Re: [Phys-L] Sequence

[Philip Keller <pkeller@holmdelschools.org>]

I spend weeks on this all as well!  It may be the thing that Hewitt said 
that I most agree with: kinematics can be a black hole right at the 
beginning of the course.

But what if the ONLY things you wanted to get across were:

PART I -- here is one kind of motion we care about.
1.  When an object moves at a constant speed in a straight line, its 
position graph is a line.
2.  In that case, the slope of that line stays constant.  That slope is 
the speed.
3.  Since the speed is not changing, the "velocity" graph is a 
horizontal line.

PART II--here is another motion we care about.
1.  Sometimes, velocity graphs increase or decrease in a linear manner.
2.  In that case, speed is changing.  The position graphs curve up 
(showing increasing speed) or down (showing decreasing speed).
3.  The slope of that linear speed graph is called "acceleration".

Suppose we stop right there (for now).  No equations.  No word 
problems.  Nothing about area under v vs t.  No discussions of positive 
vs negative velocity and acceleration.  No debates about the 
acceleration of an object at its peak.  No debates about "deceleration" 
vs negative acceleration.  No discussion of vectors. All of that can 
wait.  For now, velocity is just our fancy-pants word for speed.

That's what I think can be done in just a few periods.  But again, I was 
thinking out loud.  I have not tried this approach.



On 3/19/2014 8:22 PM, Marty Weiss wrote:
> 3 class periods?  really?  usually takes weeks to get those points across.
> all those other concepts in a short period of time?  months here!
>
> On Mar 19, 2014, at 7:53 PM, Philip Keller wrote:
>
> > Just thinking at loud here...
> >
> > What if you start by defining constant velocity and showing what its
> > position and velocity graphs look like.  Then define constant, non-zero,
> > positive acceleration and show what its position and velocity graphs look
> > like.  At this point, you have invested maybe 3 class periods if you go
> > slow.
> >
> > Then move on to forces, momentum, energy, circular motion, gravitation,
> > whatever you like, returning to kinematics as later, say before you teach
> > projectile motion but after you teach vectors.
> >
> > This way, you would have the vocabulary of kinematics in place but not the
> > equations.  You would get to say things like: when no unbalanced forces act
> > on an object, its velocity graph looks like this,or like this but never
> > like that.
> >
> > It feels to me that you could teach a lot of physics before you needed any
> > further kinematics treatment.
> _______________________________________________
> Forum for Physics Educators
> Phys-l@phys-l.org
> http://www.phys-l.org/mailman/listinfo/phys-l