[?] ParametricPlot results of a diff eqn that are interpolating functions?

Questions
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1

In the attached file, I am solving a diff eqn numerically, and the results are interpolating fnc. I am calculating two other fnc (x and y positions of a baseball) with NIntegrate. I now want to do a parametric plot, but it does not work, although the x and y pos plot with no problem. I need help!

trajectory-quadratic-air-drag.nb (40.1 KB)

2

Adjustment.

trajectory-quadratic-air-drag ver 2.nb (60 KB)

3

Thanks! So, it is some memory effect of Mathematica? or some reserved symbols? I also noticed you added First @NDSolve.. What does this do?

4

Execute:

?First

and read the text then you see What does this do.