1. Chapter 13
Work & Energy

2. Section 1
Work, Power & Machines

3. W = F x d (Newtons x meters)
Work
Work: done ONLY when a force is applied to an object, and the object moves IN THE SAME DIRECTION OF THE APPLIED FORCE
Work is calculated by multiplying the force by the distance over which the force is applied
Work = force x distance
W = F x d (Newtons x meters)
The SI unit for work is Joules (J)

4. Work Work is ZERO when an object is not moving
If you are pushing a stalled car and cannot move it, you might exert a lot of force, but no work is done
If you move it, even a little, work is done

5. Machines
Machines can help us do work, but only some of it is useful work (work the machine was designed to do)
Efficiency: the amount of useful work a machine does
Some work is done when the machine generates heat or noises, but it isn’t “useful” because it is not what the machine is intended for
A pulley used on a boat may experience a decrease in useful work if heat is generated from friction or if the pulley squeaks

6. Power
Power: the rate at which work is done, or how much work is done in a given amount of time
Power = work/time
P = W/t
The SI unit for power is watt (W)
One watt is the amount of power needed to do one joule of work in one second

7. BE CAREFUL! Power The W for work is italicized
The W for watt is not italicized
Science 360: Hockey Work, Energy and Power

8. Section 3
What is Energy?

9. Energy Energy: ability to do work
Energy is ALL around us and exists in many forms
Whenever work is done, energy is transformed or transferred from one system to another
System: a portion of the universe that is chosen for studying the changes that take place within it

10. Energy The SI unit for energy is Joules (J)
The same units as work since work is only measured when you use energy to do it
All types of energy will be either
potential
kinetic

11. Potential Energy (EP)
Potential Energy (PE): stored energy, or the energy of position
EVERYTHING has potential energy – it is converted into other forms of energy!

12. GPE = mass x free-fall acceleration x height
Potential Energy (EP)
Gravitational Potential Energy (GPE): potential energy due to an object’s height above ground
Depends on mass and height – Heavier objects will have more GPE, and items higher up will have more GPE
GPE = mass x free-fall acceleration x height
GPE = mgh
g = 9.8m/s2
GPE = m(9.8 m/s2)h

13. Kinetic Energy (EK)
Kinetic Energy (KE): energy of motion – the energy an object has because it is moving
Things as large as planets and as small as atoms have kinetic energy
Once an object begins to move, it has the ability to do work
The kinetic energy depends on the mass of the object, and the rate of acceleration

14. Kinetic Energy (EK)
Speed matters more than mass, as a small change in speed produces a large change in kinetic energy
A speeding car will do more damage because it has higher kinetic energy
An apple that is falling at 10 m/s can do more work than an apple that is falling at 1 m/s
Atoms and molecules are always moving, so they have kinetic energy
The formula for kinetic energy involves a square root, which matters when solving for speed

15. Kinetic Energy (EK)
KE = ½ x mass x speed squared (you can use 0.5 instead of ½ when doing the math)
KE = ½ mv2
SI unit is J
The kinetic energy of a golf ball is measured to be J. If the golf ball has a mass of about kg, what is its speed?
KE = J
m = kg
v = ?

16. Kinetic Energy (EK)
143.3 = ½ (0.047)v /0.047 = ½ v2  3,048.9 = ½ v2 3,048.9/0.5 = v2  6,097.9 = v2 6,097.9=v2 = 78.1 m/s
1. Plug known values into equation
2. Divide KE by mass
3. Divide by 0.5
4. Take square root of BOTH sides

17. Forms of Energy MRS CHEN 7 forms of energy that fall under KE or PE
Mechanical (KE + PE)
Radiant (light) (KE)
Sound (KE)
Chemical (PE)
Heat (thermal) (KE)
Electrical (PE)
Nuclear (PE)
MRS CHEN

18. Forms of Energy
Mechanical Energy: energy associated with the motion and position of an object (KE + PE) in an object that is used to do work. MOTION.
A car moving, a ball that has been thrown, a person falling
Energy that does not affect motion on a large scale is nonmechanical
When you eat food the energy stored in it is non-mechanical energy

19. Forms of Energy
Chemical Energy: Energy that is released or absorbed by the rearrangement of bonds between atoms
Fuels – food, batteries, gasoline
Nuclear Energy: the movement of particles in the nucleus of an atom
Fission and fusion
This is how the sun gets energy!

20. Forms of Energy
Electrical Energy: energy resulting from the movement of electrons across a circuit
Lightning
Thermal Energy: the sum of kinetic energy in all the particles of an object
Heat energy
The higher the temperature, the more thermal energy
The larger something it, the more thermal energy it has

21. Forms of Energy
Light Energy: energy produced by vibrations of charged particles
Also called electromagnetic energy or radiant energy
Visible light, X rays, UV rays, the sun
Sound Energy: vibrations travelling through the medium of the sound
Sound requires a medium (something to travel through), like a wave

22. Chemical: guitarist gets energy to perform by eating food
Electrical: used by stage lights to produce Light
Sound: produced when he plucks the strings
Mechanical: plucking the strings of the guitar
Thermal: heat in the air

23. Conservation of Energy
Section 4
Conservation of Energy

24. Law of Conservation of Energy
Energy readily changes form from one type to another, however, energy cannot be created or destroyed
When total energy in a system increases, the increase must be due to energy that enters the system
For example, if a person on a trampoline bounces higher the second time, we can conclude their legs did work to add energy to the bounce

25. Energy Transformations
Potential energy can become kinetic energy
As a car on top of a hill rolls down, the energy changes from PE to KE
Kinetic energy can become potential energy
At the bottom, the KE takes the car up the next hill; the car slows down and most of the energy turns back into PE

26. Energy Transformations
The change of kinetic to potential energy and vice versa isn’t always complete
If so, balls would always bounce to the same height and roller coasters would never stop gliding
Some mechanical energy (KE + PE) will change to other forms of energy
Some is released as a sound from the plastic compressing
The air around the ball gets warmer

27. Energy Transformations
Energy transformations happen ALL AROUND YOU!
Light energy from the sun is converted to chemical energy through photosynthesis, and electrical energy through solar panels
A TV changes electrical energy into sound and light energy
A car changes chemical energy (gas) into thermal and mechanical energy
Nuclear energy generates thermal and electrical energy
Potential energy is converted to kinetic energy when you stretch a rubber band and then release it

28. Thermodynamics
Thermodynamics: study of heat and temperature and their relation to work and energy
Thermodynamics explains energy conservation with the First Law of Thermodynamics: the net change in energy equals energy transferred as work or heat
If heat energy is added to a system, some energy stays in the system and some leaves. The energy that leaves does work on the area around it, energy that stays creates an increase in energy of the system.

29. Thermodynamics
If a pot of water is at room temperature and you add heat to the system:
1st, temperature and energy of water increases.
2nd, the system releases some energy and it works on the environment (maybe heating the air around the water, making the air rise).