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Concept question on External force

  1. Sep 29, 2013 #1
    Describe a situation in which the net external force is not zero, but its speed remains constant.

    I don't know if this situation is possible to explain.
     
  2. jcsd
  3. Sep 29, 2013 #2

    Doc Al

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    What does Newton's 1st law say?
     
  4. Sep 29, 2013 #3
    An object stays at rest, or if in motion, stays in motion at a constant velocity unless acted on by a net external force.
     
  5. Sep 29, 2013 #4

    Doc Al

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    Good. Note the key word velocity. How does that differ from speed?
     
  6. Sep 29, 2013 #5
    Velocity has both magnitude and direction but speed doesn't have direction
     
  7. Sep 29, 2013 #6

    Doc Al

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    Exactly! So see if you can try to answer the question now.

    Give an example of something with constant speed but not constant velocity.
     
    Last edited: Sep 29, 2013
  8. Sep 29, 2013 #7
    A car going in a circle.
     
  9. Sep 29, 2013 #8

    rcgldr

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    How about a car following any curved path while moving at constant speed? A section of road could be similar to a sine wave, ellipse, spiral, parabola, ..., and a car would be able to follow that path while moving at constant speed.
     
  10. Sep 29, 2013 #9

    arildno

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    Your answer is definitely correct! :smile:
    But, as the next post shows, other similar situations also exist.
     
  11. Sep 29, 2013 #10
    Ok so what is the net external force acting on the car?
     
  12. Sep 29, 2013 #11

    arildno

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    Do you remember the expression for centripetal acceleration?
     
  13. Sep 29, 2013 #12
    Yes. So centripetal force that pulls it toward the center is external?
     
  14. Sep 29, 2013 #13

    arildno

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    Certainly.
     
  15. Sep 29, 2013 #14

    rcgldr

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    That would be speed^2 / (radius of curvature), which gets complicated for paths other than a circle.

    The simple answer is that centripetal force = centripetal acceleration x mass of the car.
     
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