1. Energy, Work, & Power
SP3. Students will evaluate the forms and transformations of energy.
a. Analyze, evaluate, and apply the principle of conservation of energy and measure the components of work-energy theorem by
• describing total energy in a closed system.
• identifying different types of potential energy (PE).
• calculating kinetic energy (KE) given mass and velocity.
• relating transformations between PE and KE.
f. Analyze the relationship between temperature, internal energy, and work done in a physical system.
g. Analyze and measure power.

2. What is energy & why do we need it?
Energy – the ability to do work
Work – moving an object by exerting a force
Force – a push or a pull
Whenever force is used to move an object, energy is required
EX. When you walk, your muscles pull on your bones in order to move your body. This requires your muscles to use energy.

3. What is energy & why do we need it?
Without energy, there could be no motion
Atoms and molecules could not move
Stars could not shine
Planets could not orbit around stars
Animals could not walk, run, swim, or fly
The wind could not blow
Messages could not be sent from your brain to your body

4. Is work being done when you hold a book over your head?No
But it did take work to get it there
Whenever work is being done, energy is being used
The energy was transferred from your body to the book

5. How much work is being done?
We can measure the amount of work being done to move an object
We need to know 2 things in order to measure work:
The amount of force being used
The distance of the movement
We can calculate work using the following formula:
Work = Force x Distance (W = F d)

6. Calculating Work – Which Units?
When calculating work, use the following units:
Force is measured in newtons (N)
Distance is measure in meters (m)
Work is measured in joules (J)
1 joule equals 1 newton multiplied by 1 meter
You do about 1 joule of work when you lift an apple above your head

7. Calculating with Work –Problem #1
If you lift a 3 N book 2 meters off the floor, how much work did you do?
W = F x d
W = 3N x 2m
W = 6 J

8. Calculating with Work –Problem #2
A man pushes a couch a distance of 0.75 m. If 113 J of work is done, what is the magnitude of the force applied??
F = W ÷ d
F = 113 J / 0.75 m
F = 151 N

9. Calculating with Work –Problem #3
It requires 3480 J to move a 675 N object how far?
d = W ÷ F
d = 3480 J / 675 N
d = 5.16 m

10. Calculating with Work –Problem #4
The 3rd floor of a house is 8 m above street level. How much work is needed to move a 150 kg refrigerator to the third floor?
W = Fd
F = mg (150 kg x 9.8 m/s2)=1470 N
W = 1470 N x 8 m
W = J

11. Calculating with Work –Problem #5
A toy truck is pushed across a table 0.80 m north and pulled back 0.80 m south. If a constant horizontal force of 15 N was applied in both directions, what is the net work?
Displacement = 0 (same distance N and S)
No motion = no work done

12. Question
When you carry a heavy bag of groceries from your car to your kitchen, what does work, your arms or your legs? Explain why.
Your legs, because they move you and the groceries from the car to the kitchen. Your arms only lift and hold the groceries.

13. Energy Is Also Measured In Joules
Since energy is required to do work, it is measured using the same unit (joules)
The amount of energy required to do work is ALWAYS greater than or equal to the amount of work being done
EX. If you do 6J worth of work to lift a book, you need at least 6J of energy to do it

14. Doing Work Gives Energy to Objects
When work is done on an object:
Energy is transferred from the object doing the work to the object having work done on it
The object doing the work loses energy
The object having work done on it gains energy
EX. A student pushing a desk across the floor is doing work on the desk – energy is transferred from the student to the desk and the student loses energy

15. Why do we get tired after doing work?
We give up our energy to all the objects we touch and move around
In any energy transfer in our body, some energy is changed to heat and transferred to our environment – the energy is LOST from our body
Staying alive requires a lot of energy
Heart beating, brain sending messages, cells moving substances in and out, muscle contractions, etc.
As our body’s energy gets low, we get tired

16. What is power? Power – the rate at which work is done
More power means…
More work is done in the same amount of time
The same amount of work is done in less time
EX. A person that is a more powerful runner is faster and can run farther in the same amount of time as a less powerful runner (more work in equal time)
EX. A car with a more powerful engine can accelerate to 60mph faster than a car with a less powerful engine (equal work in less time)

17. Calculating Power Power is measure in watts (W)
We need to know 2 things in order to measure power:
The amount of work being done
The amount of time it takes to do the work
We can calculate power using the following formula:
Power = Work / Time (P = W / t)
1 watt equals 1 joule divided by 1 second

18. Calculating Power – Sample Problem 1
If an engine does 100,000 joules of work in 10 seconds, how much power did it use?
P = W / t
P = 100,000J / 10s
P = 10,000W
The engine used 10,000 watts of power

19. Calculating Power – Sample Problem 2
Because work equals force multiplied by distance, another way to write the power formula is:
Power = (Force x Distance) / Time
P = (F x d) / t
P = W / t is the same as P = (F x d) / t

20. Calculating Power – Sample Problem 2
If an engine exerts 3500 newtons of force to move a car 50 meters in 10 seconds, how much power did it use?
P = (F x d) / t
P = (3500N x 50m) / 10s
P = 175,000J / 10s
P = 17,500W
The engine used 17,500 watts of power

21. A Watt Measures Work Done and Energy Used In an Amount of Time
A Watt equals 1 Joule per second
The more watts, the more work is done each second
Joules also measure energy, so a watt also measures energy use per second
EX. A 100W light bulb uses 100J of energy each second that it is on

22. More about Energy Different Forms of Energy Electrical Chemical
Radiant
Thermal
Mechanical

23. KINETIC ENERGY (KE) -The energy of MOTION
-All moving objects have Kinetic Energy!
-Depends on: MASS and SPEED of the object
-Equation:
Kinetic Energy (KE) = ½ x mass x velocity squared
KE = 1/2mv2
-Joule (J) = SI unit for Energy

24. Sample Problem
-A jogger whose mass is 60kg is moving at a speed of 3 m/s. What is the jogger’s Kinetic Energy?
Mass = 60kg
Velocity = 3 m/s
KE = ??J
KE = 1/2mv2
KE = 1/2(60kg)(3m/s)2
KE = 270 J

25. POTENTIAL ENERGY (PE) -The energy of REST
-Objects at REST have POTENTIAL ENERGY
-Potential Energy is CHANGED into KINETIC ENERGY when MOTION occurs

26. TYPES OF POTENTIAL ENERGY
Elastic Potential Energy = Energy stored by something that can be STRETCHED or COMPRESSED
Ex: Rubber band
2. Chemical Potential Energy = Energy stored in CHEMICAL BONDS
Ex: Food, Natural gas
3. Electrical Potential Energy = Energy stored due to ELECTRICAL CHARGES
4. Nuclear Potential Energy = Energy stored in the NUCLEI OF ATOMS

27. TYPES OF POTENTIAL ENERGY
5. Gravitational Potential Energy: Energy stored by objects due to their POSTION ABOVE EARTH
-Ex: Anything with the potential to FALL
-Depends on: MASS and HEIGHT above ground
-Equation:
GPE (J) = Mass (kg) X gravity (m/s2) X height (m)
GPE = mgh

28. GPE Example Problem
-What is the GPE of a ceiling fan that has a mass of 7kg and is 4m above the ground?
GPE = mgh
Gravity = 9.8 m/s2
Mass = 7 kg
Height = 4 m
GPE = (7kg)(9.8m/s2)(4m)
= 274 kgm2/s2 = 274 J

29. To INCREASE GPE
1: INCREASE object’s HEIGHT
2. INCREASE object’s MASS

30. The Change of GPE to KE -As objects fall, GPE is changed into KE
-KE is LARGEST right before the object hits the ground, thus GPE is the SMALLEST right before hitting the ground
-Objects with MORE GPE move FASTER because they have more KE

31. Converting between KE and PE
-MECHANICAL ENERGY = Total amount of POTENTIAL and KINETIC energy in a system
Mechanical energy = PE + KE
What happens to the mechanical energy as PE and KE are converted into each other?
ME stays the same!! As PE and KE are converted, the FORM of energy changes, but the TOTAL AMOUNT STAYS THE SAME

32. The Law of Conservation of Energy
-States that: ENERGY CAN’T BE CREATED OR DESTROYED!!
-So does this mean the total amount of energy in the Universe is the same at all times???
Friction and Air Resistance
These forces can cause some mechanical energy to change into THERMAL ENERGY!!

33. TEMPERATURE
-TEMPERATURE = Measure of the AVERAGE KINETIC ENERGY of the particles in an object
-As temp. INCREASES = Average SPEED of particles INCREASE = KE INCREASES
-SI UNIT: Kelvin (K) or Celsius (0C)

34. THERMAL ENERGY
Sum of the KINETIC and POTENTIAL energy of all the particles in an object

35. THERMAL ENERGY & TEMPERATURE
-As KE INCREASES = TEMPERATURE INCREASES
THUS
-As TEMP INCREASES = THERMAL ENERGY INCREASES

36. THERMAL ENERGY & MASS -As MASS INCREASES = total KE INCREASES THUS
-AS MASS INCREASES = THERMAL ENERGY INCREASES

37. HEAT
-Thermal energy that FLOWS from something at a HIGHER TEMP to something at a LOWER TEMP
-Form of ENERGY
-Measured in JOULES (J)
***Always flows from warmer to cooler***

38. SPECIFIC HEAT
-Amount of HEAT needed to RAISE THE TEMP of 1kg of a material by 10C
-Measured in: J/(kg 0C)
-As a substance absorbs HEAT, its TEMP change depends on the composition of the substance. This is called the Specific Heat of the substance!
Ocean water v/s Beach Sand
(see next slide)

39. SPECIFIC HEAT EXAMPLE PROBLEM
1 kg of sand takes 6x less heat to raise it 10C than 1 kg of water
Which has the higher specific heat??
WATER
Which can absorb more heat with changing its temp?
WATER

40. TRANSFERRING THERMAL ENERGY
-3 Ways to transfer thermal energy from place to place
1. CONDUCTION
2. CONVECTION
3. RADIATION

41. CONDUCTION -The transfer of energy by COLLISIONS or TOUCH
-This happens because particles are in CONSTANT MOTION
-Thermal Energy is transferred by collisions between molecules with more KE to molecules with less KE
**HEAT is transferred by COLLISIONS, not by movement of matter**

42. HEAT CONDUCTORS
1. Heat moves faster by conduction in SOLIDS and LIQUIDS than in gases
WHY??
2. The BEST conductors = METALS
Gases are more spread out/collisions are less frequent
All the e – aren’t bonded to atoms, so they move more freely

43. CONVECTION
-The transfer of thermal energy in a FLUID (gas or liquid) by movement of WARMER and COOLER fluid from place to place
-Uses CONVECTION CURRENTS to transfer the energy from WARM TO COOL
-Still uses collisions to transfer as well
-As a fluid increases its temp = it EXPANDS and its DENSITY DECREASES
WHY??
-Particles have more KE = move faster = spread out

44. RADIATION -The transfer of energy by ELECTROMAGNETIC WAVES
-Waves can travel through space even though there is no matter THUS Radiation is the only form of energy transfer in space
-Energy = RADIENT ENERGY

45. RADIATION
-Radiation can be ABSORBED, REFLECTED, or TRANSMITTED thru an object
-This depends on: MATERIAL OF OBJECT
-LIGH-COLORED = Reflect more
-DARK-COLORED = Absorb more
-Radiation can be TRANSMITTED thru:
SOLIDS
LIQUIDS
GASES

46. INSULATORS -A material in which HEAT FLOWS SLOWLY
-Good Conductors = BAD INSULATORS
-Good Insulators = Wood, plastic, fiberglass, air
-Gases are usually the BEST INSULATORS

47. CONTROLLING HEAT FLOW
-You can use various materials to control heat flow
Ex: Jacket/sweater
-Living organisms have special features to help control the flow of heat
Ex: Fur, Blubber, Scales (reflect), Color

48. Transferring Energy Review: 1st: Discussed transferring energy as HEAT
-Heat flows from an object that is warmer (more ke) to an object that is cooler (less ke)
New Stuff: 2nd way to transfer energy
-Work = transfer of energy that occurs
when a force makes an object move