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I am trying to solve a differential equation that changes its behaviour depending on the value of the dependent variable. I've tried as shown below, but the computer does not understand my If clause as an "equation or set of equations".

NDSolve[{If[x[t] > 0, x''[t] == -x[t], x''[t] == -2*x[t]], x[0] == 1,      
x'[0] == 0}, x, {t, 0, 10}]

How can I make this work?

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    $\begingroup$ You can surely use WhenEvent[] as @anders describes, but otherwise, your If[] is not properly structured: NDSolve[{x''[t] == If[x[t] > 0, -x[t], -2 x[t]], x[0] == 1, x'[0] == 0}, x, {t, 0, 10}] $\endgroup$ – J. M.'s technical difficulties Feb 3 at 15:30
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You can use WhenEvent (note the use of DiscreteVariables):

sol = NDSolve[{x''[t] == a[t]*x[t], x[0] == 1, x'[0] == 0, a[0] == -1,
               WhenEvent[x[t] >= 0, a[t] -> -1], 
               WhenEvent[x[t] < 0, a[t] -> -2]},
       {x, a}, {t, 0, 10}, DiscreteVariables -> {a}]

Plot[a[t] /. sol[[1]], {t, 0, 10}]

enter image description here

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