"Error using plot, Value not a numeric scalar"

clc, close all
c=12.5; % kg/s drag coefficient
M=70; % kg mass
g=9.81; % m/s^2 gravity
delta_t=0.1; % s time step
t=0:delta_t:12; % s time overall 
x=0:500; % m displacement
n=length(t);
for i=2:n
    c1=(x(i+1)-x(i))/delta_t; % first der central
    c2=(x(i+2)-2*x(i+1)+x(i))/(delta_t.^2); % second der central
    y1=c2+((c./M).*c1)-g; % central difference 
    f1=(x(i+1)-x(i))/delta_t; % first der forward
    f2=(x(i+2)-2*x(i+1)+x(i))/(delta_t.^2); % second der forward
    y2=f2+((c./M).*f1)-g; % forward difference
    b1=(x(i)-x(i-1))/(2*delta_t); % first der backward
    b2=(x(i+1)-2*x(i)+x(i-1))/(delta_t.^2); % second der backward
    y3=b2+((c./M)*b1)-g; % backward difference
end
plot(t,y1,'b','linewidth','2')
hold on
plot(t,y2,'r','linewidth','2')
hold on
plot(t,y3,'k','linewidth','2')

Answer 1

[the cyclist]

The LineWidth parameter should just be the number 2 (without quotes), not the character '2'.

After that, you are going to run into the problem that in your for loop, you are not creating a vector y1. You are instead overwriting a single scalar value of y1 in each iteration of the loop.

Answer 2

[Asad (Mehrzad) Khoddam]

it should be y1(i), y2(i). or y3(i)