Unit 2 Constant Velocity

We started this unit by discussing how we can describe the motion of an object, specifically a battery powered buggy.  We started by making observations on the buggy and came up with the research question "Does the car move at a constant speed?"  We then proceeded to collect data by letting the red buggy run on the carpet and mark off its distance after every two seconds.   We asked whether we could use the motion detector or not and we were not able to.  We also could not use a video of the buggy to determine distance that way either.  We resorted to placing tape on the carpet at every two second intervals where one person ran after the car (it was moving rather slow) and placed tape on the floor to mark its position every two seconds.  We used a stopwatch and someone called out the two second intervals.  This was not an easy task, but it helped to form a visual of the constant speed the buggy was traveling at.  We had a visual sense that the buggy moved at a constant speed; however, we wanted data to support our claim.  We graphed a "Distance vs Time" graph and got a nice linear relationship.  We had a whiteboard discussion about this and noticed that everyone had a linear relationship and we started to discuss what the slope of the line would represent.  We moved into the discussion that distance was not a good term to describe how far the buggy traveled because, by definition, distance cannot be negative.  We discussed the difference between position and distance and defined position as "where you are relative to the zero reference line."  For the same reason, we discussed how velocity was a better description of how fast the buggy was moving because speed cannot be negative.  We know this by looking at the "speed-o-meter" on cars and they never read negative values.

We ended having a cognitive conflict when we did the next lab using a red fast buggy and a blue slow buggy.  We could use video from a tablet to help us get the position vs time graphs and then each group had a slightly difference scenario which lead to many different graphs.  Then we whiteboarded them and had a discussion about what was similar and what was different. 

We had to come up with "For every ..." statements because we had a linear relationship and we also discussed how the data was consistent with that because as you doubled the time the distance doubled, and so on.  We noticed the different graphs and different slopes and discussed how there was a positive direction and a negative direction in some of the graphs.  Some slopes were positive and some slopes were negative.  We also discussed about the 5% rule of thumb and also how there was meaning to some of the y-intercepts in these as well. 

Note:  One reason that we were not allowed to use the motion detectors was because we needed to come to the reasoning on our own that the slope of the "position vs time" graph is velocity.  The motion detector will automatically give you the velocity graph along with the position graph.

Next, we were told to make a velocity vs time graph based upon our data that we just gathered.

We noticed that this supported our original visual claim that the buggy's do travel at a constant velocity; however, velocity may be negative if it is moving in the negative direction.

More on Unit 2 in the next post :)