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Homework Help: Newtons third law?

  1. Nov 23, 2005 #1
    Two blocks standing on oneanother are pulled with a force F. There's friction between them. With a certain force F the purple block is standing still on the green one (situation 1) because of static friction. Is the netto force working on the green block F+ static friction of the purple block (because Newtons third law implies the static friction on the purple block= the opposite static friction of the purple block on the green block)?

    http://img363.imageshack.us/img363/3448/situations9qp.gif [Broken]

    And in the second situation the static friction is overcome and replaced by kinetic friction. Is the netto force working on the green block now F- kinetic friction of the purple block (again thanks to Newtons third law)?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Nov 23, 2005 #2


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    Staff Emeritus
    Science Advisor

    When the purple block does not move - there is no work done by the purple on the green block (static situtation, whereas as work is a product of force and distance). However, the purple block adds its weight to the green block, and if friction is involved between the green block and supporting surface, that is where the weight of the purple block will be manifest, i.e the friction depends on the combined weights of the purple and green blocks.

    In the second situation, the Fk acts on the purple block accelerating it to the right, but it acts oppositely on the green block, opposing the motion of the green block. Again, friction between the green block and its supporting surface would still involve the combined weight of the purple and green block.
  4. Nov 23, 2005 #3

    Doc Al

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    Staff: Mentor

    I don't understand how you've drawn your arrows representing the friction force between the blocks. Since the friction arrows are on the top block, I assume you mean to show the friction acting on that top block. If so, those friction arrows should point to the right in both cases.

    And, from Newton's 3rd law, you realize that the top and bottom blocks will exert equal and opposite friction forces on each other. Thus the friction that the top block exerts on the bottom block will be to the left in both cases.
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