code: function Spring(fromBody, toBody, length, coeff, weight) {
      this.from = fromBody;
      this.to = toBody;
      this.length = length;
      this.coeff = coeff;
  
      this.weight = typeof weight === 'number' ? weight : 1;
  }
docs: Represents a physical spring. Spring connects two bodies, has rest length
  stiffness coefficient and optional weight