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Homework Help: Pulling an object [please help ]

  1. Apr 18, 2007 #1
    Physics Of Water Skiing

    This is the problem my group was given-

    What is the minimum speed needed to pull an object attached to a rope over a water surface so that is does not sink. Investigate the relevant parameters experimentally and theoretically.

    If anyone has any ideas, of how to approach this problem, and what things need to be investigate, please post a reply. It would be greatly appreciated :)
  2. jcsd
  3. Apr 18, 2007 #2


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    Welcome to the Forums,

    Look up Archimedes principle, Bernoulli's Principle and Turbulent flow. That should keep you busy for a while :wink:
  4. Apr 19, 2007 #3
    Thanks for your help, we started looking them up today.
    Any ideas on different methods to pull an object through water at a constant velocity?
  5. Apr 19, 2007 #4
    Pulling an object [please help urgent]

    Any ideas on a method to pull an object through water at a constant velocity?

    Please remember the object needs to be pulled at a constant velocity, not at a constant acceleration.

    We thought about using this method:
    1. the object in water, is attached to a pulley with a weight on the end of it
    2. the weight is then dropped off a table ledge, causing the object to be pulled through the water (as it is attached to the pulley)

    But this method only pulls the object at a constant acceleration!

    If you could tell us how to pull an object through water at a constant velocity, it would be greatly appreciated :)
  6. Apr 19, 2007 #5


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    Wind the string onto a drum driven by an electric motor that turns at a constant rpm
  7. Apr 19, 2007 #6
    Personally I think your group needs to get to a nearby lake and do some water skiing.
  8. Apr 20, 2007 #7


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    You might find such mechanisms at air fields. They are used to launch gliders.
  9. Apr 20, 2007 #8
    well..i am weak in fluid mechanics

    but i guess the water applies a "FORCE" of friction on the object moving on its surface
    how can u pull out with constant velocity...without applying a constant acceleration against the force of friction?
  10. Apr 20, 2007 #9
    Is there a way to generate lift here? Without getting into all the complexities of turbulence, drag of various types, etc, if the object can generate an additional buoyancy force while moving thru the water by displacing fluid in a downward direction, like aircraft wings do, this would have to offset whatever non-buoyant mass is present. Thats my limited take on the question.
  11. Apr 20, 2007 #10


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    You could determine the minimum speed by observing the tension in the rope as the speed at which it is pulled increases. So you do not really need to investigate it at a constant speed.
  12. Apr 20, 2007 #11
    It seems quite simple to me, but I ain't really sure.
    So, get things like pulley, rope, different objects, and a box or an aquarium, or just a lake or pond if you have it nearby.
    So, record all the possible information about the first object<its mass, material, and shape>, name it 1.
    Start from 0 velocity and keep on increasing until it finally floats.
    Try same thing for different objects. <be sure about using similar objects like a pair that has same mass, but different shapes, or a pair that has same shapes and different masses, but same material>
    And record all the observations.
    I think you would discover out what's making the objects floating on the water, and how minimum velocity is related to the shape, mass, or material.

    For providing velocities, you can use a toy boat, or motor, or anything like that, or a hanging mass from the pulley

    Hoping this would help!
  13. Apr 20, 2007 #12
    Thank you everyone for all your help. It's really appreciated. We are taking all your ideas on board and now trying out some of your suggested experiments. To pcdagr8, we thought about your comment, and you are right, so thanks for pointing that out. Thanks to rootx, andrevdh, denverdoc, and hootenanny also! We can't thank you enough.

    Does anyone know, what the fundamental aspects are that apply to this problem. We have already started investigating Bernoulli's principle, Archimedes principle, turbulent flow, drag viscousity, surface tension and the surfing of physics...all to various degrees of success...

    Any additional suggestions on what to investigate for this topic would be greatly appreciated :)

    Thanks again
  14. Apr 20, 2007 #13
    Lift! You gotta drag a boat anchor fast to keep it skimming.
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