Metrics are best when they provide clear, quantifiable data that can be measured, tracked and analyzed over time. Metrics should also be focused on key areas of importance to your organization or project.
In physics, mass is a quantitative measure of inertia, the resistance that matter offers to change in velocity when a force is applied. It is measured in kilograms and fractions of a kilogram.
Weight
Many people use the words “weight” and “mass” interchangeably. However, the two measurements have different meanings in physics.
The SI unit of mass is the kilogram (kg), and masses are measured in kilograms and multiples or fractions of the kg. The physical prototype of the kilogram is kept in a cylinder of platinum-iridium at the International Bureau of Weights and Measures.
In physics, mass is an intrinsic property of matter and does not depend on gravity. This is why you would have the same weight on Mount Everest or in orbit.
In contrast, weight depends on the force of gravity and changes depending on where you are. For example, you will have a very different weight on Jupiter than you will in your own backyard. Consequently, a balance-type weighing scale and a load cell-based bathroom scale will give you different readings for the same object. This is why scientists insist that you should only use precision mass standards to measure your own weight.
Density
Density, defined as mass divided by volume, is a fundamental physical property of substances. It is essential to understanding and measuring the properties of all matter, including planetary bodies and stars.
For solids that have simple shapes, such as a cube or sphere, density is easy to measure by weighing the sample and dividing it by its geometric volume. This time-honored method is also effective for liquids.
In general, a substance with more mass in a given volume has higher density. Knowing density allows scientists to classify materials, and even predict their interactions with each other – like whether a certain oil will float or sink in water.
For partitioned solids such as powders and granules, density is determined using a digital or electronic hydrometer or pycnometer. Depending on the instrument, results can be expressed in different units and concentrations, which is useful for quality control or product development. In some cases, the instrument’s tare function can automatically convert results to standard units and concentrations.
Volume
Students need to understand the differences between mass and volume, which is important for chemistry studies. For example, the same substance can weigh differently on Earth and in space because of different gravitational pulls.
Measuring the volume of a sample is a critical step in determining its density. Often the process is accomplished using displacement methods such as graduated cylinders, or volumetric flasks. The most commonly used unit of measurement for volume is the cubic meter (m3), which is derived from units of length. A liter, for example, is equal to 1000cm3 (1dm3).
The shape of a sample also affects its volume. Solids have very stable shapes and volumes, while liquids and gases have unstable shapes and volumes. For these reasons, it is essential to measure an object’s volume before measuring its weight to get an accurate picture of the matter inside. In addition, it is important to know that the same measurement of a container filled with different substances will yield different results due to the difference in their densities.
Acceleration
The rate at which an object changes its velocity. Acceleration is one of the kinematic quantities (position, distance, speed and direction) that define motion.
A change in acceleration defines the rate of change of a body’s velocity, so it is also a measure of force. The quantity of force required to cause a given acceleration is called the mass of that object.
Sports announcers occasionally use the word “accelerate” to refer to someone moving fast, but in physics, acceleration has nothing to do with speed and everything to do with changing how quickly an object is moving. An object can have a constant speed, yet be accelerating, or even decelerating.
Using a ticker-timer attached to the trolley with a meter connected to it, students can actively observe that acceleration depends on net force, and that an increase in force leads to an increase in acceleration. This can be a useful introduction to the concept of proportionality, which states that two variables are related in such a way that when one variable increases or decreases, the other does the same.