Suppose an object is moving in a circular path about a certain point $O$, with an angular velocity $\omega\hat{k}$ or linear velocity $v\hat{\theta}$. This is basically describing an anticlockwise orbit.
Now consider a frame, that is rotating anticlockwise at an angular velocity $\omega_0\hat{k}$. The origin $O'$ of this rotating frame coincides with the origin of our inertial frame. Let us try to find the velocity of the object in this new frame :
$$\vec{v_r}=\vec{v_i}-(\omega_0\hat{k} \times \vec{r})$$
This is where my confusion begins. Let $(\hat{r},\hat{\theta},\hat{k})$ be the unit vectors in our inertial frame $S_i$. Similarly $(\hat{r'},\hat{\theta'},\hat{k'})$ are the unit vectors in our rotating frame $S_r$. Since the new frame is rotating about the $z$ axis of the old frame, we can write $\hat{k'}=\hat{k}$
Let $\vec{r}_i$ be the position vector in the $S_i$ frame basis, and $\vec{r}_r$ be the position vector in the $S_r$ frame basis. We have :
$$\vec{r}=\vec{r}_i=r\hat{r}=r'\hat{r}'=\vec{r}_r$$
Plugging this into our expression for velocity, we get :
$$\vec{v}_r=\omega r\hat{\theta}-(\omega_0\hat{k}\times r\hat{r})=r\omega\hat{\theta}-r\omega_0\hat{\theta}=r(\omega-\omega_0)\hat{\theta}$$
Hence, we have managed to find an expression for velocity in the $S_r$ frame. However, this is still expressed using $\hat{\theta}$ which is a unit vector in the $S_i$ frame. How do I express this velocity in the rotating frame, in terms of the rotating unit vectors ?
I suppose, in this case, it is rather simple, as the body is moving in a circle in both the frames. However, suppose the body is moving in a random direction in the inertial frame. How do I find the velocity of the object in the rotating frame, and express it using the new unit vectors ? I suppose we'd then have to express both $\vec{v}_i$ and $\vec{r}$ using the new rotating vectors and use that in the expression. How do I do that? If I have been given the vectors in the inertial frame basis, how do I find their expressions in the rotating frame basis?
When we write $$\vec{F}_{centrifugal}=-m\vec{\omega}\times(\vec{\omega}\times \vec{r})$$, we insert $\vec{\omega}=\omega\hat{k}$ and $\vec{r}=r\hat{r}$. Then we get the expression :
$$\vec{F}_{centrifugal}=m\omega^2 r\hat{r}$$
In this expression, isn't $\hat{r}$ and $\hat{k}$, unit vectors of the inertial frame ? If so, how do we express the centrifugal force and other forces using the unit vectors of the rotating frame ? How do I find a relation between $\hat{r}$ and $\hat{r}'$ ?
Any help understanding this would be very highly appreciated. Thanks.