Energy, Work, and Power What the heck is Energy, and what do “Work” and “Power” mean?

Work, Energy, and Power We use these words frequently, in Physics they have very specific meanings. Energy is something that gets transferred or moved around to do something useful with.

Energy What are different types of energy? Kinetic, Potential, chemical, Nuclear solar, thermal (heat), sound, wind, electrical (elec/magnetic), etc.

Energy Energy: you can think of Energy as a way of keeping track of this “stuff” which is transferred around. Big Idea about Energy Conservation: For any closed “system”, energy can be transferred into different forms but is never lost….Often called the Law of Conservation of Energy

Energy In this class we’ll first look at forms of mechanical energy and how to calculate these. Later we’ll look into heat as a form of energy

Energy Potential Energy (due to gravity) is energy an object has because of it’s height or location. An object located above a table has potential energy (relative to the table) because it will fall if allowed to. (falls due to gravity) Potential Energy is relative to what you call the “reference position”. Potential Energy depends on height, mass, and the pull of gravity: PE = mgh The units of energy are….mgh = force x distance = Newton-meter = Joule

Energy Quick example….. Fluffy the 20kg cat climbs to the top of a saguaro cactus (height = 12 meters) What is the cat’s potential energy relative to the ground? (answer: mgh = 2400Joules) What is Fluffy’s PE relative to a deer mouse hiding in the cactus 5 meters above the ground? (answer: mgh = 1400Joules)

Energy Our hero Fluffy now jumps from the top of the cactus to the ground. What happens to the Fluffster’s Potential Energy? As Fluffy falls she is losing Potential Energy but gaining a different energy related to her picking up speed…..we call that Kinetic Energy or KE. What are the parameters which affect the amount of KE?

Energy Kinetic Energy is the energy an object has because it is moving, so it depends on speed or velocity. Does Fluffy moving at 5 m/s have the same KE as a soccer mom driving her 50,000kg SUV at 5 m/s? Well…..no….because KE depends on speed and mass, in fact ……

Energy KE = ½ mv² (notice that velocity is squared, so KE is highly dependent on how fast an object is going.) If you are driving your car at 60 mph how much more energy is that than driving at 30mph? It’s A LOT (yes, that’s the correct answer) Doubling the speed causes a 4x increase in kinetic energy….tripling your speed makes the energy go up by 9x!

Energy A pendulum is a simple example of an object transferring energy from all Potential Energy at the top of the swing to all Kinetic Energy at the bottom of the swing.

Energy and Work What about Work? We use the word “work” in many different ways…..in physics “work” has a very specific meaning. Work is an external Force which acts for a certain distance to change the energy of an object. W = F x distance. The Force applied must be in the direction the object is moving or no “work” is done. If you push an object forward you do positive work on an object. If there is friction then it is acting in the opposite direction and is doing negative work. Work has the same units as Energy

Power Power is another word that is commonly used in many ways….in physics it has a very specific meaning. Power = energy/time work/time …so it’s the rate of using or making energy per time. Power units are joules/sec which is another unit….what?

Power Yes, Watt

Summary of equations Kin. Energy = ½ m v² Potential Energy = mgh Work = Force x distance Work …..doing work on an object changes its energy… work and energy have the same units……Joules Power = energy/time or work/time = watts