Weight of a 70 kg person on Earth using g = 9.8 m/s^2

Assumptions: the person is near Earth's surface and g = 9.8 m/s^2.
Use W = mg.
W = (70 kg)(9.8 m/s^2)
W = 686 kg*m/s^2
Since 1 kg*m/s^2 = 1 N, the person's weight is 686 N.