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Mythbusters - "Motorcycle Water Ski" 6/5/13

Discussion in 'Now Playing - TV Show Talk' started by DancnDude, Jun 7, 2013.

  1. john4200

    john4200 Active Member

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    How do you know? Are you seriously telling me that if you submerged a heavier-than-water wheel by 1/3 of its diameter and then started it spinning quickly that you think the wheel would rise to the surface?

    I find that hard to believe, since, as I already mentioned in this thread, wheels are round, so the back pushes down even if the front pushes up.

    Note also that air can be considered to be a fluid, albeit with a lower viscosity than water. So, would you claim that a powered wheel can climb into the air? Heck, why build airplanes? Just put some windscoops onto the wheels of our cars and we can fly away! :D
     
  2. Church AV Guy

    Church AV Guy Active Member

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    high desert...
    I would have liked to have see a graph of the bike speed vs. distance traveled from the water's edge. My guess is that the bike starts to slow down immediately on contact with the water and continues to lose speed until it sinks. The distance it travels is mostly determined by the initial speed as it leaves the ground. I bet. no matter the initial speed or distance traveled, it sinks at the same speed each time.

    Yes, a powered front wheel would have been an assist, at least as long as the bike is moving forward.
     
  3. cannonz

    cannonz Active Member

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    Meant to record this, forgot won't be on again till sun morning.
     
  4. Idearat

    Idearat New Member

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    If it were a real experiment, then have Jamie do the run multiple times to get the distance, then see what happens if he squeezed the clutch as soon as he got to the point where the water is more than a foot deep. Then you could see if momentum at that point was the reason it stayed up or if it was really powering over the water.

    I'd also be interested in what the front tire was doing if we could zoom in with the high speed camera. (Already deleted it). It is still spinning at 60mph, or just skidding across the water? If a tire can paddle the water for propulsion then water zooming under a freewheeling tire should be spinning from the water.
     
  5. Jonathan_S

    Jonathan_S Active Member

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    Northern...
    I don't think it would climb if evenly submerged; but in an asymmetrical submergence, for example where it's raising a bow wave, or you hit a small wavelet in the water, I think the additional torque a powered wheel has over a free-spinning one helps it climb up, instead of dig into, to that front wave.

    That would help the vehicle maintain speed and keep it from digging in.


    Also, more powered wheels has another probably larger benefit (which I should have realized and brought up earlier). As you mentioned in an earlier post it's hard to get thrust with a wheel pushing on water; in effect it's a very low friction surface. However much power you've got there's an effective limit to how much thrust any single wheel can generate. Adding a 2nd powered wheel should roughly double your traction since you can transmit the engine's torque to the water through roughly twice the contact area. That should directly translate into better ability to maintain speed (although I guess if the engine was low enough power that one wheel could handle 100% thrust then adding a 2nd wouldn't help here)
     
  6. john4200

    john4200 Active Member

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    I'm still skeptical of the `wheel climbing the wave` explanation. I already explained the main problem I have with it, but your "asymmetrical submergence" theory can address my initial refutation, so here is another. If you have a wheel spinning in water, it kicks up a lot of water spray, as anyone who has ridden a fenderless bike through a puddle well knows. To be accurate, the wheel throws the water up and backwards, but my point is that there is a significant upwards component to the momentum of the thrown water. If the wheel throws water up, then there is a corresponding downward force on the wheel (basic action-reaction, conservation of momentum, the rocket effect, whatever you want to call it).

    So, even in the asymmetrical submergence case, the back of the wheel is clearly generating some negative lift. If the wheel has large knobs (fins, paddles, whatever you want to call them), then it can also generate positive lift in the front. But I find it hard to believe that any wheel would generate net lift (i.e., the lift in the front exceeds the negative lift generated at the back) in the water.

    But I do not think either of us can prove our case with a thought experiment. Perhaps we should write a letter to Mythbusters asking them to test it? They could start with a small-scale paddle wheel and see if they can generate net lift by spinning it fast enough.

    I'm with you here. As long as we avoid the "powered wheel climbing the water argument", I can accept that it is possible for a heavier-than-water vehicle to travel over the water indefinitely, provided it generates enough thrust with "paddle wheel(s)" to offset the drag of water resistance (plus air resistance) as it skims over the water. Then the explanation that the wheel "climbs" the water is not necessary -- it is just the wheel skimming the water due to the high speed of travel.
     
  7. MarkofT

    MarkofT ****

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    Firstly, look at the vectors. The upward component to horizontal component ratio is inversely related to the speed of the wheel. IE, a wheel moving along the surface at 100 MPH will produce a rooster tail that isn't that high but extends back quite a distance. But as the wheel slows to 10 MPH, the vertical component has overtaken the horizontal component an you get air sprayed almost straight up.

    The angle shown by the rooster tail is an indication of the angle of the wake behind the wheel, starting from the bottom of the wheel. You can see by the following image that the water at the rear of the wheel may not be touching water for nearly as long as the forward part of the wheel. This would result in greater upward lift then downward lift at higher speeds.

    [​IMG]

    The spin helps the skimming by climbing the water in front. It works similar to a flat ski shape. It won't work if it gets too deep, but it will keep the wheel up if it's already there.
     
  8. john4200

    john4200 Active Member

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    How do you know? The lift at the front could still be less than the negative lift at the rear. The forces at the front and the rear are created by different types of moving water. You cannot tell which force is greater just by looking at it.

    As I said, the way to settle the question is to test it. Maybe if Mythbusters gets a letter from each of us, they would test it.
     
  9. alpacaboy

    alpacaboy Active Member

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    Something disturbs me about seeing Buster referred to as "it."

    My reaction was something like, "How mean. Buster's a 'him'. A distinct part of the Mythbusters crew. He has feeli... no, wait. Oh yeah, I guess LoadStar's right."

    But it still bothers me.
     
  10. jeff92k7

    jeff92k7 Annoyed with trolls

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    on planet earth
    Keep in mind that with forward momentum, there will be a higher pressure area on the front of the spinning wheel and a lower pressure area on the back of the spinning wheel. Thus, the downward motion at the front of the powered, spinning wheel can create more traction and lift against the water than the drag created by the upward motion of the rear of the wheel.
     
  11. john4200

    john4200 Active Member

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    Except that you have not considered the additional downward force created by the wheel throwing some of the water upwards.

    I repeat, this question cannot be settled just by looking at it. The forces are too complicated to analyze by simple physics. The way to settle the question is to do the experiment.
     
  12. verdugan

    verdugan Active Member

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    Do you cry when you see what they do to "him" during the episodes?

    :D
     
  13. jeff92k7

    jeff92k7 Annoyed with trolls

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    on planet earth
    I did consider that and even mentioned that when I said "drag created by the upward motion of the rear of the wheel". Though, granted, I didn't use the specific words "throwing the water upwards". I would have thought that would have been clear by my statement, but I guess I have to spell it out for some people.

    I stand by my statement. The higher pressure in the front would provide greater traction than any drag caused in the low pressure area at the rear of the wheel INCLUDING any water being thrown upwards. The water being pushed downwards will far exceed the water being thrown upwards.

    Jeez, this is a touchy bunch here.

    As for doing the experiment, I thought they proved it pretty well by traveling around 300+ feet (yards?) across the water. If it was solely a "rock skipping" effect, it would have worn off early. It was clear (at least on my TV), that the only reason they couldn't continue was because of a loss of momentum/buoyancy of the front wheel. Once it "dug into" the water, they weren't going any farther. The front wheel stayed up due to it's rolling motion. Once it stopped/slowed, it went under. The powered rear wheel was still spinning and lifting the rear out of the water.

    Of course, discussions like this are why I don't hang out on the mythbusters forums. It's an entertaining show with a little bit of science thrown in. Once you start to over analyze the science, it loses all the entertainment value and we end up with people arguing about it on internet forums.

    I think I'll exit this thread now and go laugh at the people who are still shocked by the red wedding.
     
  14. Hank

    Hank AC•FTW

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    Welcome to TCF. But don't let that scare you off.
     
  15. john4200

    john4200 Active Member

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    Actually, I guess you need to not get the physics incorrect. Drag is not equal to the force required to throw water up. Talking about two different pressures as definitive in determining drag or force on the wheel is absurd when the pressures around the wheel vary greatly with position.

    Your hand-waving analysis would apply equally well to a paddlewheel in the air. So, do you also claim that if you spin a paddlewheel fast enough in the air that it will climb into the sky? Where are the paddlewheel airplanes?
     
  16. Kamakzie

    Kamakzie Are you serious bro?

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    I would've liked to see them have the stunt guy try to cross the whole lake instead of Jamie..
     
  17. phox_mulder

    phox_mulder I get paid 2watch TV

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    Salt Lake...
    This.

    He seemed to be able to keep the front wheel out of the water more than Jamie was.
    Not to mention the difference in weight.


    phox
     
  18. randyb359

    randyb359 Member

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    The front of the wheel pushes the water out of its way so there is much less for the rear of the wheel to push on so the front pushing down will have a greater affect because it is pushing on more water
     
  19. john4200

    john4200 Active Member

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    Very funny. :D You should get a job doing physics standup comedy.
     
  20. jeff92k7

    jeff92k7 Annoyed with trolls

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    on planet earth
    That is utterly ridiculous and not even comparable. Are you seriously trying to compare a motorcycle wheel getting traction from a small patch of water, lifting it up into a lower pressure field (the air) with a device completely inside only one gaseous field (air)????

    By your logic, astronauts don't need spacecraft. They can just waive their arms to move around in space. Apparently the need for, and effects of, an atmosphere are irrelevant.
     

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