Answer
1. a. Start with F = $m \frac{v^{2}}{r}$.
The string tension T provides the net force F, and the length L is the radius of the path.
$$T = m \frac{v^{2}}{L}$$
$$m=\frac{T L}{v^{2}}$$
b. Plug in values.
$$m=\frac{(10 N)(2 m)}{(2 m/s)^{2}} = 5 kg$$
