I have been trying to find a solution to this, but I never reach same one that my professor provides. The problem is the following:
For an object of mass m standing still on the surface of the Earth, find what the planet's rotational speed ω should be so that the apparent weight is equal to the "real" weight (W=mg). The trivial solution (ω = 0) isn't valid.
This comes from a chapter about non inertial reference frames. The exercise doesn't mention anything about latitudes.
Since I know that the apparent force exerted on a body is equal to the real forces minus the fictitious forces, all I can come up with is: $$F_{apparent} = F_{real}-F_{fictitious} = W + N - (F_{centrifugal})$$
(W being weight, N being the normal force, and $F_{centrifugal}$ being the centrifugal force). There is no Coriolis force since the object isn't moving with respect to the Earth's surface. By setting that the apparent foce should be equal to the weight, I get: $$F_{apparent}=-mg\hat{r}+mg\hat{r}+m\omega^2r\hat{r} \Rightarrow -mg\hat{r} = -mg\hat{r}+mg\hat{r}+m\omega^2r\hat{r}$$ Therefore, $$\omega=\sqrt{g/r}$$
But the solution given by my professor is $\omega = \sqrt{2g/R_{Earth}}$, which is $\sqrt{2}$ times my solution. Where did I go wrong?