Kinetic Energy
Energy, the ability to work, appears in all naturals systems, and it comes in many forms. The identification of these forms posed a great challenge to scientists in the nineteen century. Ultimately, the recognized two very broad categories. Kinetic Energy, including heat and wave energy, is energy associated with moving objects, whereas stored energy or potential energy is energy waiting to be released.
Kinetic Energy
Think about a cannonball flying through the air. When it hits a wooden target, the ball exerts a force on the fibers in the wood, splintering and pushing them apart and creating a hole. Work has to be done to make that hole; fibers have to be moved aside, which means that a force must be exerted over the distance they move. When the cannonball hits the wood, it does work, and so a cannonball in flight clearly has the ability to do work--that is, it has energy--because of its motion. This energy of motion is what we call kinetic energy.
You can find examples of kinetic energy in nature. A fish moving through water, a bird flying, and predator catching its prey all have kinetic energy. A speeding car, a flying frisbee, a falling leaf, and anything else that moves.
Our intuition tells us that two factors govern the amount of kinetic energy contained in a any moving object. First, heavier objects that are moving have more kinetic energy than lighter ones: a bowling ball traveling at 10 m/s (a fast sprint) caries a lot more kinetic energy than a golf ball traveling at the same speed. In fact, kinetic energy is directly proportional to mass: double the mass, double the kinetic energy.
Second, the faster something is moving , the greater the force it is capable of exerting and the greater energy it posses. A high-speed collision cause much more damage than a fender bender in a parking lot. It turns out that an object's kinetic energy increases as the square of its speed. A car moving 40 mph has four times as much kinetic energy as moving 20 mph while at 60 mph a car carries nine times as much energy as at 20 mph.
