In physics, mass is a quantitative measure of inertia, an object’s resistance to change in its speed or direction when a force acts upon it. The kilogram is the SI unit of mass.

Since the discovery of the theory of relativity, the notion of mass has undergone significant revisions. It is now considered to be interchangeable with energy.

Compass

A compass is an instrument that uses magnets to determine the direction of north. It is also used to measure the bearing (the distance from the needle to the geographic north) and inclination (the angle of deviation).

The earliest compasses were probably used as backups for astronomical navigation, but they eventually became indispensable for explorers. They were often made from lodestone and were magnetised with iron needles.

Modern compasses have a needle mounted inside a capsule that is completely filled with liquid (lamp oil, mineral oil, white spirits, purified kerosene, or ethyl alcohol are common). The liquid dampens the movement of the needle and increases stability.

The inclination of the needle is measured in increments, which are recorded and converted into digital values by the signal conditioning circuit. The inclination is then transmitted in digital form to the computer. The system can also save a recording of the last detected position for future reference. This is especially useful for military purposes, since the compass dial is spaced in units called angular mils (6283 per circle). This value can be converted to bearing and distance information.

Weight

Mass is a measurement of the amount of matter in an object, and it doesn’t change no matter where you are in the universe. Weight, on the other hand, is a measure of the force exerted on an object by gravity. Objects with the same mass will have the same weight.

Historically, weight has been measured by using a balance that compares the mass of a known object to an unknown one. The result is a number that is equal to the sum of an object’s gravitational and inertial forces, which can be measured precisely with modern electronic balances.

You can also use a spring scale to measure weight. It works by seeing how much an object pushes down on a spring inside the device. This is affected by the strength of gravity in a particular location, so it isn’t as accurate as a modern bathroom scale. Nonetheless, it can still be used to compare objects on Earth.

Density

Density is a property of matter that tells how much mass there is in a given volume. Different materials have different densities, and therefore they will weigh differently. Hence, different types of metals (like iron or platinum) are more dense than other materials such as bamboo or styrofoam.

The density formula is r = m/V. This means that the density of a material equals its mass divided by its volume. Objects that have the same mass will have the same density, but objects with the same volume will have different masses.

Students can experiment with this by calculating the mass of different-sized objects and plotting their results on a graph. This will show that as the volume of an object increases, its mass also increases. This is because the particles are packed closer together in solids than in liquids or gases, which have more space between the particles. This ratio of mass to volume is called density, and it can be used in a variety of applications such as designing pipes, ships or aeroplanes.

Gravity

Gravity perplexes physicists because it doesn’t behave like any of the other three known forces, electromagnetism and the strong and weak nuclear forces. Those are all described by a mathematical framework called the Standard Model, which includes the action of particles. But gravity doesn’t have a particle to transmit it, leaving it a holy grail for scientists hoping to unify all the forces of the universe into one single theory.

To measure mass, the most straightforward approach is to use a balance, the same device you might find in a doctor’s office. The balance compares an unknown object’s resistance to motion with the force of gravity acting on it. It’s not perfect, though. The scale might read slightly different on the Moon, for example, due to differences in its gravitational acceleration. More advanced techniques, such as atomic interferometry, depend on modern ideas of quantum physics. Lasers split clouds of ultracold atoms into two waves that travel on different paths at different elevations, with the pattern of their interference revealing the object’s gravitational acceleration.