Hooke's Law
Spring scales use a spring that stretches in response to a weight according to Hooke's law of elasticity. This law holds that the length of the spring will be directly proportional to the weight so long as the weight doesn't exceed the elastic limits of the spring. This relationship may be expressed mathematically as F = -kx where F is the force exerted by the weight, k is a constant for the spring being used and x is the increase in the spring's length from its equilibrium position.
Preparation
A spring scale is commonly used in a doctor's office. Traditional spring scales use a traditional dial display and newer units use a digital display. A spring scale should lie on a flat, hard surface to ensure consistent readings. The doctor may ask you to remove your shoes and any heavy clothing for a more accurate result. You'll usually have an empty stomach when you're weighed in a doctor's office so this measurement should be your minimum weight during the day.
Operation
The doctor will instruct you to step on the scale, distribute your weight evenly and stand still. Once the weight reading on the scale stabilizes for the most part, the doctor may then read the display. Your weight will be the number that scale's arrow points to for a dial display. If the scale uses a digital display, your weight will be the displayed number. The doctor may weigh you more than once and average the results to reduce the measurement error of the scale.
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