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This is an example in Mathematica's help document.

NDSolve[{y''[t] == -9.81, y[0] == 5, y'[0] == 0,  
   WhenEvent[y[t] == 0, y'[t] -> -0.95 y'[t]]}, y, {t, 0, 10}];
Plot[y[t] /. %, {t, 0, 10}]

I do a modification like this:

NDSolve[{y''[t] == -9.81, y[0] == 5, y'[0] == 0, a = y[t]; b=y'[t];
   WhenEvent[a == 0, b -> -0.95 b]}, y, {t, 0, 10}];
Plot[y[t] /. %, {t, 0, 10}]

It doesn't work.

If I want to keep a new symbol just as here a to represent the y[t] or some expression about y[t] in the WhenEvent, what can I do to achieve this goal?

(The error may be caused by the HoldAll attribute of WhenEvent. Why I want to replace the y[t] by a? It is important to me. Bucause I will use a dynamic a in the WhenEvent )

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If you want to update a why not use a ParametricNDSolve? It might be good, if could show what you want to do. –  user21 Jul 29 '13 at 6:40

1 Answer 1

up vote 1 down vote

It's not quite clear what you are looking for, how about using a function:

test[a_?NumericQ] := a + 1
NDSolve[{y''[t] == -9.81, y[0] == 5, y'[0] == 0, 
   WhenEvent[test[y[t]] == 0, y'[t] -> -0.95 y'[t]]}, y, {t, 0, 10}];
Plot[y[t] /. %, {t, 0, 10}]
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