If an object is falling, then it is only experiencing gravitational force. The equation for gravitational force is:
Fg = mg
In which...
Fg = the force of gravity measured in Newtons
m = the mass of the object, measured in kilograms
g = the acceleration due to gravity, which is 9.8 m/s² ([dn])
Identity the variables you have.
Fg = 5N
m = ?
g = 9.8 m/s²
Rearrange the gravitational force equation for m and then substitute values.
Fg = mg
Fg / g = m
5N / 9.8 m/s² = m
0.51 kg = m (significant digit rules)
Therefore, the mass of the object is 0.5 kg