If you've ever played the guitar, lit a fire, felt the warmth of a summer sun, or jumped out of an airplane, you will be familiar with the relationship between potential and kinetic energy, even if you don't realize it.
In simple terms, kinetic energy is the energy of an object by virtue of its motion in all its forms, and potential energy is the energy inherent in an object by virtue of its relative position to other objects, its electrical charge, its chemical composition, and other factors.
The two types of energy are tightly related to one another in a way that is constantly changing but always in equilibrium. This back and forth between potential energy and kinetic energy is the key to everything from the wind in your hair to the nuclear furnace at the center of the sun and much, much more.
What are the types of kinetic and potential energy?
There are five primary forms of kinetic energy:
- Radiant energy
- Thermal energy
- Sound energy
- Electrical energy
- Mechanical energy
Radiant energy is a form of kinetic energy transferred by electromagnetic radiation, best represented by radiation or light, in that it is transmitted without the movement of mass. Examples of radiant energy include the infrared light that radiates from a hot stove and the warmth from direct sunlight.
Thermal energy, otherwise known as heat, is a form of kinetic energy produced by the motion of individual atoms colliding with each other. The more excited the atoms are, the faster they move and so the more kinetic energy they possess. When they collide with other atoms, that kinetic energy is experienced as heat.
Sound energy is a form of kinetic energy produced by the vibration of an object that travels through the displacement of material in a medium, such as air or water. Without particles to displace though, sound cannot travel, which is why there is no sound in a vacuum such as outer space. In addition, denser material can carry sound farther, like sonar in the ocean versus ringing a bell on the shore.
Electrical energy is also a form of kinetic energy, which is produced by the flow of free electrons in a circuit. This form of kinetic energy is essential to our modern world, since this is what provides the power for so much of our modern technology.
Mechanical energy is the most obvious form of kinetic energy since this is the form we see it in just about everywhere. Whether it's a prevailing wind turning the blades of a wind turbine, a bus rolling through an intersection, or a rollercoaster rolling down a slope for acceleration only to slow down again as it crests the next hill, this swinging back and forth between potential energy and kinetic energy is the most visually apparent kind of kinetic energy.
There are four main types of potential energy that can be directly converted into kinetic energy.
Chemical energy is the energy stored in the bonds between atoms that make up molecules. By breaking these bonds or altering the composition of the molecules, you can release some of this stored energy to produce kinetic energy. One of the most common ways to do this is by burning a substance as fuel to convert that energy into thermal energy, such as burning wood in a fireplace to heat a room.
Mechanical energy, while a form of kinetic energy, it is also a form of potential energy. A common form of potential mechanical energy is tension, like the compression of a spring or the twisting of a rope, which can then be released so that it spins in the opposite direction to release the inherent tension. A rubber band is also an example of potential mechanical energy bound up in the elasticity of the rubber.
Nuclear energy is another important form of potential energy. Nuclear energy refers to the tremendous amount of energy that keeps the atomic nucleus together and which can be released if an atomic nucleus is split apart or two atomic nuclei are fused together. Nuclear energy is responsible for producing radiant kinetic energy in the form of light, gamma rays, and other forms of radiation, like that from the sun's nuclear fusion or the radiation produced by the nuclear fission of an atomic bomb.
Gravitational energy is the potential energy stored in an object as a function of its distance from a center of gravity, most commonly experienced as free-falling from a given height. For example, a cup of water on the edge of a table has potential gravitational energy that is released as mechanical kinetic energy when a cat comes by and pushes it off the table's edge. Our rollercoaster example above is also a prime example of gravitational potential energy since it's the running down long, steep drops that give the coaster the kinetic energy to overcome gravity and friction to make it to the top of the next incline.
What does kinetic energy depend on?
Kinetic energy is reliant on potential energy to, well, get moving. Newton's second law of motion states that an object in motion will stay in motion in a straight line unless acted upon by an outside force, and that an object at rest strongly inclines toward staying at rest.
So, trying to roll a large boulder that is settled in the middle of a field takes a lot of external energy input, while when that same boulder gets to a hill and starts rolling down, it becomes easier to accelerate the faster it's moving. Conversely, that boulder now rolling down a hill uncontrollably will take considerably more energy to slow down or stop, which is obvious if you've ever seen a considerable mass roll down a hill and hit something. An object at complete rest meanwhile requires absolutely zero energy to decelerate or stop.
In our example of the boulder, the potential energy required by a person to move the boulder from a state of rest comes from the chemical energy inside your body that you convert through metabolism into the mechanical kinetic energy to push against the rock. From the rock's perspective, your muscles straining to get it to roll provide mechanical potential energy that then gets converted into a slowly accelerating roll.
So, one object's kinetic energy might be another object's potential energy, and this transference and conversion of energy from potential to kinetic is going in both directions at the same, often in several different forms of kinetic and potential energy all at once.
Returning to our example of a person rolling a boulder down a slope, if that slope is the side of an erupting volcano and that rock just got blown out of a magma pocket and is hundreds or thousands of degrees Fahrenheit, even approaching it will hit you with intense thermal energy, which would be all the greater if you manage to get your hands on the rock enough to start pushing.
Oh, and at that point, the skin and soft tissue in your hand become potential chemical energy as the heat of the molten rock burns your hands in a chemical reaction that releases water, carbon dioxide, and various other chemicals created by the charring of the flesh of your hand, transforming it into a form of charcoal. But at least you might have moved the boulder a little way down the side of the volcano.
Different types of potential and kinetic energy: How are they used in daily life?
The different types of kinetic energy are used in just about everything we do.
The largest example of radiant energy is that coming from the sun, which bathes the Earth in a broad spectrum of radiant energy in the form of light, heat, and other kinds of radiation. Besides using this energy to visually navigate our world in the form of sunlight, we can also capture it in photovoltaic panels and turn radiant energy into electrical energy. Of course, plants and other organisms also capture this energy, using it to drive chemical reactions that create fuel for the plant to use for growth.
Radiant energy is most associated with nuclear potential energy, but can also be produced by chemical energy, as with chemical lights and bioluminescence. It can also be a byproduct of thermal energy, like the coil burners of an electric stove.
Speaking of, thermal energy is what we use to keep ourselves warm and to cook our food. We can also use it to make metals more pliable so that we can bend and shape them to make tools. Pretty much whenever we need something heated up, we're looking at thermal energy. The most common way to get thermal energy out of potential energy is to burn fuel, but mechanical potential energy can also become thermal energy.
Since thermal energy is the result of the collision of individual atoms, any time one object his another, its atoms are hitting the other objects atoms and thermal energy is generated as a result. Friction is another way to produce thermal energy from mechanical energy.
We use sound energy to make sense of our surroundings, communicate with one another, and make music. Bats rely on sound energy for echolocation to help them identify insects to eat, and whales use sound energy to stay connected to other members of their pod and find mates across vast distances. Sound energy is a strictly mechanical process, since sound energy is really just vibration.
When vibrating within a medium of some kind, it produces sounds that we can hear, but even in a complete vacuum, the vibration of an object is still releasing sound energy even if there's no way for us to hear it.
Electrical energy is what is allowing you to read the words on this page, right now, thanks to the electronic display that converts electrical energy into different colored pixels on your screen.
Electrical energy is also what was used to transmit the digitized version of these words across fiber-optic cables, which were either piped directly into the computer you are using to read this as electrical signals or were instead converted into radio waves by a Wi-Fi transmitter which your computer was then able to translate back into electrical energy.
All of these electrons moving through a material, like copper wiring, excite the atoms they come into contact with, which causes them to move a little bit faster. This produces heat energy that either needs to be utilized, as in electric heaters, or radiated away as exhaust. Electric energy can also become chemical energy as bonds are formed with different molecules. This is essentially how we store electric energy in a chemical battery such as lithium-ion battery packs.
Even our bodies take chemical energy in the form of food, water, and oxygen and convert it through metabolism into electrical impulses in our nervous system that allows our brains to process information, relay messages, or perform work.
Finally, mechanical energy is responsible for everything from turning the key you use to lock or unlock your door, to turning a screwdriver to tighten a screw, or the movement of our arms and legs which allow us to walk. It's also responsible for various mechanical turbines, which are essential to generating the electricity we need to power all the technology we've come to rely on for the past century and a half.
Simply put, if something is performing some form of physical work, the interplay between potential energy and kinetic energy is going to be involved, which makes it one of the most ubiquitous and essential forms of energy in the universe — and also the most useful.