The study of motion, and in particular how forces (pushes and pulls) affect the motions of objects (from atoms to super galactic clusters), is referred to as Mechanics. Classical Mechanics represents primarily the work of Isaac Newton, and takes this force-centric approach. However, we can also analyze motion from an energy-centric perspective, and this we call Lagrangian Mechanics. Both methods always arrive at the same results, but each has its advantages depending on the circumstances.
Quick Discussion on Forces
Forces are pushes or pulls on one object from another object, and can occur as either
contact or a
non-contact. For example, a contact force we all know and love is the force of
friction, while a non-contact force we have all experienced is the force of
gravity.
I must confess, at this point, a dirty little secret... ALL of the contact forces we are familiar with are, in actuality, simply Electric forces! How can this be? Well, think about friction. What is it? You might say, "It's two things rubbing together, heating up, and therefore losing energy." Well, yes, that's basically it. But can two things actually touch, in order to rub together? No, they can't! You see, what you experience as pressure when you pick something up is actually the result of electrical repulsions between the electrons in your atoms and the electrons in the object you are picking up. As you are standing on the floor, you are really not in contact with the floor! You are, in fact, hovering a few Angstroms above the level of the floor because the electrons in your feet are being repulsed by the electrons in the floor. If you were to actually "touch" the floor, you'd have to overcome an incredible force, one which we can only generate in large particle accelerators! This is one of those neat facts that confirms for me how cool physics is...
Quick Discussion on Energy
When we think about the energy of an object, we are not talking about a
property of that object, like, for example, it's color or odor. Energy comes and goes, continually being transformed from one type to another. Einstein showed us that energy and mass are one in the same (E = mc
2), so mass is just another form of energy, too.
What this all means is that when we say, "Oh, that pendulum bob has a potential energy of 1.2 Joules," we can't count on it having 1.2 Joules of potential energy a few moments later. That energy may have become transformed into 0.5 Joules of kinetic energy, 0.1 Joules of heat, and 0.6 Joules of potential energy, or any other combination. One caveat: the TOTAL ENERGY always remains the same, so if the bob has 1.2 Joules of total energy, then when we add up all the various forms of energy the bob has, that number always will add up to 1.2 Joules.
Potential? Kinetic? Heat? These are three of the possible forms energy may take. More info on this in the links below, but quickly, potential energy is a measure of energy available to the object according to some reference point. For example, we may say a car has 100 Joules of potential energy at the top of a certain hill, and by this we are implying that the reference point is the bottom of the hill. But we could also use sea-level as the reference point, or any other point, so long as we are consistent. Kinetic energy is energy associated with the motion of an object. Therefore, a fast moving linebacker has more kinetic energy than a slow moving linebacker (assume they have equal masses). Heat is internal excitation of the molecules or atoms of an object, and is considered "lost" energy, although the general law of Conservation of Energy tells us that energy is not gained or lost spontaneously, but rather simply transforms from one form to another. But, since we usually cannot recover energy that transforms into heat, we consider it "lost" to us.