## Faster Than a Speeding Bubble!

April 28, 2012

by: Cindy House

### Speed of the Bubble Apparatus

Bubbles in tubes offer many advantages over spheres on ramps for velocity and acceleration experiments:

• The bubble stays in the tube! There are no escaped marbles to chase down.
• The bubble moves more slowly than a marble, permitting more accurate determination of elapsed time.
• Results are highly reproducible.
• Many data points can be collected in a short period of time.

A simple apparatus to hold and protect the tube is easy to construct from scrap and/or inexpensive materials. It enables even very young students to obtain highly reproducible data quickly. It also protects the tubes from being damaged if dropped or bumped.  Plans and suggested materials are included in this blog. The following experiment is one I use with the elementary students in our after school science club.

Equipment (per pair of students)

1 bubble tube apparatus

hook/loop strap (optional)

data tables

per class of 30 students: 3 each of 15O, 30O, 45O, 60O , 75and  90blocks.

From start to finish this experiment can be comfortably accomplished within one hour if the tubes are already installed in the apparatus.  I first demonstrate how to use the apparatus, then ask the students at what angle they think the bubble will move most quickly and why. After recording their responses we start the experiment.

Procedure:

Each pair of students selects an angle block; it makes no difference which degree value is chosen first. They push the block into the axis between the swing arm and base as far as it will go, securing it with the hook/loop strap.  The strap is optional, but it makes it easier for the children to hold the block in place when they’re tipping the apparatus.

One partner returns the bubble to the starting position before each trial by tipping the entire apparatus until the bubble gets to the end of the tube. She quickly, but gently, sets the apparatus base flat against the table as her partner prepares to start the timer when the front of the bubble touches the thirty centimeter line. He stops the timer when the front of the bubble touches the zero centimeter line.

They do three trials of each angle, then calculate and plot the average values using a bar or scatter chart.

Discussion:

When all teams finish collecting and analyzing their data, we compare and discuss the results.  The student teams found that either  45or 60produced the speediest bubbles.  Why might the same experiment produce two different answers? Does the difference between the 45and  60 results fall within the experimental error?  Is the real answer somewhere in between, perhaps  50O  or 55O ? Does the color of the tube, i.e. the viscosity of the liquid, make a difference?  What are sources of error in the experiment, for example, what would be the effect of not holding the angle block tightly against the arm/base axis during each trial? How might the experiment be changed to answer these questions? How many significant figures should be recorded for the times?

With additional time, students can determine if the speed of the bubble is constant throughout the length of the tube.  The procedure is similar to that of the first experiment. Students measure the time it takes to travel 10 centimeters, 20 centimeters, 30 centimeters, and 40 centimeters, conducting three trials for each distance.  Distance traveled versus average time is plotted on a scatter chart.  Does connecting the data points yield a straight line?  Is the speed constant? Discussion can include speed as the slope of a line, and, particularly if you have a calculator which automatically calculates this value, standard deviation.

## Science Experiments With Japanese Yen Coins

February 24, 2010

by: Ron Perkins

Who knew that a single coin could be used for so many classroom science activities!  You can demonstrate concepts such as surface tension, buoyancy, and even eddy currents with a single Japanese yen!

Surface Tension: Even though aluminum has a density of 2.7 gm/cm3, and the density of water is 1 g/cm3, aluminum yen coins can float on the surface of the water!

Surface tension is a physical property of water.  It is caused by cohesion, which is the attraction of like molecules.  Water molecules are made up of two hydrogen atoms and one oxygen atom.  The “stickiness” of water is caused by hydrogen bonding.  This hydrogen bonding pulls the water molecules towards one another and forms a sort of “skin” on the surface of the water.

Using a bent paper clip or a plastic fork, gently lower the flat side of the coin onto the surface of a pan or cup of water and remove the clip or fork. The coin should rest on the surface of the water. Plastic cups, glass bowls or baking dishes with clear sides will make it easy to see the effects of surface tension. The coin will actually slightly depress the surface of the water and can easily be viewed through the side of the dish or pan.

Adding more than one coin to the pan will result in a cluster of coins forming. Since each coin depresses the surface of the water, they will tend to slowly float together and form a regular, crystalline structure. (Imagine bowling balls on a stretched bed sheet – they will slowly roll towards each other to form the most stable structure.)

Adding a few drops of soap, such as dish detergent, will break up the surface tension of the water and cause the coins to sink.

Another great surface tension experiment you can conduct with your students is to have them initially predict the number of drops of water they can fit on the face of the yen.  Then, using a pipet, have students drop water, one drop at a time, onto the face of the coin.  They will be amazed at how many drops this small coin will hold.  This activity is perfect for discussing variables that could change the results of the experiment as a result of the experimenter’s manipulation (independent variables) .  Students can brainstorm reasons that some coins held more drops of water than another.  Examples include the side of the coin that is used, how worn the coin is, and how high above the coin the water is dropped from the pipet.  Controlling as many of these variables as possible, gives the most accurate results.

Buoyancy vs. Surface Tension: A charged rod will have different effects on floating objects, depending upon whether the object is floating due to surface tension or buoyancy (displaced water). A buoyant object will be attracted to a charged rod, while an object resting on the surface of the water will be repelled. Try charging a rod or piece of PVC pipe and bring it near to a floating aluminum coin – the coin will be repelled. To demonstrate a buoyant object being attracted to a charged rod, make a small boat out of aluminum foil and float it in the same pan as the coins. This boat will be attracted to a charged rod.

Eddy Currents: For this demonstration, you will need a strong magnet, such as one of Educational Innovations’ neodymium magnets. First, demonstrate that the yen coin is not magnetic, by trying to pick it up or stick it to the magnet. Next, set the coin on a flat surface, so that it balances upright on its edge. Very quickly move the magnet back and forth over the top of the coin without touching it. The rapid movement of the magnet will induce an eddy current, which creates a temporary magnetic field in the coin. The magnetic field in the coin is attracted to the moving magnet above, causing the coin to move.

There are so many uses for this small aluminum coin in every science classroom.  You can get yours at Educational Innovations for only \$7.95 for a package of 50 yen coins!