Describing Energy

What does energy mean to you?  Let’s Discuss (this means you don’t have to write it down) How do you commonly use the word energy to describe things? What different forms of energy do you know about? What are examples of energy you see every day?

Energy and Work  Energy: the ability to cause change Without energy nothing would ever change  Energy can also be described as the ability to do work Example: a tennis racket causes a change when it hits a tennis ball ○ The tennis racket does work on the tennis ball by applying a force through a distance ○ When this happens the racket transfers energy to the ball

Systems  System: anything around which you can imagine a boundary  This can be a single object (such as a tennis ball), or a group of objects (such as the solar system)  Work can transfer from one system to the next

Different Forms of Energy  Turn on an electric light, a dark room becomes light  Turn on an IPod, sound comes through the headphones  In both situations a change occurs but this is different than a tennis racket hitting a ball  This is because energy has many different forms These forms include: mechanical energy, electrical energy, chemical energy, and radiant energy.

Different forms of Energy cont.  Energy plays a role in every activity you do

The Energy-Money Analogy  Energy exists in a variety of forms just as money does  You can convert money into coins, dollars, $20 dollar bills, etc.  The same is true for energy!  Energy from the sun warms you and energy from the food you eat are two different forms of the same thing

Kinetic Energy  Kinetic energy: energy due to motion  Kinetic Energy Equation:  1 Joule (J) = kg x m 2 /s 2

Kinetic Energy Example  A jogger with a mass of 60.0kg is moving forward at a speed of 3.0 m/s. What is the jogger’s kinetic energy form this forward motion?  KE = ? m = 60.0 kg v = 3.0 m/s KE = ½ mv 2 KE = ½ (60kg) [(3.0m/s) 2 ] KE = ½ (60kg) (9.0 m 2 /s 2 ) KE = 270J * Note: square the velocity first before continuing to solve

Potential Energy  Potential Energy: the energy that is stored due to the interactions between objects  Examples: An apple hanging from a tree ○ Energy stored between the apple and earth b/c of gravitational force between the two Objects connected by a compressed spring ○ Energy is stored in the compression of the spring Potential Energy is like money stored in a savings account. You aren’t using it now but it is there for you to use when it is needed.

Kinetic and Potential Energy

Elastic Potential Energy  Elastic Potential Energy: energy stored by compressing or stretching an object

Chemical Potential Energy  Chemical Potential Energy: energy that is due to chemical bonds  You may notice chemical potential energy when you burn a substance When an object is burned, chemical potential energy becomes thermal energy and radiant energy

Gravitational Potential Energy  Gravitational Potential Energy (GPE) : energy that is due to the gravitational forces between two objects  Any system of objects attracted to each other have GPE

Gravitational Potential Energy Equation  The solar system also has GPE  GPE depends on mass, gravity, and height  Remember: on Earth g= 9.8 N/kg

Height and GPE  You measure heights from a reference level Ex: measure from the floor to the ceiling or measure from the top of your desk to the ceiling  This means GPE varies depending on a chosen reference level

GPE Example  A 4.0kg ceiling fan is placed 2.5m above the floor. What is the GPE of the Earth- ceiling fan system relative to the floor?  GPE = ?  m = 4.0kg  g = 9.8 N/kg  h = 2.5 m GPE = mgh GPE = (4.0kg) x (9.8N/kg) x (2.5m) GPE = 98 N x m = 98 J

In Class/Homework  For the rest of the period work on Assignment 4.2  Finish this up for homework and turn it in tomorrow by the beginning of class  If you have questions COME IN DURING TUTORIALS. Do not come in at the beginning of class and say you don’t understand.

Wrap Up  How are energy and work related?  What are the different forms of energy?  What is a system?  What is the units for energy?  What is the difference between kinetic and potential energy?