Mathematica How to find the value for phi at a specific time in Mathematica?

[bruce999]

Hello again.

I have a solution for a simple differenttial eqn. "Simpletreb=First@NDSolve[{phi''[t] == -d
Sin[phi[t]], phi[0] == (3π/4), phi'[0] == 0}, phi[t], {t, 0, 1}]"

After some plots and other calculations i need to find the value for phi at t=0.224511 so i input "phi[0.224511]/.simpletreb" but this just outputs "phi[0.224511]". I've also tried "Evaluate[phi[0.224511]/.simpletreb" with no joy.

My .nb file is here: (you have to cut and paste the link I'm afraid) "http://bruce1711.150m.com/simple trebuchet (first) final.nb"

Any ideas anyone?

 

[saltydog]

Hello again.

I have a solution for a simple differenttial eqn. "Simpletreb=First@NDSolve[{phi''[t] == -d
Sin[phi[t]], phi[0] == (3π/4), phi'[0] == 0}, phi[t], {t, 0, 1}]"

After some plots and other calculations i need to find the value for phi at t=0.224511 so i input "phi[0.224511]/.simpletreb" but this just outputs "phi[0.224511]". I've also tried "Evaluate[phi[0.224511]/.simpletreb" with no joy.

My .nb file is here: (you have to cut and paste the link I'm afraid) "http://bruce1711.150m.com/simple trebuchet (first) final.nb"

Any ideas anyone?

In general:

solution=NDSolve[{y''[x]=such and such},y,{x,0,end}];

f[x_]:=Evaulate[y[x]/.solution];

Now, can use f[x] like any other function.

 

[bruce999]

Ok i tried that and i still get:
"Evaulate[phi[0.22]]" as the answer instead of a value.

 

[bruce999]

Got it now thanks Saltydog.