Monday, February 27, 2012

Slinky Waves

Recently, EaSiEE as Pi was in the 5th grade looking at waves.  One great way to show a wave is to use slinkies.  Everyone loves playing with slinkies and they make looking at certain types of waves visually possible.  The 5th graders had just finished studying the rock cycle, the Earth's interior, and plate tectonics.  Earlier in the year they studied sound and light.  So, I decided we would look at waves which would relate to all three of those subjects.  With the help of books and the internet I came up with four stations for the students to rotate through:

  1.  Making waves with strings and ropes
  2.  Making waves with a slinky
  3.  Making nodes and anti nodes with a slinky on a string
  4.  Earthquake impacts and characteristics
Before starting, I had to get a couple of slinkies and build some devices for the stations.  I found the slinkies at Toys R Us.  They had the metal versions and the plastic one.  I bought several of the large metal versions and one plastic slinky.  The plastic ones are not really good for demonstrating wave propagation, so make sure you get the big metal ones.  I will explain why I bought the large plastic slinky later in the description of the earthquake station.

In the first station that I listed above, we used different types of string a rope and let the students generate waves.  I used light weight string, elastic string, and a light weight rope.  The elastic string was able to generate the waves with the most nodes.  If you look carefully at the photo below, you can see the rope in the middle with some waves.  This was a popular station and one that I have seen in books and one the internet.  This experiment I looked at the book Making Waves by Bernie Zubrowski.  What we found is that it is very hard to make waves with a student at each end moving the string.  It works better if one person holds the string or rope steady and then the other makes the waves.  The idea was to try and make one wave, then two, then three, and then to make as many as you could.  The idea was for each student to try and make waves with each material so that they could find out which material allowed them to make the most waves. 

Another station that was popular was the slinky station where we had the students make a P or pressure wave with the slinky.  You have one student at each end of the slinky, and have them back up so they have the slinky stretched out quite a bit.  Then as you can see below with the student on the right.  Have them cup their hands, then strike their hand with the other hand to create the pressure wave.  When I did the demonstration for all of the students before we started, the kids all oohed and ahhhed.  I was surprised that they would be so excited seeing the wave in the slinky, but I have to admit, it is pretty cool!  I also had them produce S or shear waves on the floor.  If you have them make S waves with the slinky in the air it often gets out of hand and results in very tangled slinkies.  You can see a great visual graph of P and S waves at the Purdue University site.

One internet source which is a great help in finding fun hands-on activities is the Exploratorium Snack page.  The activity I used was "Slinky in Hand".  I set up a slinky on a fishing line that was tied between two chairs.  With the slinky you can do some interesting things with compression waves.  I used a 10 pound line, although the instructions said use a 20 pound line.  I think a good thick line is indeed needed.  The first class broke the fishing line in the first few minutes.  I then tied a thin cotton string to the chairs and used that.  However, the monofilament line is better because it has a lot less friction than the string.  From the series of photos below, you can see how the students can make pressure waves with the slinky by either moving their hands toward each other in a clapping motion, or moving them together in sync.  However, we found that moving together in sync is pretty hard, and if you get two people who work well together, they can act as if they are sawing a tree and get multiple wave forms that way.

For a couple of the experiments showing the earthquakes, you need to attach the slinky's to a small block of wood.  That setup is described on a website about Earth Science at Purdue University.  If you look at the Seismic Waves writeup, you can see how to build a couple of the devices that I used.  I show a photo below of drilling a hole into a block of wood.  I then used screws and washers to secure the last slinky link to the block of wood, and then built a little house per directions on the website.

This is a photo of the final device in action.  A student is generating a wave and shaking the building and you can see how it is leaning to the left as a result of the "earthquake".

In this earthquake station I also had the metal and plastic slinky's taped together to show how the waves propagate differently through different types of material just as a earthquake would propagate through different types of rock.  This device works quite well.  It also works well to have the kids put it on the floor and generate S or shear waves and see how the propagate differently in the metal vs. the plastic.  This was a popular station.

Overall, I think this set of experiments worked well.  I think I would do a little tweaking.  I think I would use a set of whacking balls.  You can use marbles and string, but buying a set of these balls, called Newton's cradle, would be better.  They show how an seismic P waves can propagate through the earth.  
Another  experiment I might add would be to set a slinky on a piece of sandpaper and have the students move the paper quickly about 6 inches and see reaction.  You can also join two slinky's together for a taller building.  I found this experiment in Janice Van Cleave's book Earthquakes.  

Friday, February 10, 2012

Fun with Roller Coasters

We had planned to have a fun science/engineering day back in December just before winter break with the 3rd graders.  But a bug struck our family and had us down and out for the two weeks before break, so I had to cancel this session.  I spoke with the teachers and we decided to go ahead and use this one out of sequence for our session this week.  What a great time the kids had!  They all proclaimed that this was their favorite activity we have ever done (which is saying a lot since I have been doing science with these kids since kindergarten!).  I used a writeup from called Building Roller Coasters.  The grade level rating on this exercise is grade 7, but the third graders did great with this activity.  I have also seen something similar written up in Exploratopia (pg. 155).  Both activities use pipe insulation cut in half.  Each team of three was given a cup with a glass and a steel marble, a 6 foot piece of pipe insulation, 12 inches of masking tape, a pencil, and a scoring table.

Roller Coaster Scoring

Roller Coaster Features
Your Design
1 point / 12 inches

90 degree turn

180 degree turn

270 degree turn



Each marble in cup


We talked a bit about potential and kinetic energy, asking the students if they could explain what each meant, and we also talked about acceleration and deceleration.  We then let them go build.  The most points awarded was for a corkscrew, which is the most challenging to build with the six feet of pipe.  Here are a couple of examples of corkscrew designs that worked.

The most popular design by far was the loop.  It also resulted in some good learning, too.  One group in particular was struggling with their loop.  The marble was making it about 2/3 of the way up the loop and falling out.  They couldn't figure out what was wrong.  I sat with the group and started asking them questions:

Why is the marble falling out?
          Ans:  It is not going fast enough
How can you help it get up the loop?
         There was a bit of discussion among the group, I hinted that there were a couple of solutions, so finally they decided to make the ramp or beginning of the coaster higher.

They tested it again, and it made it up a bit farther up the loop, but was still dropping out.  We discussed the problem some more, but they didn't have enough track to make the coaster any higher.  After more discussion among themselves and some gentle hinting on my part, they decided to make the loop smaller.  

Voila!  It worked!

Another group that had a valuable lesson was the group below.  They built a loop with a 90 degree turn.  I asked at the end of the session if anyone had had any problems to overcome and they said yes.  They first had the track flat and the marble flew off the track.  After some trial and error and thinking on their part, they realized they had to bank the track to get it into the cup!  Good thinking girls!

The biggest challenge of the day, besides the noise (which was all very joyful), was keeping some of the groups from changing their design constantly.  Some groups felt that they had to try every possible design and in the end had nothing to share with the class.  I tried to get the groups to experiment, decide upon a design, get it working and stop to share with the class.  A couple of groups just couldn't stop experimenting, they were having too much fun!  The other challenge was that a couple of groups just wanted to get the marble down the track with no twists or turns, so they needed a little encouragement to try new things, and step out of their comfort zone.

This was a great experience for all of us, students, teachers, parents, and me!

Wednesday, February 1, 2012

Potential and Kinetic Energy for 4th grade

In January, we went in and worked with the fourth grade on an experiment from called Falling Water.  In this experiment, you take a straw, a meter stick, colored water, and some paper.  The idea is to test that a water drop falling from 90  cm has more potential and kinetic energy than the same size drop falling from 30 or 60 cm.  This idea is tested by dropping the same amount of water from three different heights.  We chose the heights of 30, 60 and 90 cm.  We marked the straws 1 inch from the bottom and the students tried to get the same amount of water for every test.  We had groups of three and four students run the experiment as a team.  Each team member dropped water from each height.  They then measured the diameter of the water drops and recorded them on a table.

Here are some students dropping the measured water from 90 cm.  You can see the measured amount of water contained in the straw.  It is a stretch for the students to reach 90 cm from the table top, so we suggest that they run it on the floor.  The second photo shows the water after the drop.  You can see it makes a nice large circle on the paper.

The students circle the drops with ink, then record the data in a table.  You can see the paper with the drops circled on the left below and two team members filling out the data table.

The students have a great time with this experiment, and overall the experiment works the way it should with two exceptions.  One, each team member makes a drop at each height.  Some teams are very careful and meticulous and their data is very tight.  Other team or team members may not be as careful about the whole process.   They may not get the same amount of water in the straw each time.  They may have problems with using the straw and keeping the water in, or they may not trelease the water from the straw with it straight up and down and may release it at a slant.  If you drop the water with the straw slanted you get a nice big oval rather than a circle, and then it is hard to pick a diameter.  The other thing that can happen is that some team members are very careful and one or two may not be as careful with the process.  In this case, you will get two good data points, and one or two that may be way off.

Even if the results are not great for every team, it presents a learning opportunity.   We discussed why some of the data point are off.  The table that the students use with sample data is shown below.

Water height at 30 cm
Water height at 60 cm
Water height at 90 cm
Diameter of first splash
70 mm
68 mm
65 mm
Diameter of second splash
45 mm
65 mm
60 mm
Diameter of third splash
49 mm
57 mm
66 mm
Average Diameter

The writeup asks that the students calculate the total of the drop diameter and the average.  First of all, the fourth grade has not studied averages yet, but some of the more advanced students can do this.  However, I disagree with this method for graphing the data.  If we use the data above, then the graph that results is shown below.  This graph shows that the drop size increases from 30 to 60, but not from 60 to 90.  So, the students could conclude that the height of the drop doesn't necessarily result in a larger circle.

However, if we have the students plot each data point, then we know there are some problems with the data.  We can see right away that the first set of data show that the drop size for two of the students is about 45 to 49, but the third point shows a drop size of 70 mm.  Right away, the students can see that something is not quite right.  It could be that one of the students used too much water, or slanted the straw and then measured the resulting oval with a very large diameter.  In the 60 cm drop, we have one data point that looks correct, but two that look a little large.  The third set of data at 90 cm looks about right.  Having the students plot all of the data rather than just the average shows where there could be problems with the experimental method, and opens up the discussion about how to run careful experiments.  If a student team is very careful, they should see plots such as the one shown for student 3.   Most of our students groups had such plots.

Overall, this is a great experiment, and one that I would recommend.  The students love it, and find it straightforward to do and results in a lot of good discussion about experimental method and the care that scientists must take with experiments as well as discussions about potential and kinetic energy.