Unable to calculate the second derivative of a function

I want to calculate the second derivative of a function. Below are the steps I am following-

syms a b;
th = sym('th(t)'); %th is a time dependent variable
y = diff(a^2*cos(th)+(b/12)*sin(th));
thd = diff(th); %derivative of th wrt time
ybythd = diff(y,thd); %derivative of y wrt thd
p = diff(ybythd); %derivative of ybythd wrt time

It calculates the following value of p

p = diff(diff((b*cos(th(t))*diff(th(t), t))/12 - a^2*sin(th(t))*diff(th(t), t), t), diff(th(t), t))

I do not see any term of d2th/dt2 in the above result. What is missing?

[Andriy Kavetsky]

y = diff(a^2*cos(th)+(b/12)*sin(th),th,2) %second derrivative ////////////////////////////////////////// y = diff(a^2*cos(a)+(b/12)*sin(a),a,2) %second derrivative from a and its result

y =

2*cos(a) - 4*a*sin(a) - (b*sin(a))/12 - a^2*cos(a)

Answer 1

[Marc Jakobi]

change

th = sym('th(t)'); %th is a time dependent variable

to

th = sym('th'); %th is a time dependent variable