1 eV = 1.6 * 10^-19 Joule
10^20 eV = 16 Joule = 16 kg*m^2/s^2
1 tennis ball = 57 g = 0.057 kg
E = 1/2 m * v^2
v = (16 * 2 / 0.057)^0.5 m/s = 23.7 m/s = 53.3 mile/hour