What force is required to keep a 10 mg water droplet moving in this circular arc?

The solution for this problem is:

With a mass of 10 mg or convert this into kilograms by:

We know that 1 mg = 1x10^-6 kilograms, so solving this will give us

10 * 10^ -5 in kilograms.

Then, we compute a force of ma F=ma where m is the mass and acceleration is m/s^2

F = 0.00001 [(2.5m/s^2)/ (0.16 m)] 

= 3.9 * 10 ^ -4 N or F=.0.0004 N