What is Energy?

Qualitative Definition of Energy:

• Energy is the currency of change.• Energy, though not a substance, is conserved

like a substance and has the capability to cause change.

• Energy is transferred from place to place, but is never destroyed.

• Tracking the changes in a system will track the energy transferred.

Can energy be used up?

• Your experience of energy probably makes it appear that energy is “consumed” or used up:

• Bouncing ball is dropped and comes to a rest• Cell phone goes dead after it runs out of battery• Spring powered toy becomes unwound and comes to a

stop.

• However, energy has not been used up, it has been transferred elsewhere.

• Total Energy is Conserved!

Energy is Like Money

• Energy can be stored in different accounts.

• Just as all money is in units of currency ($, pesos, etc.) Energy has its own units (J,C, etc.) But they all mean energy

Monetary Account Energy Account

Checking Account

Saving Account

Xbox Credits

Bovada Sports Betting

Kinetic Motion

Elastic

GravitationalWallet

Energy is Transferred like Money:

• Energy like money can be transferred in many ways:

• We will mostly calculate energy transfer via Working- Force * Displacement.

Monetary Transfer Energy Transfer

Write a Check

Hand Cash

Credit Card

iPay

Radiating

Heating

Working

Energy is like Information

• Both energy and information can be stored and transferred in different ways:

Information Storage Information Transfer

Books

Music

Memory

Pictures

Reading

iTunes

Speaking

Instagram

Mechanical Energy Accounts

• Energy = Change• As the spring stretches, what quantities change?– What can be measured?

• Both force required and the length of the spring change

• The bigger the change in the value of these quantities, the more energy transferred

(where does this energy come from?)(and where does it go to?)

Elastic Energy

• Amount of elastic energy depends on:– How far the spring is deformed (stretched or

deformed) in meters– How difficult it is to deform the spring

• More length = more energy• More force to deform= more energy

Spring Launches Cart

• Remember energy is the currency of change.

• Where does the energy come from and where does it go?– Energy stored in spring and is transferred into the

motion of the cart (kinetic energy).

Kinetic Energy (K.E.)

• Kinetic Energy is the energy associated with motion.

• Amount of K.E. depend on– How fast (speed)– How massive the object is (mass)

• Greater Speed = more KE• Greater Mass = more KE

Working

• Work = Force* Distance• Energy is transferred through work, which

happens when you put force over a distance

https://www.youtube.com/watch?v=xBnS23U_ao4

Lifting a box

• If a box is lifted at a constant speed.– Did this take energy?– Where does the energy go?

• This does require energy• The energy goes into another

energy storage called gravitational energy

Gravitational Energy

• Gravitational Energy is the energy associated with the height of an object.

• Depends on:– Height, Mass, Gravitational Field Strength

Dissipated Energy

• Energy that gets transferred into an unrecoverable account.– Heat, friction, sound, etc.

• Like banking fees or taxes

Energy Bar Graphs

• Energy Accounts:– Elastic Energy– Kinetic Energy– Gravitational Energy– Dissipated Energy

Example 1

• The spring is part of our system

• “system= spring + cart”

Example 2

• The spring is not apart of the system

• “system”=cart

Work by Spring