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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. Two main categories of energy: Kinetic Energy Potential Energy.
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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
Two main categories of energy: • Kinetic Energy • 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 = ½mv2
KE = ½mv2 • We all know what m and v are • The units of energy are kg•m2/s2 • This unit has a name, the joule (J) • So, from now on, the unit of energy is the 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 • 2. 300,000 J = 0.5(m)(20m/s)2 • 300,000 J = 200(m) • m = 1500 kg
3. 4,000 J = 0.5(80 kg)v2 • 4,000 J = (40 kg)v2 • 100 = v2 • V = 10 m/s
Potential energy (PE) is the energy an object has stored due to position.
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 • Energy stored by something that can stretch or compress • Rubber bands and springs store this kind of PE
Nuclear PE • Energy stored in the nucleus of an atom
Gravitational PE • Energy stored by objects due to their position above Earth’s surface • This one has an equation (oh, sweet joy!) • PEg = mgh
GPE = mgh • m = mass (in kg) • g = accel. due to gravity (9.8 m/s2) • 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 log burns. • 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: So, if your ME while swinging on a swing can not be destroyed, why do you stop if not pushed???? • A:. . . . . ?
