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Finding Drag(for water)

  1. Correct me if I'm wrong but to find drag for water you have to use this equation:
    0.5 * velocity^2 * water density which is 1000kg/m^3 * Drag Coefficient * Cross Sectional Area
    Velocity is supposed to be in m/s right? not km/h?


    Okay so for example if there was an imaginary planet with no air and atmosphere, but only a large body of water(ocean). If a 1*1*1 meter cube was falling and it hit the ocean at 200m/s, the drag will be:
    0.5 * 100m/s * 1000kg/m^3 * about 1 * 1m^2 = 50000
    when it hits the water

    If the cube is made out of really strong light weight material, wouldn't the cube get flung up? For example if the cube is only a gram. Is this even possible? When I think about it in my head, no matter how hard an object hits the surface of water, it would never bounce that much...

    So anyways, i feel like I am misunderstanding something... Can someone help me?
     
  2. jcsd
  3. mfb

    Staff: Mentor

    The units do not matter, as long as you keep them consistent. If different quantities are given in different units, you'll have to convert some of them.

    Don't forget units.

    It would decelerate extremely fast and then come to a rest (approximately) - unless the cube itself is elastic, then it might bounce.
     
  4. But if the force applied to the cube is 50000N wouldn't the cube be flung up because the mass is really small? So since force is m * a, if a cube is 1kg, that means that the acceleration will be 50000m/s^2. Since that is way larger than gravity which is 9.8m/s^2, wouldn't it make the cube fly up in the air? Or am I not calculating this correctly?
     
  5. HallsofIvy

    HallsofIvy 40,417
    Staff Emeritus
    Science Advisor

    Force in what direction? You were asking about drag. That will be directed opposite to the velocity. In order to have a force upward to "fling" the cube upward the velocity must be downward. But as in order to go "upward" the velocity would have to change from downward to upward so must, at some point, be 0. As soon as that is true, there is no longer any drag force.
     
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  6. Chestermiller

    Staff: Mentor

    You are not thinking about this correctly. As soon as the cube starts to contact the water, it will decelerate very quickly, its velocity will drop rapidly, and the drag force will decrease very quickly. So the force on the cube won't stay anywhere close to 50000N for more than a minute interval of time.

    Chet
     
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