Big Categories of Energy

1. Potential Energy (stored energy)

2. Kinetic Energy (energy of motion)

3. Thermal Energy (heat)

*Mechanical energy is the SUM of kinetic and potential energy*

Types of Potential Energy

1. Chemical Potential Energy

2. Gravitational Potential Energy

3. Elastic Potential Energy

4. Nuclear Potential Energy

Chemical Potential Energy

• Stored in molecular bonds
• Released or stored during chemical reactions
• Examples: Food, Batteries, Car Fuel

Gravitational Potential Energy

• Stored energy from gravity
• Depends on the height and the mass of an object 
• The more mass and/or height, the more energy

Elastic Potential Energy

• Stored energy due to an object deforming
• Stretchy and bouncy things store a lot
• Related to the deformation (change in shape) and the objects stretchy/bounciness

Nuclear Potential Energy

• Energy stored in the nucleus
• Released in nuclear reactions

Electromagnetic Radiation

• Also called radiant energy
• Energy stored in electric and magnetic fields
• Examples: light, cell phone signal, wifi

Sound Energy

• From vibrating objects
• Energy goes between elastic potential energy and kinetic potential energy
• Molecules are pushed closer together than spread apart 

Electric Energy

• Can be potential energy or kinetic energy
• What everything powered by electricity uses

Kinetic Energy

• Energy of motion
• The faster something moves, the more kinetic energy
• The more mass, the more kinetic energy

Thermal Energy

• Heat is the change in thermal energy
• The internal energy of the molecules in a substance due to temperature
• Related to the TOTAL internal kinetic energy of the molecules

Conservation of Energy

• Energy is neither created nor destroyed. It can only change forms.
• Example: Chemical potential energy is turned into kinetic and thermal energy after consumed

Calculating Gravitational Potential Energy

Eg=mgh

Eg= ?, m= ?, h= ?

& What is it measured in?

• Eg= gravitational Potential Energy (J)
• m= mass (kg)
• g= acceleration due to gravity (9.8 m/s)
• h= height (m)

*Energy is measured in Joules (J)*

Calculating Kinetic Energy

Ek=0.5mv²

Ek= ?, m= ?, v²= ?

• Ek= kinetic energy (J)
• m= mass (kg)
• v²= speed or velocity squared (m/s)²

What is Work?

• Work is the transfer of energy
• W=fd (work= force times distance)
• If the force and distance are perpendicular, no work is done
• Any force and do work
• Refer to work, force, distance triangle for formula

How to Find Work

(two methods)

1. Take the force times the distance (W=fd)

2. Figure out how much energy changed from one form to another (work=change in energy)

Work from Gravity

• Fg=mg
• Distance it falls is the height
• W=Fd=mgh
• Same as the potential energy from gravity

Power

• Power is the rate at which you do work (rate at which you transfer energy)
• P=w/t
• Power is measured in Watts
• Refer to work, power, time triangle for formulas

What is Momentum?

• Momentum= mass times velocity (p=mv)
• Momentum is a vector and points the same direction as the velocity
• Units: kg*m/s
• Momentum is the most important variable for collisions and explosions
• Refer to momentum, mass, velocity triangle for fomulas

Conservation of Momentum

• If there are no external forces, the total momentum before a collision or explosion=the total momentum after a collision or explosion
• Related to Newton's Third Law. Equal and opposite forces causes equal and opposite changes in momentum

Collision Type: Elastic Collisions

• Bouncy Collisions
• Both momentum and kinetic energy are conserved

Collision Type: Inelastic Collisions

• Sticky collisions
• Momentum is conserved
• Kinetic energy is NOT conserved

Matter & Phases of Matter

• Anything that has mass and takes up space
• Solids, liquids, gases, and plasma

Solids

• In a solid state, molecules constantly vibrate, but around a fixed location
• A solid retains its shape and size regardless of its container

Liquids

• In a liquid state, molecules move around and flow around each other
• A liquid has a definite size (volume), but changes shape depending on the container it is placed in

Gases

• In a gaseous state, molecules move around quickly and randomly bounce and hit each other
• Gases have no definite shape or size. They fill up whatever container they are in

Plasma

• So hot electrons come off atoms
• The electrons occasionally reattach and makes light
• Too hot for molecules to exist

How to get from:

Solid-Gas              Solid-Liquid              Liquid-Gas

Liquid-Solid         Gas-Liquid                 Gas-Solid

Gas-Plasma           Plasma-Gas

• Solid-Gas: Sublimation
• Solid-Liquid: Melting
• Liquid-Gas: Vaporization
• Liquid-Solid: Freezing
• Gas-Liquid: Condensation
• Gas-Solid: Deposition
• Gas-Plasma: Ionization
• Plasma-Gas: Recombination

Extreme Matter

• Bose-Einstein Condensate: at temperatures colder than outer space
• Quark Gluon Plasma: at temperatures>10,000x hotter than the center of the sun
• Degenerate Matter: At very high pressure, in neutron start

Temperature

• In Kelvin: measures average kinetic energy of molecules

Thermal Energy

• Total kinetic energy of all the molecules (temperature times mass)

Heat

• Change in thermal energy

Absolute 0 & Outer Space

• Absolute 0= -460^oF. 0 Kelvin is the lowest possible temperature where there is no molecular motion
• Outer space is 2.7K (left over energy from the big bang)

Kelvin, Celsius, & Fahrenheit

(temp water boils, freezes, absolute 0)

• Water boils: 373K, 100^oC, 212^oF
• Water Freezes: 273.15K, 0^oC, 32^oF
• Absolute 0: 0K, -273.15^oC, -459.67^oF

Practice Problem #1

How much kinetic energy does a 5.0 kg dog running at 3.0 m/s have?

Answer: 22.5J

Breakdown:

Kinetic energy= 0.5mv²   

0.5 x 5.0kg x 3.0 m/s x 3.0 m/s

22.5J

Practice Problem #2

A student drops a 1.2 kg book from a height of 1.8 meters.  How much kinetic energy does it have just before it hits the ground?

Answer: 21.17J

Breakdown: 

Gravitational potential energy=mgh

1.2kg x 9.8m/s x 1.8m

21.17J

Practice Problem #3

A student drops a 2.00 kg rock from the top of a bridge that is 10.0 meter high.  How much kinetic energy does the rock have when it has fallen 1/2 way to the ground?

Answer: 98J

Breakdown

Gravitational potential energy=mgh

We want to find it 1/2 way down.

10/2= 5.0m, so h=5.0m

2.00kg x 9.8m/s x 5.0m

98J

Practice Problem #4

A 500 kg spherical cow is launched up into the air using a cowtapult at 20 m/s. What type of energy does it have at the start when moving at 20 m/s?

Answer: Kinetic

Practice Problem #5

A 500 kg spherical cow is launched up into the air using a cowtapult at 20 m/s. How much kinetic energy does it start with?

Answer: 100,000J

Breakdown:

Kinetic energy= 0.5mv²

0.5 x 500kg x 20m/s x 20m/s

100,000J

Practice Problem #6

A 500 kg spherical cow is launched up into the air using a cowtapult at 20 m/s. How high does the cow go?

Answer: 20.41m

Breakdown: 

Conserved kinetic energy=100,000J (last problem), will convert to gravitational potential energy at top (Eg=mgh)

Rearrange potential energy formula: h= kinetic energy/mg

100,000J/ (500kg x 9.8m/s)

20.41m

Practice Problem #7

A crane lifts a 400 kg steel beam a distance of 30 meters in 60 seconds. How much work did it do?

Answer: 117,600J

Breakdown: 

Work= fd= mgh

400kg x 9.8m/s x 30m

117,600J

Practice Problem #8

A 1000 kg car coasts down a 10 meter high hill without friction. How fast is it going at the bottom of the hill?

Answer: 14m/s

Breakdown:

Kinetic energy=gravitational potential energy

0.5mv²=mgh (cancel out mass from both sides)

v=√2gh

v=√2 x 9.8m/s x 10m

v= 14m/s

Practice Problem #9

A spherical cow is shot upwards with a speed of 10 m/s and sideways with a speed of 5 m/s.  What is the maximum height the cow travels?

Answer: 5.1m

Breakdown: 

mgh=0.5mv² (rearrange and cancel out m from both sides)

h= v²_y/2g (v_y= initial vertical speed)

h= 10²/ (2 x 9.8m/s)

5.1m

Practice Problem #10

A big firecracker is put into a coconut and it explodes into two pieces. The first piece is 0.20 kg and moves right at 4.0 m/s. The second piece is 0.40 kg. What is the momentum of the first piece?

Answer: 0.8kg*m/s to the right

Breakdown:

Momentum=mv (p=mv)

0.2kg x 4.0m/s to the right

0.8kg*m/s to the right

Practice Problem #11

Anna and Elsa are ice skating on a pond. Anna is 60 kg and moving towards Elsa with a speed of 3.0 m/s. Elsa is 50 kg and standing still. Anna eventual reaches Elsa and gives her a hug. What is Anna's momentum before the hug?

Answer: 180kg*m/s

Breakdown

p=mv

60kg x 3.0m/s

180kg*m/s

Practice Problem #12

What is 39 oC in K?

Answer: 312.15K

Breakdown: 

K= ^oC + 273.15

39 + 273.15

312.15K