Composition

• A spring scale is basically a spring that is held at one end while an object is suspended from the other end of the spring. When an object is attached to the spring and released, its weight will cause the spring to stretch. The amount by which the spring stretches from its rest position is directly proportional to the weight of the attached object.
  
  

Theory

• If one end of a spring is kept fixed and the other end is pulled with a force of size F, the spring will stretch. The size of the stretch is the distance "d" that the movable end of the spring is displaced from its rest position. Hooke's law states that the force required to stretch the spring is directly proportional to the size of the stretch. This can be written as F = k x d, where "k" is the spring constant for the spring. The value of k reflects the stiffness of the spring. The harder a spring is to stretch, the larger its value of k.
  
  

Weight Units

• Weight is a measure of the size of the gravitational force acting on an object. Force and weight units are therefore identical. The most common imperial units for weight (or force) are ounces and pounds. The standard metric unit for a weight or force is the newton.
  
  

Mass Units

• The weight (w) of an object on the surface of the Earth is given by the mass of the object (m) times the acceleration of gravity (g) or w = m x g. Weight units are converted to mass units by dividing the weight by the acceleration of gravity in the appropriate units. Common metric units for mass are grams or kilograms. In the imperial system the mass unit is slugs, which is rarely used.
  
  

Unit Conversions

• The common units listed on spring scales are pounds, ounces, grams, kilograms or newtons. These units are easily converted from one type to another if the scale you are using is not calibrated in the units you prefer. Familiar conversions are 1 pound equals 16 ounces and 1 kilogram equals 1,000 grams. The equivalency that a 1 kilogram mass weighs 2.2 pounds is also well-known.
  
  Less familiar conversions involve changing metric force or weight units into metric mass units. Use w = m x g for these conversions. For metric conversions, the acceleration of gravity, g, is 9.8 meters per second squared. If this value of g is used, then the mass unit must be kilograms (kg) and the weight unit newtons (N). For example, a spring scale reading of 5.0 newtons gives a mass reading of m = w/g = 5.0/9.8 or 0.51 kilograms.