Energy
Energy is one of the fundamental conserved physical quantities found in nature. Kinetic energy is the energy of motion resulting from the mass and speed of a moving object. Potential energy is the energy resulting from the position of an object. The sum of the potential and kinetic energy of an object is its mechanical energy. Energy is a scalar rather than a vector quantity.
Kinetic Energy
A moving object has a certain amount of energy associated with its motion, which is its kinetic energy. A more massive object has more energy when it is moving. Similarly the faster an object is moving the more energy it has. So the formula for kinetic energy must include both the mass and speed of an object. The kinetic energy of an object is one half its mass multiplied by its speed squared or
KE = 1/2mv^2 (v^2 indicates v squared).
If the mass is in kilograms and the speed in meters per second, then the kinetic energy, like all energy, is in units of kilograms meters squared per second squared, which physicists define as a joule. To get a very rough feel for how much energy a joule is, think of it as the amount of energy required to lift an apple from the floor to a table. Another way to get a feel for a joule is to realize that a watt is a joule per second, so a 100 watt light bulb uses 100 joules of energy every second. A joule is not a large amount of energy.
Work Energy Theorem
According to the work energy theorem the net work done on an object equals the change in its kinetic energy: Net work = Final Kinetic Energy - Initial Kinetic Energy.
This theorem is very useful for solving physics problems, but there is a subtle point that confuses many students. The work involved is the net or total work of all the forces acting on the object. The work energy theorem does not give the work performed by just one of the forces acting on an object.
If for example Bill is sliding a refrigerator across the kitchen floor at a constant speed, the net work done on the refrigerator is zero because the kinetic energy does not change. However Bill is doing quite a bit of work. Bill does a positive work on the refrigerator, and the friction from the floor does the same amount of negative work. The work by Bill and the friction add to give zero total work. In this case trying to use the work energy theorem to find the work Bill does will give the wrong answer. Use the work energy theorem to solve problems only when they ask for the net or total work.
Heat and Random Kinetic Energy
When the temperature increases, the random motions of individual atoms and molecules are faster. Water molecules at a low temperature do not move randomly very much, so we have ice. As the heat energy and therefore temperature increases, the molecules have greater random motion, so the ice melts and becomes liquid water. With further temperature increases, the water becomes steam and the molecules are moving around much more rapidly.
A higher temperature means the average speed of random molecular motion is greater. So heat or thermal energy is simply random kinetic energy at the atomic or molecular level.
Whether it is random or not, kinetic energy is the energy of motion.
Further Reading
Knight, R.D., Physics for Scientists and Engineers with Modern Physics, Pearson, 2004.
