gnu units can do that

  You have: .5 * (10 m/s^2) * (15 s)^2
  You want: 
          Definition: 1125 m
  You have: sqrt((6.673E-11 N*m^2/kg^2) * (5.98E24 kg) / (6780 km))
  You want: 
          Definition: 7671.783 m / s