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Surface Tension Physics Experiments & Tricks

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Quarters in a Full Glass of Water

This is a neat trick! Ask friends how many quarters can go in a completely full glass of water before it overflows. The answer will generally be one or two. Then follow the steps below to prove them wrong.

Needed materials:

  • 10 to 12 Quarters
  • glass full of water
The glass should be filled to the very rim, with a slightly convex shape to the surface of the liquid.

Slowly, and with a steady hand, bring the quarters one at a time to the center of the glass. Place the narrow edge of the quarter in the water and let go. (This minimizes disruption to the surface, and avoids forming unnecessary waves that can cause overflow.)

As you continue with more quarters, you will be astonished how convex the water becomes on top of the glass without overflowing!

Possible Variant: Perform this experiment with identical glasses, but use different types of coins in each glass. Use the results of how many can go in to determine a ratio of the volumes of different coins.

Floating Needle

Another nice surface tension trick, this one makes it so that a needle will float on the surface of a glass of water. There are two variants of this trick, both impressive in their own right.

Needed materials:

  • fork (variant 1)
  • piece of tissue paper (variant 2)
  • sewing needle
  • glass full of water
Variant 1 Trick

Place the needle on the fork, gently lowering it into the glass of water. Carefully pull the fork out, and it is possible to leave the needle floating on the surface of the water.

This trick requires a real steady hand and some practice, because you must remove the fork in such a way that portions of the needle do not get wet ... or the needle will sink. You can rub the needle between your fingers beforehand to "oil" it increase your success chances.

Variant 2 Trick

Place the sewing needle on a small piece of tissue paper (large enough to hold the needle). The needle is placed on the tissue paper. The tissue paper will become soaked with water and sink to the bottom of the glass, leaving the needle floating on the surface.

Put Out Candle with a Soap Bubble

This trick demonstrates how much force is caused by the surface tension in a soap bubble.

Needed materials:

  • lit candle (NOTE: Do not play with matches without parental approval and supervision!)
  • funnel
  • detergent or soap-bubble solution
Coat the funnel mouth (the large end) with the detergent or bubble solution, then blow a bubble using the small end of the funnel. With practice, you should be able to get a nice big bubble, about 12 inches in diameter.

Place your thumb over the small end of the funnel. Carefully bring it toward the candle. Remove your thumb, and the surface tension of the soap bubble will cause it to contract, forcing air out through the funnel. The air forced out by the bubble should be enough to put out the candle.

For a somewhat related experiment, see the Rocket Balloon.

Motorized Paper Fish

This experiment from the 1800's was quite popular, as it shows what seems to be sudden movement caused by no actual observable forces.

Needed materials:

  • piece of paper
  • scissors
  • vegetable oil or liquid dishwasher detergent
  • a large bowl or loaf cake pan full of water
In addition, you will need a pattern for the Paper Fish. To spare you my attempt at artistry, check out this example of how the fish should look. Print it out - the key feature is the hole in the center and the narrow opening from the hole to the back of the fish.

Once you have your Paper Fish pattern cut out, place it on the water container so it floats on the surface. Put a drop of the oil or detergent in the hole in the middle of the fish.

The detergent or oil will cause the surface tension in that hole to drop. This will cause the fish to propel forward, leaving a trail of the oil as it moves across the water, not stopping until the oil has lowered the surface tension of the entire bowl.

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