Unit 5-3: The Conservation of Energy

Conservation of Energy The past two sections have focused on what energy is, but the more important concept is how it transforms. That is the focus of this section: –Transferring energy from one object to another. –Transforming energy from one form to another.

Conservation of Energy Imagine this example: –An archer nocks an arrow and pulls the bowstring back to his chin. The archer has done work by pulling back on the bowstring and that energy was transferred into the bow in the form of SPE. When the arrow is loosed, all the SPE transfers into the arrow and the arrow gains that energy as KE.

Conservation of Energy The Law of Conservation of Energy: –Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes. This is a universal law, it is true no matter where in the universe you are.

Conservation of Energy Every system is subject to the law of conservation of energy (COE): –A rock falling off a cliff –A person leaping from a building (why would you do this?) –A pendulum swinging –A pinball machine

Conservation of Energy For our calculations, we will be focusing on the changes in potential and kinetic energy. –We will also be assuming that there is no energy that leaves the object due to friction, heat, or sound. We will assume that the total energy (TE) of a system is always the sum of the kinetic and potential energy.

Conservation of Energy What does work have to do with it? –Work is the transfer of energy into or out of an object. –When work is done on something, it gains energy. That energy remains with the object until work is done to remove it. –So if you do work (ex. Lift a ball up), that object now has more energy.

Conservation of Energy We can determine the kinetic and potential energy of any point that we have enough information. In this example, Point A. The KE for point A is zero because the car is not moving initially. The GPE is 337,500J. The TE is 337,500J as well. What is Vb?

Conservation of Energy The first step is to calculate the potential and kinetic energy at any single point. –From that we determine the total energy of the system. Once we have the total energy, we can figure out the potential and kinetic at any point by using a little inference.

Conservation of Energy Examples of inference: –If the object is halfway down, we can assume that the potential energy and kinetic energy are evenly split. –If the object is at the bottom or ground level, the GPE is zero. –When a spring is fully compressed, there is no KE. –If the object is 1/4 of the way, or 1/3, the potential and kinetic will be split up from the total accordingly.

Conservation of Energy Once you know the total energy and can figure out how much potential and kinetic there is at a point, you can solve for the height, velocity, spring constant, etc. The only new part of this lesson is the conservation aspect.

Examples

Machines A machine is a device used to multiply force or simply change its direction. Conservation of energy is present in every single machine out there. Machines do not do more work, they simply provide more force (over a smaller distance) Remember, energy must be conserved at all times.

Energy for Life If we think big enough, the universe is one giant system. –In that system, energy is conserved. –The amount of energy that was present at the very beginning of the universe is exactly equal to the amount of energy there is today. The only thing different is the form of that energy.

Energy for Life On a smaller scale, we can look at the energy needed for life to exist on planet Earth. –When we look at it that way, we find that all life on planet earth is solar powered. –How can this be? As energy comes to earth in the form of solar radiation, it is absorbed by green plants that use that energy to grow. Then herbivores eat those plants and grow. Carnivores then eat the herbivores and grow. We eat the plants, herbivores, and carnivores. Eventually the plants and animals die and decompose, which is in turn recycled back into the planet for more plates to grow. That sounds familiar…