[Phys-L] Re: Energy is primary and fundamental?

[Robert.Cohen at PO-BOX.ESU.EDU (Robert Cohen)]

On Wednesday, August 17, 2005 5:27 PM, Brian Blais wrote:

> To do the problem above (which I wouldn't just jump into) I
> would do the
> following:
>
> v_avg=(vf+vo)/2 = vf/2  (in this case)
>
> mgh + 0 = 0 + 1/2 m vf^2    (eq 1)
>
> v_avg*t = h = vf *t/2
>
> solve for vf, plug into (eq 1) and solve for h.  I would plug
> in the value of "t" early, to reduce the number of symbols.

I was going to comment on how how one knows that v_avg=(vf+vo)/2 = vf/2
(in this case) but I see just now that rlamont has beaten me to it.

Personally, I find that forces are useful when I need to know the
*direction* of motion and not just the magnitude.  Consider, for
example, the classic projectile problem.  Of course, one could argue
that such a problem is not real useful for certain students.  For this
reason, I also think energy is a useful starting point for a
one-semester non-majors course but I like the force-first approach for
the two-semester sequence (since we will eventually get to both ideas).
Like others, I have found that students have difficulty with vectors and
acceleration but wouldn't they have the same problem regardless of which
method you use?  Besides, it has become less and less of a problem as I
continue to redesign my sequence (e.g., I cover forces before vectors
and acceleration).

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301