Where Did This Kinetic Energy Come From?
Introducing Kinetic Energy
Kinetic energy is the energy of motion. Objects and particles that are moving have kinetic energy. The faster an object moves, the more kinetic energy it has. Some examples of kinetic energy in everyday life include:
- A ball rolling across the floor
- A bicycle moving down the street
- Atoms and molecules vibrating and moving in warm objects
- Wind blowing across the land
Kinetic energy depends on the mass and velocity of an object. The more massive an object is and the faster it moves, the greater its kinetic energy. Kinetic energy is a property of moving objects and particles and is one of the different forms of energy in our universe.
Sources of Kinetic Energy
There are several common sources that can transfer energy into kinetic energy:
Gravitational Potential Energy
When an object falls due to gravity, its gravitational potential energy is converted into kinetic energy. For example, a ball held at a height has gravitational potential energy that transforms into kinetic energy as it accelerates downward. The higher the starting point, the greater the kinetic energy produced.
Chemical Potential Energy
Chemical potential energy stored in the molecular bonds of substances like gasoline, batteries, and food can be released to produce kinetic energy. For instance, the chemical energy in gasoline ignites in a car engine to propel the pistons and wheels. Explosives and rocket fuels also rely on chemical potential energy.
Mechanical Work
Applying a force over a distance does mechanical work that can increase an object’s kinetic energy. For example, paddle wheels or turbines convert the mechanical work of flowing fluids like water or steam into rotational kinetic energy. Muscle exertion also does mechanical work to propel our bodies.
Gravitational Potential Energy
Gravitational potential energy is the energy stored in an object due to its height above the ground. The higher the object is above the ground, the more potential energy it possesses. This is because the object has the potential to release energy and do work if it falls.
The potential energy an object has due to gravity depends on two factors – its mass (m) and its height above the ground (h). The gravitational potential energy can be calculated using the equation:
Gravitational Potential Energy = mgh
Where g is the gravitational acceleration (9.8 m/s2 on Earth). This shows that as the height (h) increases, the gravitational potential energy increases linearly.
For example, if a 1kg book is at a height of 2m above the ground, its gravitational potential energy would be:
Gravitational Potential Energy = 1kg x 9.8m/s2 x 2m = 19.6 J
If the book was moved to a height of 5m above the ground, its gravitational potential energy would increase to:
Gravitational Potential Energy = 1kg x 9.8m/s2 x 5m = 49 J
This demonstrates that increasing the height of an object significantly increases its gravitational potential energy. This stored energy can be released to do work, such as accelerating an object when allowed to fall.
Chemical Potential Energy
Chemical potential energy is the energy stored in the bonds between atoms and molecules. Atoms bond together to form molecules and compounds. These chemical bonds require energy to break or rearrange. Therefore, chemical compounds store potential energy that can be released during chemical reactions.
For example, the hydrocarbon molecules in gasoline, food, wood and other organic matter contain high amounts of chemical potential energy. When gasoline combusts in a car engine, the hydrocarbon molecules break apart, releasing energy that powers the engine. Similarly, the metabolic breakdown of food molecules like fats and carbohydrates provides energy for the human body.
The amount of potential energy stored in a molecule’s bonds depends on the strength of those bonds. Strong chemical bonds like those in fossil fuels, explosives and battery materials store more potential energy per unit weight than weaker bonds between water molecules or oxygen molecules.
Engineers and chemists can calculate the energy stored in chemical bonds using bond energy values. By carefully selecting reactants, they can design chemical reactions and processes to release energy when needed for human purposes, whether it’s powering a vehicle or generating electricity.
Mechanical Work
One of the most common ways that kinetic energy is transferred is through mechanical work. Mechanical work occurs when a force is applied to an object, causing it to move and imparting kinetic energy. For example, when you push a cart, pull back on a slingshot, or lift weights upward, you are doing work that gives those objects kinetic energy.
The amount of kinetic energy depends on the amount of force applied and the distance over which it is applied. Pushing harder on an object or pushing it over a longer distance will impart more kinetic energy. Mechanical work is happening all around us – from throwing a ball to rowing a boat to pressing the pedals on a bicycle. In each case, our muscles are applying force to create motion, thus transferring kinetic energy.
Machines can also perform mechanical work to generate kinetic energy. For instance, turbines convert the mechanical work of flowing fluids like water, steam, or air into rotational kinetic energy. Pistons and crankshafts in car engines transform small explosive forces into the kinetic energy that drives the wheels. Mechanical work is one of the most useful and practical ways we create kinetic energy for real-world applications.
Thermal Energy
Thermal energy refers to the kinetic energy associated with the random motion of atoms and molecules. As an object is heated, its atoms and molecules vibrate and move faster, increasing their kinetic energy. This increased molecular motion is directly related to an increase in thermal energy and temperature.
Heat is the transfer of thermal energy between objects due to their temperature difference. Heat flows spontaneously from higher temperature to lower temperature objects. As heat is transferred into an object, the added energy is distributed among the atoms and molecules as kinetic energy, increasing their random motions. This causes the temperature of the object to rise.
Some examples of heat transferring kinetic energy include:
- Heating water on a stove increases the water molecules’ kinetic energy, raising its temperature.
- Rubbing your hands together converts mechanical work into thermal energy through friction, warming your hands.
- Sittings near a fireplace, the radiant heat from the fire is absorbed by your skin, increasing the kinetic energy of your body’s molecules.
Electromagnetic Radiation
Electromagnetic radiation such as visible light, ultraviolet light, infrared radiation, microwaves, radio waves, and X-rays all carry kinetic energy as they travel through space. This kinetic energy is directly proportional to the frequency of the electromagnetic wave. Higher frequency electromagnetic waves like X-rays have more energy than lower frequency waves like radio waves.
When electromagnetic radiation strikes matter, it can transfer some or all of its kinetic energy to electrons, a process known as the photoelectric effect. If the radiation is of high enough frequency, its kinetic energy can eject electrons completely from the matter, called photoemission. This is how solar cells work – sunlight knocks electrons free which creates an electric current.
An everyday example of the kinetic energy of electromagnetic radiation is the warming sensation we feel on our skin when we are exposed to sunlight. The kinetic energy of the absorbed photons gets transferred to the molecules in our skin, increasing their thermal motion and temperature.
Nuclear Fission/Fusion
Nuclear fission and fusion reactions convert a fraction of the mass of atomic nuclei into enormous amounts of kinetic energy. These processes follow Einstein’s famous equation E=mc2, which shows that mass and energy are equivalent. A small amount of mass can be converted into a huge amount of energy.
In nuclear fission, a heavy unstable nucleus like uranium or plutonium splits into two smaller nuclei, releasing neutrons, photons, and approximately 0.1% of the parent nucleus’s mass in the form of kinetic energy. Nuclear power plants use fission to generate electricity.
Nuclear fusion works in the opposite way – small nuclei are fused into larger ones, releasing energy. Hydrogen bombs use fusion of hydrogen isotopes like deuterium and tritium. In the Sun, fusion of hydrogen into helium converts about 0.7% of the hydrogen mass into energy.
Both fission and fusion reactions liberate energy from tiny amounts of consumed mass. Even a slight reduction in mass results in an enormous release of kinetic energy, as described by Einstein’s famous theory of relativity.
Kinetic Energy in Nature
Kinetic energy is abundant in many natural processes on Earth and throughout the universe. Here are some examples:
Weather phenomena like wind, storms, and tornadoes involve enormous amounts of kinetic energy. Wind is simply air molecules in motion, which have kinetic energy. More extreme weather events occur when kinetic energy is concentrated in certain areas due to pressure and temperature differences in the atmosphere and oceans.
Ocean waves are created by wind transferring kinetic energy to the water. As waves approach shore, they build up potential energy until they break, releasing their kinetic energy. The tidal cycles are caused by the kinetic energy of the Earth-Moon gravitational system.
Volcanic eruptions release incredible amounts of kinetic energy pent up in molten rock (magma) below the surface. The kinetic energy propels lava, ash, and gases out of the volcano at high speeds.
Earthquakes are the release of kinetic energy built up between tectonic plates shifting deep underground. The kinetic energy travels through the earth in seismic waves, causing the ground to shake.
Photosynthesis in plants absorbs kinetic energy from sunlight and converts it into chemical potential energy in the form of glucose molecules.
Applications of Kinetic Energy
Kinetic energy has many important practical applications in power generation, transportation, and industrial processes. Here are some of the main ways kinetic energy is harnessed and used:
Power Generation – Kinetic energy is used to generate electricity in hydroelectric dams, where the kinetic energy of falling water turns turbines connected to generators. Wind turbines also utilize kinetic energy from the wind to produce electricity. Fossil fuel and nuclear power plants convert thermal energy from combustion or nuclear reactions into kinetic energy that spins turbines for power generation.
Transportation – The motion of cars, trucks, trains, planes and ships is powered by kinetic energy. Their engines transform chemical potential energy from fuel into kinetic energy of motion. Roller coasters rely solely on gravitational potential energy being converted into kinetic energy.
Industrial Machinery – All sorts of industrial machinery, from assembly lines to jackhammers, rely on kinetic energy to operate. Electric motors transform electrical energy into the kinetic motion that powers the machines. Pneumatic and hydraulic tools use kinetic energy of pressurized gases or liquids.
Kinetic energy is an essential driver of technologies and processes that power our modern world. Anywhere motion, speed or mechanical power is needed, kinetic energy is being harnessed and applied in useful ways. Understanding the transformation and utilization of kinetic energy has enabled great advances in transportation, manufacturing, and energy production.
