At the top of the loop, the forces you experience are gravity and the normal reaction force from the cart. Here, these are both pointing directly downwards. The total centripetal force experienced due to the circular motion is therefore equal to the sum of these:
$$F_c = F_g + F_n \Rightarrow \frac{mv^2}{r} = mg + F_n$$
When the cart's at the smallest possible speed with you remaining in the cart, we have that at the top of the loop $F_n = 0$ i..e your seat is exerting no force on you. At this minimum speed: $$\frac{mv^2}{r} = mg \Rightarrow v^2=gr \Rightarrow v=\sqrt{gr}$$
In our example, $v=\sqrt{10g}\mathrm{ms}^{-1} \approx 10\mathrm{ms}^{-1}$.
Note this answer is independent of mass.