The Physics of Scales

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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.

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This law states that the restoring force exerted by an elastic object is proportional to how far it has been distorted from its equilibrium (F= -k*x). Bloomfield states that all things obey Hooke’s law, whether you’re pulling on a rope or jumping on the trampoline. As long as the force applied doesn’t exceed the elastic limit, it will return to its original position. Once the force crosses that limit permanent damage occurs (p. 83).

A bathroom scale works similar to the produce scale. The difference is in the use of multiple levers under the pad area to stretch a spring which in turn registers the weight of the object on the scale. Spring scales are quite simple, but still there are problems that can cause inaccurate readings. One of these becomes evident when weighing large objects, such as a baby elephant. Not only is it a large animal that might overload the scale. It is impossible to get all four of its feet on the same scale. If you take into account the objects center of gravity, you can use multiple scales and then add the outcomes for the total weight. If the center of gravity of an object is closer to one scale than the others, the closer scale will register more of the weight and could possibly be damaged.

Another problem in spring scales is motion; all springs exhibit harmonic motion properties. So in order to accurately read a scale you must wait until there is not any motion present. If you fail to wait for motion to stop than the spring is going to be stretching and relaxing, thus causing the dial to fluctuate as well. Other large objects that need weighed are semi-trucks. Instead of driving the semis’ onto multiple bathroom scales, other scale systems have been developed.

These systems include scales using load cells and other similar devices. Measurement Specialists is a company that specializes in load cells, at their website
www.measurespec.com a load cell is described as “a force transducer that takes force and registers it into an electrical signal” (p. 2). A load cell is made up of at least one strain gage, which can be set to measure either the compression of or the tension on the load cell. Multiple strain gages can be wired together to form a Wheatstone-bridge configuration. Which has four legs and as “the input current is applied to the bridge; the output becomes a voltage proportional to the force on the cell” (p. 3). At www.travel.howstuffworks.com this strain gage is described as “a thin wire that transmits electrical current and is attached to the beam, ring or column” (p. 1). As a force is applied to the cell, the strain gage is compressed or altered and thus changes the resistance in the wire. The signal from each cell is transmitted to a junction box that measures the variance in the current and calculates the weight the scale is supporting (p. 1).

Along these same lines there are other scales that use similar techniques. These techniques are listed on the previous website. They are the bending plate system and a piezoelectric system. The first uses metal plates with a wire gauge to measure the amount of stress it is under. Next, the piezoelectric system has sensors in a conducting material that detect changes in the voltage and displays the load on the system (p. 2).
In the process of deciding what type of scale one needs, there are three different ways to get the weight of a truck. These include:

1)One-axle, where the truck must slowly move a
crossed the scale to weigh each set of axles.

2)One-stop, which weighs the truck all at once, all
it needs to do is come to a stop.

3)Weigh-in-motion, which is exactly what the name implies. Some systems have even been installed on highways and monitor the weight as the trucks are at speed (www.travel.howstuffworks.com, p. 2).

The application that you are buying a scale for should influence what type of method you need.

So whether you are weighing a watermelon for the family-get-together or transporting watermelons to the grocery store. Next time you’re at or on a scale, you will be able to answer the question asked by many inquiring minds. How do scales work?

Works Cited

Bloomfield, L. (1997). How Things Work: The Physics of
Everyday Life. New York: Wiley & Sons.

Cutnell, J. D., & Johnson, K. W. (2001). Physics (5th ed.).
New York: Wiley & Sons.

How stuff works, (2003). How do truck weigh stations work?. Retrieved April 15, 2003, from http://travel.howstuffworks.com/question626.htm

Measurement Specialists. (2001, June). Load Cell Overview. Retrieved April 10, 2003, from http://www.measurespec.com/tips/principles.htm


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