Find the mass of a body if a spring with a stiffness of 30 N / m stretches 4 cm under its weight?

k = 30 N / m.

x = 4 cm = 0.04 m.

g = 10 m / s2.

m -?

The condition for the balance of a body on a spring is the balancing of all the forces that act on it. The body is acted upon by the force of gravity Ft, directed vertically downward, and the force of elasticity of the spring Ffr, directed vertically upward.

Fт = Fпр – equilibrium condition.

Let us express the force of gravity by the formula: Ft = m * g.

We express the elastic force according to Hooke’s law: Fel = k * x.

m * g = k * x.

m = k * x / g.

m = 30 N / m * 0.04 m / 10 m / s2 = 0.12 kg.

Answer: body weight is m = 0.12 kg.