Students often confuse mass and weight. They might use a balance to guess an object’s mass and compare it with others, but that only gives them an indirect measurement of the object’s heaviness.

The fundamental SI base units are based on immutable properties of the universe. Their names combine prefixes and units that are easy for kids to remember.

Weight

Mass is a quantity describing the amount of matter in an object. It is a fundamental property of matter and the base SI unit is kilogram (kg).

Weight, on the other hand, measures the force of gravitational attraction on an object. This is a vector quantity, with magnitude directed toward the center of the Earth or other gravity well and is often measured by an ordinary balance, such as a spring balance. It is also expressed in kilograms and grams. The term ‘weight’ can also be used to describe an inertia, that is the object’s tendency to resist changes in its state of motion – think of the puck sliding on an air hockey table until some force acts on it.

Unlike weight, which depends on location, mass does not change. However, it is possible to measure both by using a triple-beam balance that compares the unknown object with one of two pans filled with known masses and is unaffected by local variations in gravitational acceleration.

Acceleration

In everyday life, we can use the term “acceleration” to refer to speeding up or slowing down. In physics, acceleration is actually the rate at which an object changes its velocity over time and has both magnitude and direction, making it a vector quantity.

An object in circular motion (like a satellite orbiting the earth) is also accelerated by change of direction but not by change of speed. This acceleration is called centripetal acceleration and it is proportional to the mass of the object.

NIST scientists have developed a system that uses the radiation pressure that a weak laser beam exerts on an attached high-reflectivity mirror to measure force and mass. This system, which is portable and self-calibrating, offers an excellent opportunity for students to learn fundamental physics concepts and the complexities of measurement.

Gravity

In a laboratory, gravity is a challenge to measure. The weakest of the four fundamental forces, it is far more difficult to measure than the other three, which are mediated by quantum particles. This is despite the fact that it plays a crucial role in the long-range trajectories of objects in the solar system and the structures and evolution of stars and galaxies.

It took a flash of insight from Newton to elevate gravity from an inscrutable force that acted on everything from raindrops to cannonballs to a measurable phenomenon. He derived the expression F = G (mass times acceleration) and established a value for the gravitational constant that is within 1% of its modern-day value.

Since then, physicists have struggled to measure G with increasing accuracy. One classic technique uses a torsion balance that measures the twisting of an inner carousel with respect to an outer disk when masses on both are moved. More recently, researchers have turned to quantum physics and clouds of ultra-cold atoms to try to determine the value of G.

Transducers

The ability to measure mass is crucial for many scientific applications. Traditionally, mass measurement has been accomplished using a balance. However, the need for more accurate measurements led to the development of new mass-measuring instruments. These devices use a system of rods or pistons to counteract the force of gravity, allowing for more precise readings.

The transducer converts the physical i/p quantity into an electrical signal, which can be easily read by a meter. Generally, the main function of a sensor is to change the physical signal into a normalized current output.

The performance of a transducer depends on its ability to translate the mechanical input into an output that is stable and can be used to measure the input signal. The main factors that influence this conversion are the sensitivity, cross sensitivity, hysteresis and operating range of the device. Also, the speed at which the device can translate a physical input into an output signal is important.

A weighing process involves measuring the amount of material in an object. In most cases, this is done using a balance.

A typical balance consists of a pivoted horizontal lever with arms of equal length – the beam. It can determine mass by placing the unknown substance in one pan & standard masses in the other pan until the system achieves equilibrium.

Measurement

From food packaging to production line weighing, accurate measurements help make business processes efficient. But how can you ensure your weighing equipment delivers the results you need?

Vibration:

The vibration transmitted through process equipment or around the weighing system can disturb the load cells. This can cause the weighing system instrumentation to produce inaccurate readings or even fail altogether. Ensure your weighing system is isolated from vibration sources when possible, or select a weighing system with sensor instrumentation that eliminates vibration effects.

The sensitivity of an optical balance is determined by comparison (or substitution) weighing with standard masses that are calibrated in advance. The sensitivity weights should be selected to minimize the influence of air buoyancy on the measurement and their handling must be carefully controlled to prevent grease or oily films from contaminating the mass. This will affect the ability to correctly measure an unknown object and will lead to poor repeatability. To correct for this, a cornerload test should be performed periodically.

Accuracy

Weighing accuracy is the closeness of a measurement to a known value. It is affected by four factors:

The major weighing component in all electronic weighing systems is the load cell (also called a load sensor or transducer). This piece of machined metal bends under weight, and its bending is sensed by strain gauges bonded to the cell at points that correspond to load areas. The gauges output an electrical signal that the weighing system’s controller interprets as the weight reading.

Any change to the signal or any interference in the weighing process can throw off the weight results. This can include vibration from equipment placed near the scale, electromagnetic fields from nearby power lines or radio signals, and temperature changes.

Control

A number of things can affect weighing accuracy. These include shock loading (such as dropping heavy materials on a scale that exceeds its max rated capacity) and wind loading (small amounts of air movement caused by things such as air ducts or air conditioning). Vibrations can also cause strain to sensitive load cells, which send an electrical signal that can be misinterpreted by the weight controller.

The response time of a load cell is important when an application involves rapid readings such as a high-speed checkweighing or rotary filling machine. Load cell responses can be influenced by temperature, wire resistance, electromagnetic interference and moisture.

Large temperature changes can impact a load cell’s sensitivity, changing its output range and requiring new calibrations. If a weighing system is designed to rezero (zero out) the weight of a container before each weighing cycle, this problem is less severe. Other random fluctuations can be digitally averaged by a weight controller.

Safety

When weighing hazardous chemicals, safety is an important factor. It is difficult to prevent accidental minor or major spills, but weighing systems can be designed to mitigate exposure risks by using a fume hood or specialised isolator.

When direct weighing a substance, the balance is first zeroed by placing a piece of clean weighing paper on the pan. Then the mass of the weighing substance is displayed by subtracting the first reading from the second. A process called taring is often used to minimise weighing errors.

The precision of a balance is also influenced by the environmental pressure around the system. Pressure differentials can affect the accuracy of a balance, so weighing equipment should be placed in an area with consistent ambient air pressure. Also, vibration can cause issues with accuracy. This is because vibration can make a weight signal inaccurate by causing the weigher to recognise the movement of nearby equipment as an actual weighing load.