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?