CHAPTER 4 SECTION 2
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 changeWithout energy nothing would ever change
Energy can also be described as the ability to do workExample: 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 m2/s2
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 kgv = 3.0 m/s
KE = ½ mv2
KE = ½ (60kg) [(3.0m/s)2]KE = ½ (60kg) (9.0 m2/s2)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 substanceWhen 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 = mghGPE = (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?