Gravitational force (weight) = (mass) times (acceleration of gravity).
On or near the surface of the Earth, the acceleration of gravity is
about 9.8 m/s².
Within that region . . . (500 newton) = (mass) x (9.8 m/s²)
Divide each side by 9.8m/s²: Mass = (500 newton) / (9.8 m/s²)
Mass = 51 kilogram (rounded)