# I am trying to determine the angular acceleration by applying the central difference technique?

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daniel choudhry on 28 Sep 2020
Answered: Nagasai Bharat on 1 Oct 2020
I am trying to applied the central difference technique formula into my code but I am having trouble with the output. I am trying to determine the angular acceleration of by using the knee angle. I need to plot my ang_acc vs time . I am also using this formula: (x_(i+2)-2x_i+x_(i-2))/(4∆t^2 )
fs = 69.9;
t = [1/fs:1/fs:length(ankle)/fs];
fc = [.1:.1:(fs/2)-.1]';
n = length(fs);
t=106;
%% Step 1: Calculate Knee Angle
% hip -knee represent the end points of the thigh segment angle
%fibula-ankle represent the end points of the leg segment angle
theta21_thigh = atan2(hip(:,2)-knee(:,2),hip(:,1)-knee(:,1));
theta43_leg = atan2(fibula(:,2)-ankle(:,2),fibula(:,1)-ankle(:,1));
theta_knee = theta21_thigh - theta43_leg;
%% Step 2: Angular acceleration of the knee (central difference):
angular_acc = ((theta_knee(3:end))-(2*theta_knee)+(theta_knee(1:end-2))) ./(4*(106).^2);

Sindar on 28 Sep 2020
is it throwing an error?
may be as simple as:
angular_acc = ((theta_knee(3:end))-(2*theta_knee(2:end-1))+(theta_knee(1:end-2))) ./(4*(106).^2);
(if you want the first and last point using forward/backward difference, you could just define those separately)
daniel choudhry on 28 Sep 2020
yes it is throwing an error

Nagasai Bharat on 1 Oct 2020
Hi,
The error is due to the difference in the sizes of the operands i.e., knee_angle has a size different from knee_angle(1:end-2) and knee_angle(3:end).
Please look into the dimensions to eliminate the error.