Some more stuff you should know

Physics of the Mundane

Adam Johnston

“Energy” is a physical quantity that objects can have in various forms.  When the concept of energy was first contrived, it was not really understood as a physical kind of “stuff”, but as simply an easy way of keeping track of what an object is doing and/or what it is capable of doing.  The key to this concept is that the quantity of energy is conserved:  As long as you can keep track of where the energy is going and what form it is in, you can describe a great deal about what kinds of things can happen.

Some examples of energy include:

Many others exist.  The point is that once an object has a certain amount of energy, this amount (but not the form) is conserved.  Thus, an object which falls from a building has its potential energy change to the same amount of kinetic energy as it falls; an wheel rolling down a ramp has its potential energy change to kinetic and rotational kinetic energies as the wheel picks up speed. 

If we are to transfer energy in or out of an object (say, by pushing on it), we are doing “work”.  This amount of work is equal to the amount of energy lost or gained by the object.  You do an amount of work equal to the amount of kinetic energy of a ball that you are throwing.  The ground does an amount of work to your car that is equal to the kinetic energy it had before you started applying the brakes to bring the car to a stop.