Energy It’s all about change

Energy is…  The ability to cause change

 Energy comes in many different forms  It can be saved and spent, just like money

TTTTwo main categories of energy: KKKKinetic Energy PPPPotential Energy

 Kinetic energy (KE) is the energy a moving object has because it is moving  KE can be calculated with the following equation:  KE = ½mv 2

KE = ½mv 2  We all know what m and v are  The units of energy are kgm 2 /s 2  This unit has a name, the joule (J)  So, from now on, the unit of energy is the joule

James Prescott Joule

 Now try the practice problems on the bottom of page 102 in your textbooks.

 1. KE = 0.5(0.15kg)(40m/s) 2 –KE = 600 J  ,000 J = 0.5(m)(20m/s) 2 –300,000 J = 200(m) –m = 1500 kg

 3. 4,000 J = 0.5(80 kg)v 2 –4,000 J = (40 kg)v 2 –100 = v 2 –V = 10 m/s

 Potential energy (PE) is the energy an object has stored due to position.

 There are many different types of Potential Energy:

Electric PE  The potential energy stored in electrons that are gathered at one end of a circuit, ready to move to the other end.  Found in batteries and made by generators

Chemical PE  Energy stored in the bonds between the atoms of many molecules  Food and gasoline both store chemical PE

Elastic PE EEEEnergy stored by something that can stretch or compress RRRRubber bands and springs store this kind of PE

Nuclear PE  Energy stored in the nucleus of an atom

Gravitational PE EEEEnergy stored by objects due to their position above Earth’s surface TTTThis one has an equation (oh, sweet joy!) PPPPEg = mgh

GPE = mgh  m = mass (in kg)  g = accel. due to gravity (9.8 m/s 2 )  h = height above the surface (in meters)  Unit, the joule (J)

Think about this…..  An object is falling. What happens to its gravitational PE and its KE?

 As it falls, height decreases, so GPE decreases  Also, as it falls, it accelerates, increasing velocity, so KE increases  In fact, every joule of GPE lost is gained as KE!!

 Tell Mr. Beamon to draw the flower pot and the apartment building.

 This brings up the next topic…..  Energy can change forms!  Of course you knew this already, since you are now watching a change in energy, from Electric PE to light

 Figure out what kinds of energy changes occur in the following situations:

 An electric space heater is in use.  Electric PE converted into thermal energy

 A propane space heater is in use.  Chemical PE converted to thermal energy

AAAA log burns. CCCChemical PE to thermal energy and light energy

 A car engine is running.  Chemical PE to thermal and KE  An object falls.  GPE to KE

 The sum of KE and GPE is called mechanical energy (ME)  While an object is moving because of gravity, ME does not change (assuming no friction of course)

 When an object falls we know GPE is converted to KE, and ME does not change.  How about when an object is thrown in the air?

 As a projectile rises, it gets higher, increasing GPE  Gravity slows it down, decreasing KE  At the top of its path, it stops for one instant, so it has a KE of zero…it is also at its max. height, so GPE is at a maximum!!

 On the way down, its GPE decreases (decreasing height) while its KE increases (increasing velocity)  The whole time, ME stays the same

 Pendulum motion…swinging weights!  As a pendulum swings, KE is converted to GPE and back again  ME does not change

The Law of Conservation of Energy  Energy can neither be created nor destroyed, only transformed  Basically, this means the total energy of the Universe can never change…it is fixed!

QQQQ: So, if your ME while swinging on a swing can not be destroyed, why do you stop if not pushed???? AAAA:..... ?