Day 4: What Students Talk About When They Use the Motion Detector

After just quickly introducing the four ways to represent constant velocity motion of a particle (see my last post), I sent the students out to try to walk various position-time graphs, and then use the computer to find the velocity-time graph, and then represent that motion as a motion map and as words.

Some questions my students were thinking about today:

  • Is going towards the detector the positive direction or is going away from the detector the positive direction? (Student tries it.) Wait, really? Why is going away positive?
  • Do I ignore that first bit where I'm not moving yet, or do I draw it on my velocity-time graph? 
  • Can I just call where I start x = 0? (As in, if I start where I normally start a little ways away from the detector and don't move, can't I call that point zero?)
  • How does the velocity change in an instant? Doesn't it take time?
  • How can I tell which way to draw the arrows on my motion map?
  • How does the slope of the position-time graph relate to the velocity of the object? (One student came up with a great rule for this. I don't want to put his or her name in here because I didn't ask for permission, but every time I came back to that group, I always went back to that rule, naming it after the student who came up with it.)