MATLAB: Problem with syms and symfun

Question

Hi guys,

I had my first code run which gives me the equations of motion.

eqx1 = M1*y1dd - F + K*(s0 + y1 - y2 - r*sin(ph))
eqx2 = F + M2*y2dd - K*(s0 + y1 - y2 - r*sin(ph))
eqx3 = I*phdd + F*r*cos(ph) - K*r*cos(ph)*(s0 + y1 - y2 - r*sin(ph))

where all the variables were defined as symm

syms M1 M2 I K;
syms t y1 y1d y1dd y2 y2d y2dd ph phd phdd;

Now, I tried replacing the y1d, y1dd, y2d, y2dd etc with diff(y1,t), diff(y1,t,2) respectively.

fx1=subs(eqx1,{y1,y1d,y1dd,y2,y2d,y2dd,ph,phd,phdd},{y1(t),diff(y1,t),diff(y1,t,2),y2(t),diff(y2,t),diff(y2,t,2),ph(t),diff(ph,t),diff(ph,t,2)})==0

Output:

 fx1=M1*D(D(y1))(t) - F + K*(s0 + y1 - y2 - r*sin(ph)) == 0

now when I run

diff(fx1), i get 
>> diff(fx1)
ans =
K == 0
>> diff(fx1,t)
ans =
M1*diff(D(D(y1))(t), t) == 0

Now, I tried directly defining f (same as fx1)

>> f = M1*diff(y1,t,2) - F + K*(s0 + y1(t) - y2(t) - r*sin(ph(t))) == 0
f(t) =
M1*D(D(y1))(t) - F + K*(s0 + y1(t) - y2(t) - r*sin(ph(t))) == 0
>> diff(f)
ans(t) =
M1*D(D(D(y1)))(t) - K*(D(y2)(t) - D(y1)(t) + r*D(ph)(t)*cos(ph(t))) == 0

how do I get the above for fx1.

How do I come from fx1 to f

Answer 1

syms fx1(t) 
fx1(t)  = subs(....)