Gas station without pumps

2011 October 14

Physics Lab 3

The Physics Lab 2 post and Physics class progress described some experiments we were going to do this week with the ultrasonic range finder(s).

What we actually did, after spending a fair amount of time discussing the schedule for the year and expectations (one of the students has not started yet, because of the amount of time he was putting into his Stanford essays—I hope he gets in after all the effort he has expended, was to drop balls and try measuring the drop with both the Maxbotix sensor and a video camera.

We tried both a ping-pong ball and a slightly larger plastic ball.  With both of them the Maxbotix sensor only detected them intermittently, and we go no useful data from the rangefinder.

The HD video from the camera was more usable, but I was not aware that Tracker could use MTS (AVCHD format) files, since it greyed them out. I wasted a lot of time converting the movies to .mov format using iMovie, a piece of software that I have come to hate for its slowness, inefficient use of disk, rigid insistence on a specific directory structure, and generally unfriendly and unintuitive user interface. I’ve gone so far as to buy Premiere Elements, with the hope that it is not so awful (but I’ve not tried to use it yet, so it might be just as bad).

It turned out that my son had not installed the Tracker software on the desktop machine, so we had to get out my laptop and try analyzing the frames there.  I remembered how to calibrate and get the autotracking set up, but I forgot how to specify the beginning and ending frame (I eventually figured it out, but only after some false starts).  The data was pretty clean, though autotracking did lose the ball at one point because of some really stupid projections about where it ought to be.  Redoing the autotracking fixed that problem.  The errors in the tracking due to motion blur were not noticeable in the position plot, made small wiggles in the velocity plot, and made huge wiggles in the acceleration plot.  We did get a chance to talk about how a ±1mm error in position results in a ±3 cm/sec error in velocity and a ±90 cm/sec2 error in acceleration, when using 30 frames per second.

I tried redoing the autotracking using the AVCHD format directly from the camera, but it turns out that AVCHD uses interlaced images, so one gets two pictures of the ball in each frame. When the ball is motionless, this makes a nice circle, but when the ball is moving fast, you get two striped circles that may overlap. Tracker has trouble handling the interlacing, though a really clever algorithm could take advantage of it to get effectively double the frame rate when tracking large objects moving with reasonably constant acceleration.

Bottom line was that neither the ultrasonic rangefinder nor Tracker resulted in fast, painless measurement. We’ll have to try again next week, perhaps with bigger targets for the rangefinder. I initially thought that brighter light for the video would reduce motion blur by reducing shutter time, but it seems that the motion blur is due to interlacing, not to a slow shutter, so changing the lighting won’t help. Algorithmic changes to Tracker would be needed.

What to do in next week’s lab

  1. Analyze the balldrop clips (in .mov format). For calibration, the distance we measured between the top and bottom stile of the file cabinet was 112cm. Since the ball was a few cm in front of the file cabinet, there may be some perspective error in using that measurement to calibrate the drop. Get Tracker to give you position, velocity, and acceleration plots. Use the fluctuation in the acceleration estimates to estimate the errors in the velocity and position measurements.
  2. Write a Vpython program that simulates the motion of the falling ball including the initial pause before dropping, but not including the bounces.

Other homework

  • Read Chapter 3.
  • Work problems 3.P.36, 3.P.40, 3.P.43, 3.P.46, 3.P.52, 3.P.65, 3.P.72.
  • Do computational problem 3.P.76. Note that the computational problems for Chapter 3 are not independent of each other, and you should read all the preceding problems to get hints for this problem.


  1. Just curious… This sounds like an engineering lab, not a physics lab. Is that the intent?

    Comment by Ron G. — 2011 October 15 @ 00:01 | Reply

    • Because I’ve been an engineering professor for 29 years, all my pedagogical designs probably come out looking like engineering.

      What, in your mind, distinguishes a physics lab from an engineering lab?

      In my mind, physics is about building simple models for physical phenomena, and intro physics labs are for measuring simple phenomena for which simple models are adequate. Because we are having to cobble together labs, we are spending more time on calibration of measuring tools than I had expected.

      I was planning to have the students measure spring constants next (actually, to plot force vs. stretch for a number of small springs, and see whether a straight-line fit is a good model).

      Comment by gasstationwithoutpumps — 2011 October 15 @ 08:07 | Reply

      • I remember when I was in high school physics I did a project where I shot a ball using electromagnets and used a photoresister(CdS cell)/light setup with input to a TRS-80 to measure the velocity (by noting the time when the ball passed the CdS sensor and blocked the light). Then I used physics formulas to measure where the ball should land based on the launch angle and speed and compared it to the experimental results. Way too much time was spent on the engineering. In retrospect there were some opportunities to look at the physics of the CdS cell and the delay in change in measured resistance, chances to look at the electromagnet forces that created the velocity… but I didn’t explore those — so I’d say I really did an engineering project. Anyway, the point is if you spend moist of the time thinking about engineering issues unrelated to physics (e.g. frame analysis software) it’s probably a great engineering project, but doesn’t create many opportunities to critically think about the physics principles.

        Comment by Ron G. — 2011 October 18 @ 16:56 | Reply

        • Good points. I agree that the initial labs have been mainly engineering. It is hard to come up with much interesting to do with constant velocity (or even constant acceleration) models. I’m also a bit worried about boring my son, whose 5th grade science fair project involved computing the height of a bounce given the coefficient of restitution of the ball.

          I’ll do a post today for the next lab (involving spring constants) that should have more physics and less engineering.

          Comment by gasstationwithoutpumps — 2011 October 18 @ 17:12 | Reply

  2. […] Friday in our physics lab, we tested using Tracker Video Analysis and Modeling Tool for Physics Education from Cabrillo […]

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  3. […] students have not yet analyzed the ball-drop clips we recorded 2 weeks ago. The assignment in Physics Lab 3 […]

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