Kinetic energy is the energy of motion. This can be the motion of large objects (macroscopic kinetic energy), or the movement of small atoms and molecules (microscopic kinetic energy). Macroscopic kinetic energy is "high quality" energy, while microscopic kinetic energy is more disordered and "low-quality."
There's a simulation to play with at potential energy that shows the interaction of gravitational potential energy, kinetic energy and spring energy. A simulation below shows how energy flows back and forth between kinetic energy and gravitational potential energy and another simulation further below shows how friction causes macroscopic kinetic energy to become microscopic kinetic energy.
Rotational kinetic energy is also a form of kinetic energy that comes from an object spinning.
Macroscopic kinetic energy
This is the most obvious form of energy as it is the easiest to observe. This is the energy possessed by moving objects. The larger an object is or the faster it moves, the more kinetic energy it has. The sum of potential energy and macroscopic kinetic energy is called mechanical energy and stays constant for a system when there are only conservative forces (no non-conservative forces).
Kinetic energy is calculated using the following formula:
- The more mass a moving object has, the more kinetic energy it will possess at the same speed. A 2000 kg car moving at 14 m/s has twice as much kinetic energy as a 1000 kg car moving at an equivalent 14 m/s.
- Because the velocity term in this formula is squared, velocity has a much larger effect than mass does on kinetic energy. A car moving at twice the speed of another car of identical mass will have 22 or four times as much kinetic energy. A car moving at three times the base speed will have 32 or NINE times the original kinetic energy!
Some ways to harness macroscopic kinetic energy include:
Wind power harnesses the kinetic energy possessed by moving bodies of air (wind), converting it into electricity. Wind itself is created initially through complex patterns of changes in thermal energy as the atmosphere and oceans are heated and cooled by the sun. (The sun actually doesn't cool objects, but the sun never shines on an object on Earth all the time!)
Tidal power harnesses the energy of moving water as it moves back and forth due to tides
PhET: Energy Skate Park
The University of Colorado has graciously allowed us to use the following PhET simulation. Explore this simulation to see how gravitational potential energy and kinetic energy go back and forth but keep mechanical energy the same. Notice how mechanical energy can be lost and turned into thermal energy, but the total amount of energy still stays the same:
Microscopic kinetic energy
Thermal energy (temperature) is a special type of kinetic energy. It is not the energy of a whole object itself moving - it is the total energy of motion, rotation, and vibration of the atoms and molecules inside an object. In a gas or gas mixture, like air, the motion (and rotation) of individual gas particles makes up this energy. In a solid, like a table, the thermal energy exists as vibration of atoms or molecules. Total thermal energy also includes some atomic forms of potential energy, but the kinetic energy of particles is the easiest to focus on. The temperature of an object is determined by its total microscopic kinetic energy.
While not all of microscopic kinetic energy can be turned into useful work, a heat engine can get some of the thermal energy and turn it into useful work (although this is limited by the second law of thermodynamics).
The University of Colorado has graciously allowed us to use the following PhET simulation. This simulation explores how macroscopic kinetic energy becomes microscopic kinetic energy:
To learn more about kinetic energy please see hyperphysics.
- Wolfson, Energy, Environment and Climate, Second ed. New York, USA: W.W. Norton, 2010