| MadSci Network: Physics |
Well, being an experimentalist, I tried your experiment. I too noted that the marble tends to go to the opposite side when dropped near an edge. And also, that dropping at the center, the marble drifted to one side or another. I tried the experiment with two spheres (a b-b and a marble) and with 2 tubes(1 inch and 2 inches). I am certain that the effect has to do with the relative density of the sphere being dropped into the water. Because boyant forces and drag depend on the size, or more appropiately the surface area of the object. The denser b-b has less surface area/mass ratio so the effects from the water will be smaller. This was very true. The b-b basically fell straight down. The marble being less dense had a larger surface area/mass ratio will be easier for the water to perturb. If the objects were not spheres, this simple ratio of surface area to mass would not be correct. A nail, or something cone shaped would probably tumble around on its way down. I believe that one of the biggest effects is spin on the sphere. Sort of like a knuckleball. A knuckleball is thrown with very little spin. Any slight disturbance will throw the ball off course. In the case of a knuckleball it is because of the irregular surface that it deflects. I think it this case it is because of edge effects of the water that the sphere deflects away from the wall. The water near a wall is less free to flow. This will create a pressure difference across the the surface of the sphere which causes the deflection. I tried to spin the sphere clockwise or counterclockwise relative to the falling axis, and this helped some. If you've ever seen a football throw this is the same idea. A ball with good spin will fly true, while a football with less spin will wobble (sometimes called a lame duck). In guns this is called rifling. When golf first began the balls where very smooth. When a ball was cut, it tended to fly better or at least more predictably. Gradually, people figured out that a dippled or rough uniform surface can be more stable than a smooth one given the right conditions (usually high spin). Anyway, because of the density of the marble, any non-uniform effect will cause the marble to deflect. Like near an edge, or just a slight spin, being ever so slightly off center, or just not being perfectly round. Another effect to consider is turbulence. If the ratio of speed to viscosity is high, then you can get turbulence. Turbulence is hard to predict. Any small deviation can possible cause big effects. With a smooth, nearly perfect spherical marble, turbulence could likely happen in water. I hope this is an acceptable answer for you. I recommend you look up information on knuckleballs, golf balls, ballistics, and the Magnus effect for further ideas. Tom Cull