# Teaching Kids About Mass Measurement

Kids learn faster when they’re inquisitive, and teaching them about mass is a great way to get them interested in science. Introduce them to the metric system — the units for length, volume and weight with easy-to-remember prefixes like decimal and hecto.

Kids can practice calculating the differences between weight and mass by using a balance. Explain that an object’s mass stays the same regardless of its shape or location, while its weight changes depending on the amount of gravitational acceleration exerted.

## Density

Density is a physical property that tells you how tightly packed a substance’s molecules are in a given volume. This is why some materials are heavier than others. For example, wood is more dense than Styrofoam.

The density of an object is defined as its mass divided by its volume. The symbol for density is rho “” or Latin letter “d.” The formula for density is m/v, where m represents the mass and v is the volume. Density is usually measured in grams per cubic centimeters cube (g/cm3) or kilograms per liter, but there are many other units used.

Density is an important concept to understand because it relates to the properties of solids, liquids and gases. Knowing about the density of a material can help you determine whether a solid, such as gold, will sink in water or if a liquid, such as alcohol, will rise when placed on ice. The density of a substance can also be used to distinguish between different minerals.

## Weight

People use the words weight and mass interchangeably, but they mean different things. Weight is a measure of the force of gravity on an object, while mass is an amount of matter in an object.

The SI unit of mass is kilograms (kg), but it’s also measured in multiples and fractions of the kilogram, including grams (g). You can balance objects with a scale to find their mass. For example, a pineapple has the same mass as a wooden baseball bat, but one is lighter than the other.

A more scientific way to measure an object’s weight is by calculating its acceleration under gravity, using the formula m x g. However, this method requires a very precise measurement of the gravitational field strength and an accurate mass measurement. It’s usually not done in everyday life, and it’s difficult for humans to perform accurately. Moreover, there are many other ways to calculate an object’s mass that don’t require knowing its weight.

## Acceleration

Acceleration is a vector quantity that measures how much an object changes its velocity. The SI unit for acceleration is velocity units divided by time units, a/t.

For example, a passenger on a moving bus will tend to lean backwards because of the inertia that keeps the vehicle at rest. If the bus accelerates, however, the distance between the passengers and the dashboard will decrease because the inertia is inversely proportional to the change in acceleration.

NIST researchers have developed a simple optomechanical accelerometer that is capable of sensing accelerations as tiny as 32 billionths of a g, the force exerted by Earth’s gravity. This is a lower measurement uncertainty than all other commercial accelerometers of similar size, frequency response and bandwidth that are currently available. The device is expected to enable a variety of applications that require displacement measurements such as structural health monitoring (SHM) and earthquake engineering. The optomechanical accelerometer is part of NIST on a Chip, which brings the Institute’s cutting-edge measurement science directly to partners in commerce, industry and government.

## Passive Gravitational Mass

Mass is the property of a physical body that measures its resistance to acceleration (change in its state of motion) when a net force is applied. It also determines the strength of its gravitational attraction to other objects. It is measured by allowing a test object to fall freely and measuring its free-fall acceleration. This is a very precise measurement that enables us to distinguish between active and passive gravitational mass.

A test object’s active gravitational mass depends on the distance r between it and a second test object. Its passive gravitational mass, on the other hand, is independent of r and of the material composition of both test objects.

This implies that, according to the weak equivalence principle of Newton’s law of universal gravitation, passive gravitational mass must be proportional to inertial mass for all objects, regardless of their material composition. This is what is referred to in metrological terms as the “eotvos principle.”

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