Videos of “More on surface tension – floating Ping-Pong balls”

Here are the videos of the “More of surface tension – floating Ping-Pong balls” demonstration. The full videos are linked from the GIF captions.

Cup partially full with ball drawn to the edge of the cup

Video Apr 22, 11 17 01 AM 00_00_30-00_00_36

Cup over full with ball drawn tot he middle of the cup. 

 

Video Apr 22, 11 18 39 AM 00_00_27-00_00_34


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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More on surface tension – floating Ping-Pong balls

This is not a new demonstration but rather an extension of a previous post “Surface tension – floating Ping-Pong balls” The equipment is just a ping-pong ball, a cup and some water.

Photo Apr 22, 11 16 28 AM

In the previous demonstration the ball was placed in the middle of the cup and was dragged to the side by a surface tension imbalance. Soap was then added as a surfactant to reduce the imbalance and allow the ball to float near the middle of the cup. In this extension (see, for example, various Martin Gardner books 1,2) the first part of the demonstration is the same as in “Surface tension – floating Ping-Pong balls“. However, in the second part, the ball is held in the middle by changing the curvature of the water surface rather than reducing the surface tension.  The water surface curvature is changed by overfilling the cup so that the water surface curves up above the lip of the cup. In this case the minimum area occurs when the ball is centered in the cup. Again, see John Bush’s lecture notes here for a more formal discussion of surface tension.


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Other fluids education resources III: YouTube channels

Prior posts on other resources (1, 2) linked  to particular websites, books, blogs, and videos. Here I just want to highlight a couple of YouTube channels that have some great fluid mechanics content. In general they tend to focus on fairly accessible stuff like surface tension and buoyancy though there is also other content as well. In no particular order they are:

Physics Girl (on twitter @thephysicsgirl) posts videos covering a very broad range of physics much of which is fluid mechanics. Her videos cover topics such as surface tension, vortex dynamics, and the Coanda effect. There are also some cool videos on calculating Pi with a dart board and momentum from dropping stacked balls.

Veritasium (on twitter at @veritasium) is another general physics YouTube channel with a couple of very nice fluids videos. One of the best is this set on buoyancy forces (second & third). There is also one on jet packs. A lot of it is not fluids but there are a ton of other interesting videos including this one on our willingness to take risks.

Benjamin Drew teaches at the University of Western England. He has posted a large number of his fluid mechanics lectures online. The lectures are well organized and, given the number of posted videos, there is quite broad coverage. He has also posted a bunch of lectures from other courses he teaches on a broad range of subjects.

Vsauce (on twitter @tweetsauce) doesn’t do much on fluid mechanics other than this video on water. However, there are a lot of interesting general science videos. Well worth following.

With the exception of Ben Drew’s videos there is not a whole lot of math or theory in this set but there they are all well produced, well presented, and well worth following.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Video of “Surface tension – floating Ping-Pong balls” demonstration

Here are the videos from the “Surface tension – floating Ping-Pong balls” demonstration. The full videos are linked from the GIF titles

Placing the ball with no dish soap

no soap

Placing the ball with food coloring showing a rapid surface flow toward the edge of the cup.

viz

Placing the ball after the soap showing the ball staying relatively still.

soap

 

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Surface tension – floating Ping-Pong balls

Here is another really simple surface tension demonstration that I found out about from Ben Sill. It can be done on an overhead projector (OHP) or just with a streaming video camera. The projection technology is not central.

Equipment

  1. A Ping-Pong ball
  2. A cup of water (or a round glass bowl if using an OHP).
  3. Some dish-soap
  4. Food coloring (optional)
  5. A student volunteer

Photo Aug 11, 1 57 41 PM

Demonstration

  1. Fill the cup with water and let it settle so that it is not moving
  2. Ask the student to place the ping-pong ball in the center of the cup. This should be very difficult as it will quickly be drawn to the side of the cup.
  3. (optional flow visualization) Put a small drop of food coloring on the ball and place it drop down in the water. You should see some of the food coloring fall down (it is denser than water) and some get pushed out away from the ball near the surface (see analysis section below).
  4. Put a little of the dish soap on the surface of the water and then place the ball there. It will still eventually move off center but a lot more slowly.

Analysis

Placing the Ping-Pong ball at the surface of the water creates curvature in the water surface all the way around the ball. The surface tension will act to minimize the total surface area. Unless the ball is placed exactly in the middle of the cup (impossible) there will be an asymmetry in the surface curvature that will draw the ball toward the side nearest to the ball, thus minimizing the total surface area.

st balls

When the soap is added the surface tension is reduced (soap is a surfactant) and the path to minimum surface area is less steep. As such, the ball will drift much more slowly such that it appears to be stable in the center of the cup. For a more formal discussion of surface tension see John Bush’s lecture notes here.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.