1 | initial version |

The problem is that `diff(a_V_ra,t)`

is giving you a column vector and `vector([0,0,-g])`

is giving you a row vector. Further, Sage sees the first as a matrix and the second as a vector. You can fix this using `.column()`

as you did earlier in your code.

```
a_a_ra = diff(a_V_ra,t)+vector([0,0,-g]).column()
```

gives:

```
[ cos(psi)*D[0](v)(t)]
[-sin(psi)*D[0](v)(t)]
[ -g]
```

My full code is:

```
var('psi t g')
v = function('v',t)
R_ra=matrix([[cos(psi),sin(psi),0],[-sin(psi),cos(psi),0],[0,0,1]])
r_V_ra = vector([v,0,0])
a_V_ra = R_ra*r_V_ra.column()
a_a_ra = diff(a_V_ra,t)+vector([0,0,-g]).column()
```

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