Hands on Science Sunday: Ticker-tape timer for measuring motion

by Stephanie Chasteen on June 21, 2009

picture copyright Don Rathjen of Exploratorium

picture copyright Don Rathjen of Exploratorium

A pretty standard lab for introductory physics is to chart what constant speed (or constant acceleration) looks like, and graph it versus time.  There are all different ways to do this, but one is to use a ticker-tape timer, which I think is wonderfully cool.  The idea is to attach a piece of ticker tape to whatever object you want to graph the speed of, and then have some apparatus (a ticker tape timer) that makes marks on the ticker tape at regular time intervals.  If the ticker-tape timer made a mark every second (which would be convenient), then the distance between marks on the ticker tape would represent the distance traveled per second.  Generally they’re much faster, and timer speeds are measured in hertz.  Whatever the units, though, you now have a graphical representation of distance per time!

To make a graph, you can then cut up the ticker tape.  Choose a unit of time that you want to use on your x-axis — for example, 10 “hits” of the ticker tape timer.  That will be the unit of time.  (It’s generally arbitrary, since the timer doesn’t hit every second).  Chop your marked ticker-tape into 10-mark sections.  The horizontal axis, then, is time, and the vertical is distance per unit time.

If it was moving at constant acceleration, it will look like this:

If it sped up and then slowed down it will look like this:

All images are taken from the Practical Physics website, which has a lot of experiments using ticker-timers.  Go to their experiments website and search for “ticker” to see labs on measuring time, velocity, and other aspects of motion.

Of course, what is this “ticker timer” that will actually make the marks for you?  If you’re a tinkerer, there is a wonderful write-up for a DIY ticker timer from the Exploratorium (PDF) by Don Rathjen.  It’s a fussy thing to make, though, so really only for the type of person who reads those detailed instructions and thinks “Hey, I want to make that!” (and if you are, you’ll love Don’s instructions for making a clock escapement).  Still, I recommend downloading the instructions, because they give some really cool examples of how you can use these, including how to make displacement and acceleration graphs (instead of just velocity graphs) and how to measure “g”.

If you have the money, though, Arbor does sell (for $140) a spark timer that does the same thing.  Here is their ticker-timer lab (written for advanced and introductory levels) with links to all the needed materials.

Useful for all sorts of things, they also sell cheap constant acceleration cars and constant velocity cars.  (New, improved!  It actually goes straight!).

And, if you really need it to be on the cheap, there are some online applets for constant velocity and constant acceleration that were recommended by a physics teacher.

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{ 4 comments… read them below or add one }

avatar Stich June 22, 2009 at 9:22 pm

Very cool! You would be surprised how practical some of these devices are for special effects sometimes in haunted houses. My friend Edwin, who now works at National Instruments, has countless things like this lying around the lab. Figuring out how you can make an interactive play more fun with technology is a super cool geek learning experience.

avatar Captain Skellett June 23, 2009 at 2:41 am

Ahh yes… I remember doing this in high school. Got so sick of counting dots after the first week though, and trying to tie the tape to the little wooden car with varying weights on it. Then again physics was never my forte :P

avatar Carina June 23, 2009 at 12:28 pm

The post reminds me of a neat alternative to ticker-tape measurements. Use a flashing LED and a digital camera and attach the blinking LED to a moving object. Record the full trajectory you are interested in by taking a photograph with the shutter open long enough to capute the entire scene. You will see a series of lines corresponding to the “on” state of the LED. It is described in a paper in The Physics Teacher from Oct 2008; the paper is free to read without subscription.

http://scitation.aip.org/journals/doc/PHTEAH-ft/vol_46/iss_7/395_1.html

The authors use it to measure g, but I suppose you could use it for any (not too slow) movement, although I haven’t tried it yet.

avatar sciencegeekgirl June 23, 2009 at 2:26 pm

They’ve done those sort of LED measurements at the Exploratorium as well, though you can also use the LEDs to visualize motion, in the same way. Go to http://apps.exploratorium.edu/blogs/sebastianm/2008/09/15/a-different-physics-class/ to see some pictures that a teacher sent in of pictures she made in her classroom. It’s pretty neat stuff. There is a link for instructions too.

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