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The first example on the WhenEvent help page is

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}];

Say I want to stop the integration at the first bounce, I would put

NDSolve[{y''[t] == -9.81, y[0] == 5, y'[0] == 0, 
   WhenEvent[y[t] == 0,"StopIntegration"]}, y, {t, 0, 10}];

That, however, does not stop the integration (i.e. it goes on to t=10), neither does the condition y[t] < 0. What am I doing wrong?

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1 Answer 1

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Try this:

ySol = NDSolveValue[{y''[t] == -9.81, y[0] == 5, y'[0] == 0, 
   WhenEvent[y[t] == 0, {tMax = t, "StopIntegration"}]}, 
  y, {t, 0, 10}];
Plot[ySol[x], {x, 0, tMax}]

And in tMax you'll get 1.00964, that's slightly larger than t when y[t] = 0.

plot

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  • $\begingroup$ great, that works! $\endgroup$
    – jacob
    Commented Aug 7, 2014 at 13:14
  • $\begingroup$ @jacob You're code works too, I think you just want to plot to 10 and it will plot, because it's an InterpolationFunction and can plot outside the defined range. $\endgroup$
    – m0nhawk
    Commented Aug 7, 2014 at 13:15
  • $\begingroup$ ok thanks, that's what was confusing me $\endgroup$
    – jacob
    Commented Aug 7, 2014 at 13:28

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