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Friction on the ramp vs in-water?

  1. Jun 10, 2008 #1
    Hi, I'm new to this forum.

    I'm currently doing a project on the coefficients of different materials used in sports so as to study the effects they have in their respective sports.

    Because we lack the equipment and knowledge to calculate drag, i'd like to ask whether a material having high friction on a ramp would equate to proportionately high friction in water/air?

    I base this on the fact that friction includes surface roughness, and since water and air can get into these small 'pockets' in the material, it would somehow create drag?

    Can anyone please verify this? And it would help a great load if there is any references as i need to answer to the skeptical judges. :biggrin:
     
  2. jcsd
  3. Jun 10, 2008 #2

    mgb_phys

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    Sorry I'm not quite clear on your question.
    You are asking if there are materials that will have high friction between two surfaces when they are dry but lower friction when they are wet and why?

    The simplest picture is a sandwich of surface-water-water-surface.
    One layer of water sticks to each of the surfaces and remains fixed, moving with it. The sliding motion is then between the two layers of water (or oil) which since liquids have low shear strength doesn't take much energy.
     
  4. Jun 10, 2008 #3
    Nono, i'm asking, if something have high friction on the sliding ramp, would it also mean that it has high friction in water?

    Eg. sandpaper on the ramp would be high friction, but does that automatically mean the friction is quite high in water?
     
  5. Jun 10, 2008 #4
    No. You're really talking two entirely different phenomena (although technically related). The amount of friction a block experiences when sliding down a ramp is more or less unrelated to how much 'drag' it would experience in water. The drag is mostly a function of its shape (as opposed to its 'roughness'). As for air drag vs. water drag I'm sure at your level it's a perfectly acceptable comparison (although technically water is 'stickier' then air I believe). You could perhaps put a bat in a water tank and push it with a constant measureable force and observed it's acceleration and compare that to the calculated acceleration (as a function of its mass and the force) in a vaccuum to get an idea of the relative drag between bat shapes. (how aerodynamic they are)
     
  6. Jun 10, 2008 #5
    In fact, for a very rough estimate you could simply drop different bats in water and see how long it takes them to hit the bottom compared to 9.81. However the problem with this is that the bats are not going to fall as they would be swung (i.e. they're going to try and orient vertically as the sink). Oh and also certain bats might actually float which would kinda ruin this.
     
  7. Jun 10, 2008 #6
    simple!!

    surface friction only depends on surfaces in contact.

    Drag is the sum of pressure drag and form drag(viscous drag), and both change with the relative velocity between the fluid and velocity. The total drag is sum of these two. Whereas viscous drag drops with velocity, the pressure drag increases with velocity. SO at a certain point, the total drag is minimum. there are lots of ways of minimizing both type of drags. For example, a rough surface always doesn't mean more drag, it can reduce pressure drag.

    So a body having a frictional coefficient on a surface may have smaller, same or larger friction(drag) in water
     
  8. Jun 10, 2008 #7
    After a tedious amount of research, i seem to have stumbled across something called 'skin friction' which seems to support what i have explained.

    Eg. "Skin friction drag is caused by the actual contact of the air particles against the surface of the aircraft. This is the same as the friction between any two objects or substances. Because skin friction drag is an interaction between a solid (the airplane surface) and a gas (the air), the magnitude of skin friction drag depends on the properties of both the solid and the gas. For the solid airplane, skin fiction drag can be reduced, and airspeed can be increased somewhat, by keeping an aircraft's surface highly polished and clean."

    Reference: http://www.centennialofflight.gov/essay/Theories_of_Flight/drag/TH4.htm
    http://www.allstar.fiu.edu/aero/drag3.htm


    What do you guys think? Does the surface of a material really have an effect on the drag it experiences?
     
  9. Jun 10, 2008 #8
    Well we've already gone over this. Yes it does (as I said with the water vs. air that water is a bit 'stickier') but 99.999% of the design of a bat goes into weight vs. aerodynamic considerations (it's shape) and the effect that the material has on its frictions vs. air is extraordinarily negligble compared to the importance of its shape (in things like jet engines this is less so). So by far and away the number one factor will be shape.
     
  10. Jun 10, 2008 #9
    I assume you're thinking that doing experiments to measure the friction of a material for different materials will be much easier to do then actually comparing drag for your project and that's your prerogative (and probably correct) but to actually determine the effectiveness of various sports equipment it's drag you want to know about and skin friction is a very very very minor consideration in comparison. Especially since the choice of which material that a bit of sports equipment is going to be made of is entirely a factor of the materials weight vs. its properties under stress and not at all due to its skin friction.
     
  11. Jun 12, 2008 #10
    Point noted. Thank you very much for your replies, it was clear and much informative. :)
     
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