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.