The air you breathe, the food you eat, and even the keyboard you type on have mass. Understanding how to measure these substances can give you greater insight into our everyday world.
The basic SI unit of mass is the kilogram, a cylinder of special metal that serves as the world’s standard. The second measurement of an object is its volume, which is the amount of space it takes up.
Weight
Although mass and weight may seem to have the same meaning, they are actually two very different physical properties. Mass is a measure of the amount of matter—the number and type of atoms that make up an object—whereas weight is a measurement of the force of gravity acting on the object.
It is important to distinguish between these properties as they are used in science and commerce. For example, a scale measures the “weight” of an object by measuring how much it pulls down on and stretches a spring. However, the same object would have a different weight on the Moon than on Earth because of the gravitational pull in a different frame of reference.
In routine laboratory use, the term weight is usually replaced with the word density when describing an object’s value in a high-precision scale or balance. In these cases, the precision scales are calibrated with a stainless steel standard to provide conventional mass measurements (true mass minus an unknown degree of buoyancy). This method is more accurate than using an arbitrary block of platinum for calibration, but it still depends on gravity in a different frame of reference.
Density
The density of a sample is its mass divided by its volume, a ratio that remains constant regardless of the size of the sample. It is an intensive property, and for this reason numerous analytical methods exist for measuring it. For liquid samples, for example, a volumetric flask with one marking that corresponds to a particular volume is filled up to the line and weighed.
Knowing how to measure density can help students and professionals across STEM fields understand the fundamental characteristics of matter at a variety of scales and volumes. Whether it’s comparing the weight of stainless steel and feathers or observing how oil floats in water, density is an essential concept in physics, chemistry, biology, geology and even cooking. It is, in fact, the principle that enables the Eureka moment celebrated by Archimedes. It is why, for instance, a dense block of metal will be heavier than an equal-sized piece of bread. This is why it’s important to master the skill of density measurement.
Velocity
Velocity is a key concept in physics, especially when it comes to motion. It is the combination of an object’s speed and direction that defines its kinetic energy, the amount of force it takes to accelerate an object. This is a result of the law of conservation of linear momentum, which states that an object’s total energy (kinetic plus potential) is equal to its mass times velocity.
Unlike speed, which is a scalar quantity that indicates distance per unit time, velocity is a vector quantity and requires both magnitude and direction. If an object’s velocity is changing, it must be accelerating.
Several methods can be used to measure fluid velocity, including optical Doppler velocimetry and acoustic Doppler velocimetry. Optical Doppler velocimetry is usually limited to fluids that are transparent, while acoustic Doppler spectroscopy is suitable for most liquids and gases. However, in some cases the temperature of a flowing fluid can cause heating and vaporization of the particles, which can affect the measurement.
Gravity
Gravity is the force that attracts matter toward the center of the earth. It is also the force that causes two spheres of identical mass to be attracted toward each other, with a magnitude proportional to the square of the distance between them.
Scientists have a variety of ways to measure gravity, including allowing objects to fall freely and measuring their acceleration. The rate of acceleration at a given location on the surface of the earth varies slightly from place to place, depending on latitude and features such as mountains or hills.
It is important to distinguish between mass and weight, which are not the same thing. A spring balance scale compares an object’s inertial mass to a standard reference weight held in Sevres, France. Engineers involved in disciplines requiring the measurement of weight loading (force on a structure due to gravity) convert an object’s mass into a force in newtons by using classic Newtonian laws.