This page, while of technical interest, is not required for the execution of the Daredevil Design Project.
There have been several assumptions made with regards to the format of the 'best-fit' curve, and some other mathematical properties of the lesson, which you are here to have explained.
To begin, there is the question of why was
.00128*x^2+6.6
, a quadratic equation, chosen to fill-in the
'best-fit' curve in DataFlyer? Well, it's not just because it happens to be
right for the test data. The initial equations for the marble's run are as
follows:
PEa + KEa + KEra =
κ(PEb)
Where κ
is some fraction of the initial
potential energy. All energies on the left are subscripted 'after', including
the kinetic energy subbed 'rotational'. The potential energy on the right is
'before'. Converting the equations into its longer form:
mghl + ½mv2 + KEr
= κmghi
Now, factor m
& g
:
hl + (½v2)/g +
KEr/gm = κhi
Now, solve for hi
:
hi = 1/κ(hl +
1/2g(vl2) + KEr/mg)
= 1/2κg(Vl2) + C
With constant C
taking over for
KEr/mg
, since the rotational energy lost is more or
less constant across multiple runs. The rotational loss is really a part of
ΔE
, the change/loss in energy in the system. Some of that
loss is also in the form of sound and heat, but that has been ignored for our
purposes. We're not really worrying about the loss due to rotation either,
since it is also more or less constant through multiple runs.
Now, you should be able to see where the
x2
term for the quadratic comes in: from the
Vl2
term. And, the initial fraction
.00128
is the decimal result of 1/2κmg
. Knowing
that gravity, the marble's mass, and 2 do not change, we could solve for
κ
. The large constant C
turns out to be
6.6
in our test runs.
Now that you have the background for the chosen 'best-fit' terms, you can go back to the web lesson.