Remember that the Weight of any mass is
Weight = (mass) x (gravity)
and gravity on Earth is 9.8 m/s² .
Now just fill in the numbers we know:
49 Newtons = (mass) x (9.8 m/s²)
49 kg-m/s² = (mass) x (9.8 m/s²)
Divide each side by (9.8 m/s²) , and we have
mass = (49 kg-m/s²) / (9.8 m/s²)
mass = (49 kg-m/s²) x (s²/9.8m)
mass = (49/9.8) x (kg-m-s²/s²-m)
mass = 5 kg
