.

    3. The attempt at a solution

    None of block 1, block 2, the table, or the earth are accelerating, so Newton II tells us the net force on each of the 4 objects must be 0. I'll identify forces in opposite pairs a la Newton III:
    • Gravitational force of earth exerted on block 1
    • Gravitational force of block 1 exerted on earth
    • Gravitational force of earth exerted on block 2
    • Gravitational force of block 2 exerted on earth
    • Gravitational force of earth exerted on table
    • Gravitational force of table exerted on earth
    • Normal force of table exerted on block 1
    • Normal force of block 1 exerted on table
    • Normal force of earth exerted on table
    • Normal force of table exerted on earth
    • Tension of rope exerted on block 1 due to block 2
    • Tension of rope exerted on block 2 due to block 1
    • Force of friction exerted on block 1 due to table
    • ????? Force of friction exerted on table due to block 1 ?????
    • Force of friction exerted on table due to earth
    • ????? Force of friction exerted on earth due to table ?????
    As you might have guessed from all the question marks, I'm confused about the opposite pairs to the friction forces. Obviously I need to include them for the table and the earth to be in equilibrium, but I don't really know what they mean.

    When I say, "The table exerts a force of friction on block 1," what I mean is that the block would be moving laterally across the table, except that the friction force of the table's surface opposes that motion.

    But if I try to form the analogous statement, it does not seem to me to be the case that the earth would be moving laterally beneath the table, except that the friction force of the table's feet opposes that motion. What gives?
     
  2. jcsd
  3. Sep 29, 2015 #2

    Student100

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    Gold Member

    Have you drawn your force diagrams for each object?
     
  4. Sep 29, 2015 #3

    jbriggs444

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    Friction is a force that opposes the relative motion between two surfaces that are in contact. It does not matter which direction the surfaces are moving or would be moving relative to some particular rest frame. What matters is which direction the surfaces are moving or would be moving relative to each other.
     
  5. Sep 29, 2015 #4
    Cool. I think that clears up my confusion.
     
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