Terminal speed is the speed wherein an object falling will reach a constant velocity. At this point, Fg is equal to Fd. (Drag coefficient for a sphere is 0.47 and density for this object was said to be 1.25kg/m^3)
F_g = m x g
F_d = 1/2 x Cd x rho x A x v^2
1/2 x Cd x rho x A x v^2 = m x g
m = 1/2 x Cd x rho x A x v^2 / g
m = 1/2 x 0.47 x 1.25 x (.065^2/4) x pi x 28^2 / 9.81
m = 0.0779 kg
