PocketLab HotRod Ramp: A Study in Rotational and Translational Motion

Submitted by Rich on Mon, 03/26/2018 - 01:31

This project will get your physical science/physics students involved in a number of Next Generation Science Standards, particularly in the NGSS science and engineering practices.  This investigation provides a nice opportunity for the students to (1) suggest hypotheses, (2) design an experiment to test their hypotheses, (3) analyze and interpret their data, and (4) use principles of physics to explain their observations quantitatively.

A PocketLab HotRod is placed on a narrow ramp so that the wheels overhang the sides of the ramp and the entire system rolls down on its axles without slipping. When the front wheels and axle get near the bottom of the inclined plane, the front wheels come in contact with the surface of the table top. Challenge the students to (1) hypothesize events that occur after this, and (2) calculate changes in the HotRod’s translational speed as well as rotational velocity of the front wheels.

The photo below shows the setup used by the author.  A meter stick makes a good ramp.  Voyager is mounted on one of the front wheels of the HotRod.  The orientation of Voyager on the wheel indicates that the Z angular velocity would be of interest in the analysis.

The HotRod ramp

 

The photo below shows that some masses have been added to the center of the other front wheel of the HotRod in order to provide symmetry and keep the HotRod from turning while rolling down the ramp.  The mass of Voyager plus its 3D printed holder is about 24 grams.  Regarding the height of the ramp, students will need to make sure that it is not so high that the angular velocity exceeds the 2000 ⁰/s limit for Voyager.  This limit is easily recognized as the angular velocity will suddenly flatten out at this value and remain there until it once again goes below the 2000 ⁰/s limit.

Weights on the other front wheel of the HotRod

 

The combined data/video from the PocketLab app below shows a run made by the author.  The data rate was set to 50 points/second.

 

The Excel graph shown below was constructed using data from the csv file created by the PocketLab app.  Your students will likely come up with a very similar graph. Six critical points labeled A through F have been identified on the graph.

Excel graph of Z angular velocity

 

If and only if your students need more direction, the following ordered list of questions are suggested for their analysis:

  1. Explain what is happening during each of the following intervals:
    1. The interval from A to B
    2. The interval from B to C
    3. The interval from C to D
    4. The interval from D to E
    5. The interval from E to F
  2. Why is there a fairly regular but small periodic variation in the angular velocity during the interval from A to B?
  3. What is the angular velocity of the front wheels just before making contact with the table top?
  4. What is the translational speed of the HotRod just before making contact with the table top?
  5. What is the angular velocity of the front wheels during the interval from C to D?
  6. What is the translational speed of the HotRod during the interval from C to D?
  7. Explain the physics of why the translational speed increased when the front wheels contacted the table top.
  8. What is the angular velocity of the wheels during the interval from E to F?
  9. What is the translation speed of the HotRod during the interval from E to F?

Explain why the translational speed has increased when the rear wheels contacted the tabletop.

Image
Typical plot of z-angular velocity vs. time