Demo: How can something move down if I'm pulling it up?

 

The picture above is one frame of a video from class yesterday.  Hanging from the string is a PASCO cart with the "visual accelerometer" attachment.  Before this demonstration, I spent some time convincing the class that the lights on the cart correspond to the direction of acceleration.  Thus, everyone is well convinced in this screenshot that the cart has an upward acceleration.

We've used Vernier force probes before, too.  I showed the class that the cart weighs 7 N.   They can immediately see in the screenshot that the force probe reading, which is also the force of the string on the cart, is more like 11 N.  They're well convinced now, experimentally, that the upward force of the string on the cart is greater than the downward force of the earth on the cart.  They know that the net force must be 4 N, upward.  

The class is, unfortunately, also well convinced that the cart is moving upward.  Oops.

"BOUX," I've said for years.  The net force is not in the direction of motion -- the net force is in the direction of acceleration.  And we just spent a whole unit discussing how acceleration is in the opposite direction of motion when something slows down.  This cart could be moving down but slowing down.

For the first time, though, I can give immediate, undeniable experimental evidence.  I can just run the video forward for a few frames -- the class sees that the cart was, undeniably, moving downward.  Even though the acceleration and the net force was upward.  Cool.

Hints:  This was one of my early attempts at obtaining this kind of video.  I discovered I needed to use the "slo-mo" function on my iphone 6.  Try to lower (or raise) the cart such that the string never goes slack -- I didn't do that well in this trial.  

There is, in fact, a couple frames of lag between the appearance of the green dots and the force graph displaying greater than 7 N.  That's the computer processing and displaying the output from the probe, which is not instantaneous.  You can even see screen flicker in the video, since I'm recording at ~100 fps while the screen refreshes at 60 fps.  But the video is still convincing.

And finally, you can certainly get the students themselves to make this video.  Tell 'em to pull out their phones and record.  Chances are that at least one student gets a good recording.  Then have that student show a screenshot like the one above to classmates, explaining carefully the relevance of the green dots, the relevance of the force probe reading, and how the video shows the cart moving down, not up.