# Year 4 – Conversions and Standard Measurement Units

Year 4 is a time when children begin to learn about conversions and standard measurement units. They will also begin to understand the relationship between an object’s mass and its weight.

The most common method for measuring mass is by using a balance. However, this is only accurate in no-gravity, no-friction environments.

## Inertial mass

In classical physics, mass is the measure of an object’s resistance to acceleration. The greater its mass, the more force is required to change its speed. This is why a heavy object requires more force to accelerate than a light one. In addition, an object’s passive gravitational mass determines the strength of its reaction to gravity and was Einstein’s starting point for his theory of space and time: general relativity.

However, a surprising fact is that an object’s inertial mass and its weight are not the same. Inertial mass is a physical property of an object that defines its inertia, while gravitational mass determines how much an object weighs. Many experiments have been carried out to test whether or not the inertial and gravitational masses are the same, but they have all yielded the same results. Moreover, the two values have been used to calibrate all other SI base units. So, why the distinction between them?

## Weight

For many centuries, the term “weight” was used as a synonym for mass. Galileo was instrumental in demonstrating that, in the absence of air resistance, all objects fall with the same acceleration, or weight. This led Newton to develop his second law of motion, which states that the force of gravity acting on a given mass is proportional to its mass.

While this definition is technically correct, it does not accurately convey the true nature of mass. In fact, weight is actually a measure of force, and it depends on the object’s location. A kilogram of mass will weigh differently on Mars or the International Space Station than it does on Earth.

This is why a balance is the best way to measure mass. A simple double-pan balance, such as the one shown below, can measure masses up to two kilograms (kg). The counterweights are placed at opposite ends of a horizontal bar connecting the two pans. The scale is calibrated by sliding the largest counterweight up to a pointer in the middle of the balance.

## Mass density

Density is the mass of a substance per unit volume, and it is used in many quality and process control applications. A variety of instruments are available for measuring density, including pycnometers and digital hydrometers. For accurate measurements, calibration weights traceable to the international prototype kilogram are required.

The most common method for measuring mass is with a balance. This type of instrument works well in no-gravity environments because changes in gravitational acceleration affect both masses on the balance equally.

Another measure of mass is by using a refractometer to determine the specific gravity of a solution. This is also used in quality and process control applications to ensure proper consistency of a liquid product. For example, a lower specific gravity will indicate that a sample has more impurities or a higher viscosity. This is an indication that the production process needs to be adjusted. The higher the specific gravity, the more refined the product is likely to be.

## Force

While mass measures an object’s tendency to stay put or move in a straight line (its inertia), force causes objects to change their states of motion. For example, when you push on a puck on an air hockey table, the puck moves in a new direction because of the force exerted on it.

A force can be opposed or unopposed. The unopposed force is weight, which is the downward force on an object due to gravity. If no opposing force acts on an object at rest or moving at constant speed, the object remains at rest or continues to move at a constant velocity (Newton’s first law).

Force is measured in units like kilograms and grams because they are part of the metric system, a standardized system used by scientists all over the world. Force is calculated by multiplying an object’s mass into its acceleration, which is expressed as m/a. The standard unit for force is the newton, which can also be shortened to N.

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