Writing differential equation for plot

Can anyone help me how to write differential below equation for plot

d2x/dt2 = -sign(x + dx/dt)

I tried in below way and it is failing

ode = diff(x,t,2) == -sign(x + diff(x,t));

Answer 1

[Ameer Hamza]

Try this

syms x(t)

d1x = diff(x);
d2x = diff(x,2);
ode = d2x == -sign(x + d1x);
cond = [x(0)==1 d1x(0)==0]
sol = dsolve(ode, cond)

This requires the Symbolic Math toolbox.

[vaibhav gupta]

I am getting error on running this way

Array indices must be positive integers or logical values.

Error in sym/subsref (line 898)

R_tilde = builtin('subsref',L_tilde,Idx);

Error in Test2 (line 10)

plot(sol(t));

[Ameer Hamza]

Use following line to create plot

fplot(sol)

[vaibhav gupta]

Thank you! What is difference between plot vs fplot? Why plot was failing?

[Ameer Hamza]

fplot() is used when you want to plot a function handle or s symbolic function. On the other hand, plot() requires x, y data points. To use the plot() function here, you will need to define an x vector and calculate the y-values yourself. fplot() take cares of all these calculations

syms x(t)

d1x = diff(x);
d2x = diff(x,2);
ode = d2x == -sign(x + d1x);
cond = [x(0)==1 d1x(0)==0];
sol = dsolve(ode, cond);

xv = linspace(-5, 5);
yv = subs(sol, t, xv);

plot(xv, yv);

[vaibhav gupta]

Thank you for explaining difference. If i want to use ode45 function, how can i do that?