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An Experiment in Rotational Dynamics that Emphasizes the NGSS Science and Engineering Practices

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Submitted by Rich on Tue, 06/20/2017 - 23:12

Here is a PocketLab based project that will get your physical science and physics students involved in many of the Next Generation Science Standards, particularly in the NGSS science and engineering practices.

Two wheels and a wood axle from the PocketLab Maker Kit are placed on a narrow inclined plane so that the red wheels overhang the sides of the inclined plane and the entire system rolls down on the wood axle without any slipping.  When the wheels and axle get near the bottom of the inclined plane, the wheels come in contact with the surface of the table top.  Challenge the students to hypothesize what will happen next.

The photo below shows a snapshot at the instant the red wheels contact a piece of cardboard on the table top.  Cardboard was used to provide more friction as the table top was quite slippery.  PocketLabs are mounted on both of the wheels to provide some symmetry, though only one of the PocketLabs is actually used.  A small piece of wood about the same mass as a PocketLab could be used as a replacement for the unused PocketLab.

Table Setup

The photo below is an enlargement that more clearly shows the two red wheels, axle, and mounted PocketLabs on the inclined plane and just reaching contact with the surface of the cardboard.

Table setup close up

With hypotheses in hand, you can now either have the students design an experiment, without the use of PocketLab, to test their hypotheses, or show the video below.  This video shows what happens but does not provide any superimposed data from PocketLab.  If you use this video, then challenge the students to provide explanations for what happens when the wheels contact the surface.

Now it's time for the students to get quantitative by using PocketLab to collect some angular velocity data as the systems rolls down the incline and contacts the cardboard on the table.  Alternately, you can make use of the video below that contains combined video and data from the PocketLab app.  The orientation of PocketLab on the red wheel indicates that the Z angular velocity is of interest in the analysis.

The angular velocity vs. time graph below was made in Excel from Z angular velocity data in the csv file created by the PocketLab app.  The csv file used by the author is attached for your reference and for use by you and your students.


There are several discussion questions that relate to this graph:

1.  What do each of the points A, B, C, and D represent in the motion of the wheels and axle?
2.  Why is there a sort of sine wave feature in angular velocity from points A to B,  and from points C to D?
3.  What is the angular velocity of the wheels and axle system just before making contact with the table top?
4.  What is the speed of the center-of-mass of the system just before making contact with the table top?
5.  What is the angular velocity of the wheels and axle system at point C?  
6.  What is the speed of the center-of-mass of the system at point C?
7.  Explain the physics of why the speed of the center-of-mass of the system increased upon contacting the table top.

Angular velocity graph
Grade Level