Weights and balances

Last updated: April 24, 2009.

How many times do you weigh things in the course of a day? If you're on a diet, chances are you stand on scales each morning to check your progress. If you're having breakfast, you might weigh out what to eat by filling a bowl with cereal. If you're sending a letter or a parcel, you probably take it to a Post Office to have it weighed. When you buy things from a grocery store, the price you pay for most goods will be based on their weight. In some countries, even the money in your pocket is based on a system of weight. (British and Irish pounds, for example, originally weighed exactly one pound.) With so much of our world driven by weight, accurate ways of weighing are very important. Let's take a closer look at the world of weight!

Photo: These traditional scales are on a railroad station. They can weigh objects up to 192kg (24 stones) —enough to hold about two average adults.

Weight and mass

Before we go any further, let's be clear about the difference between weight and mass. Most of the time, when we're talking about weight, we actually mean mass. Kilograms, pounds, stones, ounces, and grams are all units of mass, not weight. So what is the difference?

• Mass is the amount of matter something is made from. Big things are generally more massive than small ones. If you have a lump of iron or copper and take it to different places on Earth (or even the Moon) to measure it's mass, you'll always get the same result.
• Weight is a measurement of how much the force of gravity acts on a mass of a given size. The force of gravity varies slightly all over Earth so, while your lump of iron has the same mass, its weight varies: it might weigh a little bit more in Bangladesh than it does in Tibet. What about on the Moon? Gravity is about one sixth the strength on the Moon as it is on Earth. So things weigh only one sixth as much on the Moon as they do on Earth, even though their mass is exactly the same in both places.

Photo: Your mass is exactly the same on Earth and on the Moon. Your weight is about six times more on Earth because Earth's gravity is six times greater. Photo by courtesy of Great Images in NASA.

If you use metric (and SI) units, you measure mass in kilograms (kg) and weight in newtons (N) and convert mass to weight by multiplying by approximately 10 (because the strength of gravity on Earth is roughly 10 newtons/kg). Most of the time, it's acceptable to refer to weights in mass units (such as kilograms or pounds) because any mass on Earth converts to a weight in pretty much the same way. You never hear people say things like "I weigh 700 newtons" even though—scientifically speaking—they really should!

How can you measure weight?

You can find something's weight by measuring how much gravity acts on it using an instrument called a balance. Old-fashioned pan balances (sometimes called scales) literally involve balancing two scale pans with known weights in one pan and the item you want to weigh in the other. In a slightly different kind of balance called a steelyard, you hang a pan from one end of a metal arm and move a weight along the other end, much like a see-saw, until you find a balance point. Steelyards were invented in Roman times but are still used today. Physicians and nurses still use them to weigh small babies.

Photo: More ways to weigh. Left: using a steelyard to measure the weight of letters. You put the letters on the pan, move the sliding weight until the arm is horizontal, and then read the weight off the scale. Right: A young child stands on the glass platform of some electronic scales. These show your weight immediately on the digital readout at the top (the bottom of the photo as you look at it). Photos by Joshua Adam Nuzzo courtesy of US Navy.

Many cooks use spring balances instead of pan balances and weights. You place an item to be weighed on the top of a moving platform and it pushes downward, compressing a spring inside and turning a pointer around a dial. Before you use a balance like this, you have to be sure the pointer is aligned with the zero mark by turning an adjustment screw.

Even more convenient than spring balances are electronic balances that give weights instantly as a digital readout. Scales people use to weigh themselves often work this way. You stand on a platform and your weight, pushing down, compresses a pressure sensor called a piezoelectric transducer. This is a kind of crystal that makes an electric current when you squeeze it: the harder you push, the more current it makes. So the heavier you are, the more current flows in the transducer. An electronic circuit connected to the transducer measures the current and converts it into a weight measurement in kilograms, pounds, stone, or any other unit you select.

Photo: An electronic top-pan balance measures accurately and shows the result on a digital display. Photo courtesy of NASA Glenn Research Center (NASA-GRC).

Large things (such as trucks) are obviously much too big to weigh with ordinary scales or balances, but it's still important to weigh them to check, for example, that they're not too heavy to carry on airplanes or ships. Trucks are weighed by driving them onto metal roadways called weighbridges, which are supported by hydraulic rams. The heavier the truck, the greater the force on the rams and the harder they have to push upward to balance the truck's weight exactly. You can calculate the truck's weight from the hydraulic pressure of the rams. If you know the truck's kerb weight (unladen weight), which is often painted on the side of the vehicle, you can easily calculate the weight of its cargo by subtraction.

Photo: It's important to know what units you're using when you weigh things. Most scales have two dials that give you a measurement in either metric units (such as kilograms) or Imperial ones (pounds and stones). On these scales, you read the red scale to measure in kilograms and the black scale for stones and pounds.