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I am new to mathematica and I am having this issue in this piece of code. 

x=0.1Sin[[Pi]/8t+[Pi]/4] (* this is my original function*)
v=D[x,t] (* took the 1st derivative and got 0.0392699Cos[pi/4+pi/8t)

Once I got the expression for the 1st derivative, I solved for the equation and got (-6,2) as a solution. The problem starts when I take derivative of v to get an expression for acceleration. a=D[v,t] The above code gives me -6 is not valid variable error. Any kind of help would be greatly appreciated. FWIW,I am doing this in mathematica online.

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Here is one way to be explicit about what is a variable and what is a function:

Clear[x,v]
x[t_] := 0.1 Sin[Pi/8 t + Pi/4];
v[t_] = D[x[t], t]

Now you can get the acceleration by:

D[v[t], t]
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  • $\begingroup$ thank you for the reply. took your advice and defined my functions, everything was good up until acceleration. It either gives me 0 or some kind of pattern error. Is there something I am missing? $\endgroup$
    – Isaac Ayele
    Sep 6, 2018 at 17:23
  • $\begingroup$ When I type D[v[t], t], I get -0.0154213 Sin[[Pi]/4 + ([Pi] t)/8]. If you are getting something different, restart the kernel (Evaluation->Quit Kernel->Local). $\endgroup$
    – bill s
    Sep 6, 2018 at 17:46
  • $\begingroup$ this is a little strange. I can reproduce the solution whenever I skip using the print function for the two solution I got. When I use the print function, Print["(c)solution=", {t = -6, 2} "s"]; as soon as I add the print function I get the error message. Is there a better way to print the two solutions? $\endgroup$
    – Isaac Ayele
    Sep 7, 2018 at 13:55
  • $\begingroup$ I guess you must be using Print incorrectly. But why Print anything? Why not just look at or list the values you want? $\endgroup$
    – bill s
    Sep 7, 2018 at 14:17
  • $\begingroup$ because the prof. wants us to use the Print function to display our answers. This seems to be working for now, print["(c) Solution= ", Solution]. Thank you for all your help. $\endgroup$
    – Isaac Ayele
    Sep 7, 2018 at 14:32

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