Many people are confused about the differences between mass, weight and force. This comic-book video animation series from the BBC explains what mass is, and how it differs from weight.

Children learn fastest in a relaxed atmosphere, so this is an excellent way to introduce the concept of mass. It’s important to understand that an object’s mass does not change with its shape, size or location – but its weight does depend on the strength of gravity in different places.

Definition

Mass, measured in kilograms (kg), is the amount of matter in an object. It is a measure of substance, unlike weight, which depends on the gravitational force of the Earth and can vary depending on where you are in the universe.

You might be surprised to learn that a pineapple and a baseball bat have the same mass! It is also important to understand the difference between mass and weight. Kids are naturally inquisitive, so introducing them to these concepts early will help them understand more difficult topics like mathematics and physics later on.

The international prototype kilogram, kept at NIST, is the standard unit for mass measurement. Other units that can be used for mass are grams and tonne. The kilogram is part of the metric system and an essential measurement for global trade.

Methods

Traditionally, very small masses and forces have been measured with sets of carefully calibrated metal weights. This approach is no longer practical for many modern applications such as manufacturing, pharmaceutical development and spacecraft sample-collection missions.

Alternatively, mass can be measured with a balance. The unknown mass of a body is contrasted against the known estimation of its mass to determine the value of an unknown force (F) required to produce that mass (m). A balance also works in space and places with no gravity as changes in gravity influence both masses on the scale equally.

NIST scientists are developing a highly accurate method of measuring extremely small masses and forces without calibration weights using radiation pressure that a laser beam exerts on an attached high-reflectivity mirror, balancing this force against an electrostatic force on a capacitor. This system is expected to be used as the new standard for the kilogram in 2018.

Accuracy

The most common way to measure mass is with a balance. We use balances all the time to weigh things. However, the scales don’t always work the same on different planets. That’s because of the forces of gravity that are acting on the mass.

Scientists and engineers know that there are two distinct concepts to consider when taking measurements: accuracy and precision. Accuracy describes how close a measurement is to the true value of a quantity, while precision refers to the degree to which repeated measurements under identical conditions show similar results.

In manufacturing, for example, high accuracy is important because it allows parts to fit together easily and smoothly. But consistency is more critical, so manufacturers often choose to focus on precision rather than accuracy.

Applications

Mass measurement is a vital part of daily life and technology. For example, hardware items are often sold by weight, influencing pricing and portioning. Food manufacturing and pharmaceuticals rely on precise measurements for quality control. Farmers adjust fertilizer applications based on mass analysis results.

Traditionally, the most common method of measuring mass is with a balance. A balance compares an unknown mass with a known one, reading the result as a force in newtons that counteracts gravity. This approach works in space and also in places where gravity is a factor.

Other techniques measure a sample’s change of mass as it goes through a deposition, etch, or clean process. Such dynamic measurements can detect shifts in chemical equilibria or reveal assembly/disassembly steps of complex molecules.

Future developments

In many fields of science, mass measurement is essential for accurate and repeatable experiments. It is the foundation of discoveries in physics, biology and chemistry. It also drives industrial efficiency, product quality and safety. For example, it is critical for the precise administration of drug dosages in healthcare, which impacts patient safety and treatment efficacy.

NIST scientists helped redefine the kilogram, the fundamental unit of mass in the International System of Units (SI), in 2019. The new definition replaces a physical object—a hunk of platinum-iridium alloy called Le Grand K that’s kept in France—and is based on fundamental properties of nature that are inherently stable over time.

Researchers are developing instruments to directly measure these constants, such as devices known as watt balances and methods that use atom-counting techniques like charge detection mass spectrometry or nanoelectromechanical systems mass spectrometry. These developments will enable even more precise mass measurements.

Weighing is an essential process used in a variety of industries. From checking the correct number of items in a box to making sure that kegs and gas cylinders are filled, weighing is used across all stages of production.

It is crucial that all users follow proper weighing procedures to ensure accuracy and avoid error. Some factors that impact weighing precision include air currents, temperature changes, electrical interference and vibration, as well as static electricity.

Weight determination

It is important to thoroughly assess manual interventions in weighing processes before transitioning to automated systems. This helps reduce potential for gaps in efficiency that automation might introduce, and ensures that new weighing systems deliver the desired results. This process allows managers to recognize areas that would benefit most from the addition of automation and pinpoint inefficiencies that could be resolved by introducing automated systems.

For example, many manufacturers use a weighing system to measure the level of material in silos or tanks. This information is used to determine when it is time to reorder materials. In these applications, it is crucial that the weighing system be accurate enough to provide a highly reliable level indication.

The most critical factor in achieving this is the minimum weight threshold – a rule that dictates the accuracy that can be achieved with a particular instrument. This is different from legal tolerances, which consider only manufacturer specifications and do not take into account user-specific weighing requirements.

Check weighing

Check weighing is a common production process for food manufacturers. It allows you to verify that the product weight matches the label on the package and ensures compliance with government regulations. It also helps you minimize product waste and maximize plant efficiency.

A checkweigher is a static or in-motion scale that compares the weight of an item to a specific limit, usually based on your agreed product guidelines. If the product weighs outside of that range, it will be rejected from the line. It is commonly used in automated production lines to monitor the accuracy of filling systems and ensure that all packages are within tolerances specified by USDA/FSIS and NIST.

Weighing accuracy depends on several factors, including mechanical and environmental conditions. To prevent these factors from influencing your system’s performance, choose components that are specifically suited for your application and take steps to control environmental and mechanical forces acting on the equipment. For example, Hardy check weighing systems utilize vibration isolation technology to ignore the mechanical noise generated by conveyor belts.

Direct weighing

When a solid sample must be transferred to a container, it is important that the correct amount make it to the new container. Otherwise, it will remain on the weighing bottle rim or the beaker’s edge and produce an error in the weighed results. The tried-and-true method to avoid this problem is known as weighing by difference, or direct weighing.

To perform this weighing process, the empty balance is tared (reads zero with nothing on the pan). The weighing bottle or beaker is placed on the scale and then poured. The tared balance reading is then subtracted from the reading after the pouring to determine the exact mass of the sample.

This method eliminates the need for a container and also allows for a precise tare reading without having to manually subtract it. Exact zeroing is not essential because any errors in the initial weighing will cancel out when the two measurements are compared. This technique is especially useful in chemistry when accurate stoichiometry must be achieved.

Batch weighing

In addition to providing quality control and improved production efficiency, automated weighing processes save money. They also eliminate the need for excessive manual labor that can cause human errors and decrease production time. In addition, batch weighing systems provide precise measurements to meet regulatory requirements. This helps businesses avoid costly penalties and maintain product consistency.

The type of bulk material and its processing characteristics directly impact the weighing system design. Fragile powders, for example, require gentle weighing while denser granules may negatively impact a gain-in-weight system with tight variance targets. Flowability can also affect measurement. An experienced scale and blending systems provider can help select an ideal weighing system for a specific bulk material.

Dynamic weighing systems seamlessly integrate into fast-paced continuous processing equipment like extruders. This method enables rapid dosing and blending with no loss of accuracy. This is different from static weighing that measures at rest, which requires the material to be stopped and the vessel or hopper to be empty for accurate measurements.