First, it was the sniffles. Then the coughing started. When you woke up in the middle of the night with a burning fever, you knew what was in your future: a trip to the doctor's office!
After reading magazines in the waiting room, you're finally called back to see the doctor. You're greeted by a nice nurse who takes you to a room to get your vital statistics. After taking your blood pressure, you're asked to step onto a device to find out how much you weigh. What are we talking about? A scale, of course!
Scales are valuable tools that help us keep track of our weight. In addition to doctors' offices, you'll also find scales in most homes…probably behind the bathroom door, under the bed, or in the closet!
Have you ever WONDERed exactly how a scale works, though? You can step on a rug and it won't tell you how much you weigh. So how does a scale know how to convert the force you apply to it into a number of pounds?
Weight is simply the amount of force on an object due to gravity. Weight is different than mass, which is a measure of the amount of matter in an object. Your mass stays the same whether you're on Earth or the Moon. Since the Moon has less gravity than Earth, however, you weigh less on the Moon than you do Earth.
The earliest scale was the balance scale. Dating back over 5,000 years, it was likely invented in ancient Egypt. Balance scales worked by comparing an object of unknown weight to a standard, known weight.
Of course, when you go into the doctor's office or your bathroom, you don't step onto one side of a balance scale and see how you compare to a big reference weight on the other side. Instead, you probably step onto a device known as a spring scale.
If you were to open up a standard spring scale like you'd find in most bathrooms, you'd see that the platform you stand on is connected to a dial via levers and a spring. The more you weigh, the more the platform is pushed down, which in turn moves the dial farther.
The dial eventually points to a number line representing your weight based upon calculations that rely on Hooke's law. Hooke's law relates force (your weight) to how much the spring stretches.
Of course, if you have a newer scale, it may have a digital screen rather than a dial. Newer scales might still use springs, in which case the calculation based upon Hooke's law is converted to a digital number that's transmitted to the screen.
Many modern digital scales no longer use springs, though. Instead, they feature pressure sensors called piezoelectric transducers. These devices produce an electric current when compressed. The more you weigh, the more current that is produced. A connected electronic circuit measures the current and converts it to an equivalent weight measurement.