The Physics of Scales

The Physics of Scales

To the dismay of some and a necessity for others, scales can be found about everywhere in today’s society. It never fails; if you go to the doctor you will step on the scale. They are at the grocery store, in most bathrooms and even in some of our favorite stores. Were we might even pay a quarter to have a machine tell us our weight, out loud. We as a society are obsessed with our weight. You’re asked for your weight when renewing your driver’s license. Every time you get in an elevator and see the little sign saying do not exceed *** lbs, the quarter-pounder with cheese and milkshake you ate for lunch, is brought back to your memory. A scale receives more mental and physical abuse than any other appliance that has ever been invented. All this abuse stems from a lesson each of use were taught at a young age, which is that we should always tell the truth. Well let’s look a little deeper into the scale and see the physics involved in how two different types’ of scales weigh objects.

There are two general kinds of scales. The first is a spring scale and the other type uses a load cell to electronically register a weight. Spring scales are the most common type of scale. The scale in your bathroom and those found in the produce department of your favorite grocery store are examples of spring scales.

This summer when you go to weigh that fat juicy watermelon, think about the mechanics of how the scale works. The basket is attached to a spring that stretches in response to the weight of the melon or other objects placed in it. The weight of the melon creates a downward force. This causes the spring to stretch and increase its upward force, which equalizes the difference between the two forces. As the spring is stretched, a dial calibrated to the spring registers a weight. When designing scales one needs to take into account that every spring has a different spring constant (k). Bloomfield (1997) defines k as “a measure of the spring’s stiffness. The larger the spring constant-that is, the stiffer the spring-the larger the restoring forces the spring exerts” (p. 82).

In analyzing the force associated with a certain spring, whether it is in you pen or under your truck, Hooke’s Law applies.