CSI: Motion Experiment, Episode 1: The Detector Is Blind.

A reader is having an issue with motion detectors and semi-quantitative experimental results:

Hello there! I hope your year has started off well. Mine has. I am very happy with my results from last year, implementing your teaching/activity techniques with the students. Thank you very much for all of your guidance.

I am noticing that during the in class lab exercises kinematics - those are the ones where we have a track, cart, and motion detector, and double the time, then double the acceleration, then double the initial velocity - that the double time experiment yields very high percent errors for most students. I thought it might be a sensor issue, but I zeroed the sensors and did the experiment myself on the setups giving trouble, and I am getting the same thing the kids are. It's supposed to be a multiplier of 4 for the distance, but I am getting 2.6, 2.9, 3.3...very low to what it should be. I'm now thinking it may be an issue with friction, because usually the lower the incline the worse the result, but too high and we don't have enough space to get a good time interval (I only have 1 meter long tracks). I am using the metal pasco tracks with the plastic pasco carts. The carts are on the older side, so I'm thinking that friction with the wheels might be throwing off results. 

Do you have any input on this? Do you experience the same thing? 

Hey!  I've got some thoughts.  I don't think it's a friction problem - the friction is there even for the smaller time, and in any case is not going to cause that much of a difference between what's predicted and what's measured.

One issue might be the 15 cm blind spot* for the sonic detector.  Depending on whether you're moving toward or away from the detector, you may well be missing a significant fraction of the distance the cart traveled.  After all, the blind spot is 15% of the entire track length!    Another less likely issue is that I've had students measure (and thus double) the time from when the detector was started, not the time that the cart was moving.  

*The green Vernier detectors that I use have a 15 cm blind spot.  The decade-old blue Vernier detectors have a 40 cm blind spot.  I don't know what the PASCO detector blind spot length is, but it exists.

The way I'd suggest doing this particular problem is in one trial.  Set the detector at the high end of the track.  Place the cart's back end 15 cm from the detector.  Press play, WAIT FOR THE CLICKS, and let the cart go once the detector has started running.  Use the position time graph made by the detector.  Check the distance traveled after the cart has moved, say, for 0.30 s.  (Not when the labquest says t = 0.30 s, 0.30 s after the cart begins to move.  You can check the velocity-time graph to see when the vertical axis value first moves away from zero.)  Then check the distance traveled after the cart has moved for 0.60 s.  I'll bet you come closer to a factor of four increase.

Do you have a PASCO smartcart?  Those will get much better data; and, you can use them on a long plank of wood instead of on the track - if the cart moves slightly off a straight line, the motion encoder in the smartcart's wheels will still work.

Hope this helps!

Epilogue:  Turns out the reader didn't know about the motion detector's blind spot, and so was especially confused that the cart moving toward the detector gave a consistently different result than a cart moving away from the detector.  She solved the problem using the experimental suggestion above.  Awesome.