FFT, derivative,acceleration

I have displacement results.I am using Matlab and my friend is using Phayton.

We are plotting Acceleration-Frequency graphs and they are different and his graph is really closed to experiment results.

For acceleration-Hz graph I am using FFT to change time to Hz.

And I think maybe I am doing sth wrong when I am applying FFT.

If someone can check, It can be very helpful.

   ts=1/t_step;
   N1=2^nextpow2(length(acc_theta));
   freq1=(0:1:(N1-1)/2)*ts/N1;
   ft1=fft( acc_theta,N1)/N1*2;
   fft1=abs(ft1(1:floor(N1/2)));