# Day 50: projectile Whiteboardathon and Transitioning to v-t graphs

Today was a heavy WB post lab discussion.  Each group prepared a WB for the horizontal component (both the x-t and the vx-t graphs) and another for the vertical component.  The post lab discussion usually goes like this:

Hertting: So when we look at all the horizontal position graphs, what is similar about all of the them?

Ss:  They are all linear.

Hertting:  OK, that’s a really good thing right?

Ss:  ???

Hertting:  Why would it be a good thing that the horizontal position graph is linear?  What does it tell us?

Ss:  The horizontal component is constant velocity.

Hertting: Ba Bam!!!

Then we discuss why even though the graphs are all linear, they seem to be a few varieties.  The students usually see right away that this is dependent on where they set the origin (in the VA) and which direction the object was tossed.

On the the vertical component:

Hertting:  So when we look at all the vertical position graphs, what is similar about all of the them?

Ss: They are all non-linear.

Hertting:  Good, so what does that tell us?

Ss: The vertical component is not constant velocity.

Hertting:  Good, is it constant acceleration?

Ss;  Yes

Hertting: How do you know?

Ss:  The vertical position graph is non-linear.

Hertting:  So ANY nonlinear position graph is constant acceleration?

Ss: ??? Well, no…

Hertting: So, how can you tell if it is constant acceleration?

Ss:  Look at the vertical velocity graph.

Hertting:  And ….

Ss: Well, this one is linear so it has a constant slope and the slope of a velocity graph is defined as the acceleration.

Hertting:  Nice… what value is it?

Ss: Is it it freeall?

Hertting:  Is that what our data shows?

Ss;  Well sort of?

Hertting:  What might cause it not to be free fall?

Ss:  We still stink at Logger Pro?

Hertting: (he he) Well, maybe just a bit.  What gets int he way of some of our projectiles?

Ss:  Air.

Hertting:  Good.

SIDE BAR:  I may have paraphrased that a bit… but not too much… lots of leading questions.  Take a look at this board:

It was of a bounce pass with a basketball from the right to the left.  it is the horizontal position graph.  I asked the class where the bounce shows up in the graph?  After a bit, they realized it doesn’t because it is constant velocity the whole time.  The actual LP graph was pretty linear as drawn.

General Physics:

We reviewed the video analysis process as a group and then we transitioned to velocity-time graphs using the video analysis as I did with the advanced students.  This is no doubt the best method I have used to transition to the v-t graph.  FV Nov 2013–VA to Transition x-t to v-t is what I shared with the Phox Share group.  It is really slick.