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.

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