It is generally a good idea to gain an understanding of the \"size\" of units. consider the following objects and calculate their kinetic energy in si units.

2.53 mph = 2.53*1760 yards/hr = 2.53*1760/3600 yards/second = 1.2369 yards/second 
1 yard = 36 inches = 36*2.54 cm = 36.2.54/100 m = 0.9144 m 
1.2369 yards/second = 1.2369*0.9144 m/s = 1.131 m/s 
34.00 milligrams = 34,00*10^-3 g = 34.00*10^-6 kg = 3.400*10^-5 kg 
momentum = mass*velocity = 3.400*10^-5*1.131 = 3.8454*10^-5 kgm/s (= 3.85*10^-5 kgm/s to 3 sig figs)