We are all familiar with the idea of energy, but it was highly debated until the 1850’s. Energy is what sustains life, but yet it is still difficult to define. Perhaps because energy is only observable when it is being transferred from one place to another, or transformed from one type to another. We will begin energy by looking at a related concept, work. Work The change in an objects motion is related to both the force that acts upon it, and the amount of time that the force acts on the object. In this instance, time is a factor. When we study work, time is not a variable. Work uses the quantities of force and distance. When we do work, we lift a load against earth’s gravity. The heavier the load, or higher we lift it, the more work that is done. Two variables are involved in every case of work being done: the application of a force the movement of something Work = force x distance If we lift two loads up 1 story of a building, we do 2x as much work as lifting only one load up the same distance, because the force required was 2x as great as only one trip. Similarly, if we lift one-load two stories as opposed to one, we do 2x the work, because the distance is 2x as great. Work involves both a force and a distance. A weight lifter holding a barbell above his head does no work on the barbell, because it is not moving. He is doing work on his muscles, which are stretching and squeezing. The initial lift of the barbell, however, was/is work. There are generally two categories of work, one is the work done against another force, for example pulling back a bowstring. The other is work done to change the speed of an object, for example speeding it up, or slowing it down. Work combines Newtons (force) with meters (distance) to give us the joule. One joule of work is done when 1N is exerted over 1m. Power Work does not deal with time. To discuss how fast work is done, we use power. Power is the rate at which work is done. Power= work/time High-powered engines do work rapidly, and therefore have a lot of power. The unit of power is the watt. One watt of power is used when one joule of work is done is one second. A kilowatt is 1000 watts. In the U.S. we customarily rate engines using horsepower, but the metric system of rating engines uses kilowatts. One horsepower is the same as 0.75 kilowatts. Mechanical Energy Energy is also measured in joules. It is that certain something that allows an object to do work. It has many forms. One of the most common is mechanical energy. This is the energy due to the position of an object, or the movement of an object. There are two types of mechanical energy: kinetic and potential. Potential Energy Energy that is stored and held is potential energy. Examples are a stretched rubber band, a drawn back bow, a boulder at the top of a hill, or a paused roller coaster at the top of a hill. All types of fuel contain potential energy, such as batteries, fossil fuels, and food. Elevated objects that would fall because of gravity have gravitational potential energy. Example: water tower, pile drivers ram. The gravitational energy possessed by an object is equal to the amount of work it took to lift it against gravity. We can figure potential energy by multiplying the weight by the height. PE=weight x height Kinetic Energy If an object is moving, it can do work. This energy of motion is called kinetic energy. Kinetic energy is dependent on the mass of an object as well as its speed. KE= ½ mass x speed2 or, KE = ½ mv2 When you throw a ball, it has the ability to hit something and push it, therefore doing work on that object. If there is no change in energy, no work occurs. This is the work-energy theorem. Conservation of Energy It is important to understand how energy transforms from one type to another more than what energy really is. Imagine this scenario: As you draw back the stone in a slingshot, you do work stretching the rubber band. The rubber band then has potential energy. When released, the stone has kinetic energy equal to this potential energy. It delivers this energy to a target. Energy changes from one form to another without loss or gain. The law of conservation of energy says: Energy cannot be created nor destroyed. It can be transformed from one form to another, but the total amount of energy never changes.