1. The Currency of the Universe
Energy
The Currency of the Universe
Standards:
2a Students will be able to calculate the kinetic energy of an object using KE=1/2 mv2.
2b Students will be able to calculate the gravitational potential energy of an object using the equation PE=mg delta h where delta h is the change in elevation.
2h* Students will be able to solve conservation of energy problems with springs and capacitors.
NGSS
HS-PS3-1 Create a computational model to calculate the change of energy of one component in a system when the change of another component and energy flows into and out of the system are known.
HS-PS3-3 Design, build and refine a device that works within given constraints to convert one form of energy into another.
This is a good direct lecture hitting the major points of energy that can be done during a late start schedule. Has useful examples in check questions and discusses there being one energy but many expressions of this energy. Revised in 2016

2. Energy in General
Energy is the ability to cause change in a body or system (changes in height, speed, temp, stretch or squash).
Energy is a scalar quantity measured in Joules=N m = kg m2/s2
Energy is expressed in different forms and can be transformed from one form to another without loss within a system or body.
Energy is conserved for a single body or system when there is no outside net force.
Question: Define energy. Is energy a scalar or vector quantity? What units are used to measure energy? What is unique about transformations from one form of energy to another? When is energy conserved?
Activities:
Quickly run through these generalities about energy… it’s the quantity that causes change in objects and systems, it’s a scalar measured in Joules.
Talk about how there is one energy but many different expressions of energy. There’s energy expressed motion of an object, in the temperature of an object, there is stored energy something has because it is in a gravitational, electric or magnetic field, there is energy stored due to nuclear forces and expressed as mass itself. There is one energy but many ways to experience it.
Discuss that energy can change how it is expressed w/o loss. That is the energy expressed as motion can compress a spring and become the energy stored in a spring which can then be released and propel a ball so the energy is then stored in a gravitational field and so forth. All this is done without using up the energy.
Discuss how energy is a conserved quantity. Specifically it is conserved when there are no outside forces. This is similar to momentum and means energy is always conserved if you make your system big enough. But energy is also conserved not only for a system of objects (like momentum), but also for single objects.
VIDEO: Honda Commercial or Mr. Wizard Video if time.

3. Mechanical Energy Energy Expression Definition Equation Kinetic Energy
Energy expressed in motion of an object
KE = ½ mv2
Gravitational Potential Energy
Energy stored due to an object’s position in a gravitational field
DPEg =mg Dh
Elastic Potential Energy
Energy stored in the stretch or squashing of an elastic object
DPEe = ½ k Dx2
Questions: List the three expressions of energy that are considered mechanical energy. Define each and state the equation used to find their magnitudes. When is mechanical energy conserved?
Activities:
Write definition on board and then add names.
Then give equations and discuss doubling one variable and what it does to others. Include doublling velocity, elevation and displacement. Also ½ mass, spring constant, and displacement.
Point out that Gravitational PE equation is only for near Earth’s surface and really is weight x elevation change.
State that sum of these three expressions of energy is called mechanical energy and that it is conserved for single objects and systems of objects when there is no force and even when there is an outside force as long as its gravity, elastic force or other conservative force.
Discuss how can get different answers for what is the PEg of an object but that the change in PE will be agreed upon whether zero height is table, floor, or center of Earth.

4. Check question
Each of the balls to right has a speed of 1 m/s and a mass of 2 kg. The directions of their velocities are indicated by the arrows. What is the total kinetic energy of the system of balls?
Energy is a scalar quantity so answer is 3 Joules

5. Check Questions: Some Energy Math
A car traveling at 40 mph has a certain kinetic energy. How much more KE does it have when it travels at 80 mph?
A firefighter has a certain gravitational PE when he climbs half way up a ladder. How much more PE will he have when he is at the top of the ladder?
An elastic ball stores a certain amount of PE when it is compressed 1 mm. How much more PE will it store when it is compressed 2 mm?
4 times, 2 times, 4 times

6. Work (W)
Work is an energy transfer where an outside net force changes the energy of a body or system.
Work is a scalar quantity w/ same units as energy.
Work = net force x distance object moves in direction of force = Fnet d (cos q)
Work is positive if force is in direction of motion and increases energy.
Work is negative if force is in opposite direction of motion and decreases energy.
Force and force components perpendicular to motion DO NO WORK
Question: What is mechanical work? Is it a scalar or vector quantity and what units are used to measure it? How is work related to the mechanical energy of a body or system? When is work considered positive and when is it considered negative? How much work does a force or force component which is perpendicular to an object’s motion do on the object?
Activities:
Draw three boxes in motion to the right. Put a force in the same, opposite and perpendicular to consecutive boxes. Ask what each force does to the speed and the kinetic energy of the box. Say a net force that causes a change in energy is called a work. Define positive, negative and no work cases.
For physics write W= F d where F and d must be in same or opposite direction of distance force is applied. For physics E write equation Work = Fd cos (angle) and show how it predicts this result.
This is same result as a force changes velocity, an impulse changes momentum and now a work changes energy. A NET FORCE CAUSES A CHANGE IN MOTION WHETHER THAT MOTION IS MEASRED WITH ACCELERATION, MOMENTUM or KINETIC ENERGY.

7. Work and Energy Relationships
Mechanical Energy
Kinetic Energy
Elastic Potential Energy
Gravitational Potential Energy
Positive Work
Negative Work
Question: What is the relationship between work and energy?
Activities:
Draw a wealth circle with accounts for savings, checking and savings bonds. Represent an ATM with an arrow in for deposits and out for withdrawals.
Draw Mechanical energy circle with accounts for kinetic energy, PEE and PEg
Write wealth equation and then mechanical energy equation for two circles and discuss similarities. ATM changes wealth, work changes mechanical energy. Discuss how ATM does money transfers while Work does energy transfers. These transfers can go into one, two or all accounts.
Physics E write and compare Work-KE, Work-ME and Work-Total energy equations. Discuss weight lifter holding barbells above head and how work is done chemical level causing raising of lifter’s temperature. Discuss when each equation is true.
Show above picture and derive the energy equation.
Go through money analogy with checking account, savings account and treasury bonds standing in for kinetic energy, gravitational potential energy and elastic potential energy.
Analogize positive work as deposits which increase your wealth and negative work as debits that deduct from your wealth. Write wealth equation.
Write the wealth equation and compare it to the Energy equation. Discuss how like the many money accounts there are many energy accounts, Money can be moved from one account to another w/o loss like energy can move between kinetic, PE g and PE e accounts w/o loss. There is one kind of money just like there is one type of energy but how it is expressed can be different.
Discuss how will add another energy transfer method and energy account when talk about thermal energy.
Discuss how book only gives first term in its work-energy equation and why this is incomplete. Write full kinetic energy-work equation and discuss how it is useful when dealing with a force that is constant over a distance.
W=DKE + DPEg + DPEE

8. Check Question
A boy pulls a wagon using a handle a distance of 10 m. The boy pulls with a force of 5 N at an angle such that the vertical component of the pull is 3 N and the horizontal is 4 N. What is the work the boy does on the wagon?
50 N m
40 N m
30 N m
3 N component is perpendicular to direction of motion and thus does NO work.
Work = 4N x 10 m = 40 Nm = 40 J

9. Check Question
A car traveling at 20 miles per hour slams on its breaks and skids to a stop in 20 feet. How far will it take the car to stop if it is traveling at 40 miles per hour?
20 feet
40 feet
80 feet
Double the speed means 4x the energy means 4 x the distance = 80 feet.

10. Check Question
A 20 kg satellite orbits the Earth in a circular orbit. The satellite remains 10 km above the Earth and retains a speed of 7.0 km/s throughout its orbit. What is the work done by gravity on the satellite?
0 Joules
140 Joules
490 Joules
Force of gravity and motion are perpendicular. Zero Work done by gravity. Object doesn’t change speed or height.

11. Check Question
Mr. DeVoe lifts up a trash can and moves it to the curb every Thursday night. State whether work is done by Mr. DeVoe’s lifting force and if the work is positive or negative.
Picking can up
Moving can horizontally at constant speed
Slowing can down
Lowering can

12. Power (P) Power is the rate at which energy is used.
If machine A has twice the power of machine B, it can get the same amount of work done in half the time or twice the amount of work done in the same time.
Power = Work / time for work to be done
Scalar quantity with units of Joules/ second =Watt
Question: Define power. Explain what a more powerful machine can do vs. a less powerful one. Is power a scalar or vector quantity and what units are used to measure it? Write the mathematical relationship between power, work and time.

13. Check Question
A 400 N student climbs a flight of stairs that is 10 m tall. The student does this climb in 10 seconds.
How much work does the student do during this climb?
What type of energy does the work change?
What is the power of the student?