1. What you should know about ENERGY
If it wasn't for Thomas Edison, we'd all be watching TV by the light of a candle.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

2. This lecture will help you understand:
Energy
Work
Power
Mechanical Energy : Potential and Kinetic
Conservation of Energy
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

3. Ch07 - Energy - Revised: 2/21/10
What is energy?
The ability to do work.
Energy
Mover of things
Observed when it is being transferred or being transformed
Not spooky life force stuff. There is no such thing as good or bad energy, (Sorry Cesar Millan).
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

4. Circular definition of work/ energy
Property of a system that enables it to do work.
Work
So, the word work, in science terms, means something transferred energy somehow.
Units for work and energy in science terms are Joules, in human terms they are Calories or kWh
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

5. Ch07 - Energy - Revised: 2/21/10
Derivation of W into E
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

6. Ch07 - Energy - Revised: 2/21/10
Work
involves force and distance (but really displacement).
Work = Force  displacement
W = F x d
J = N x m (units)
J = kg * m/s2 x m = kg m2/s2
whenever useful mechanical work is done:
Something changes the energy associated with that thing
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

7. Ch07 - Energy - Revised: 2/21/10
But there is a catch
The force must be in the same direction as the motion
Is this dude doing any work?
What if he was dragging the bag?
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

8. Ch07 - Energy - Revised: 2/21/10
But there is a catch
You might be saying, “Grist you are nuts. If I carried a heavy bag a long way I would get tired. So, I did work!”
Nope! You may have exerted effort, but you did not change the energy of the bag.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

9. Can you do negative work?
Yes, when F & d are in opposite directions.
Ex: setting down a bag of groceries.
W = F cos ϑ d = F cos (180) d = F (-1) d = -Fd = -W
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

10. Ch07 - Energy - Revised: 2/21/10
Work
CHECK YOUR NEIGHBOR
If you push against a stationary brick wall for several minutes, you might get tired, but did you do any work?
A. yes.
no.
Both of the above.
None of the above.
A. on the wall.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

11. Ch07 - Energy - Revised: 2/21/10
An Example of Work
A weightlifter raising a barbell from the floor does work on the barbell. Increasing it’s height increases its potential energy. If he dropped it on your foot it would do work on your foot.
Does he do work while holding it over his head?
Does he do work while holding it up and walking around?
Does he do work if it’s on the ground and he drags it around?
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

12. Ch07 - Energy - Revised: 2/21/10
Power
Power:
Measure of how fast work is done
In equation form:
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

13. Ch07 - Energy - Revised: 2/21/10
Units of Power
Unit of power
WATT is the unit of power?
Named after James Watt, developer of the steam engine
Horsepower
Named after a horse that probably carried heavy stuff or pulled things, also eater of grass and maker of manure
750 Watts = 1 horsepower
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

14. Ch07 - Energy - Revised: 2/21/10
Power Examples
A worker uses more power running up the stairs than climbing the same stairs slowly. T or F
If he were to run the 2 flights of stairs would he produce more power than running just one? Yes / No
If you drove your car twice as far as usual, would your car’s engine become more powerful? Yes / No
An engine with twice the power can do twice the work of one engine in the same amount of time. T / F
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

15. Ch07 - Energy - Revised: 2/21/10
Power
CHECK YOUR NEIGHBOR
A job can be done slowly or quickly. Both may require the same amount of work, but different amounts of
A. energy.
momentum.
power.
impulse.
C. power.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

16. Ch07 - Energy - Revised: 2/21/10
Mechanical Energy
Mechanical energy is due to position or to motion, or both.
There are two forms of mechanical energy:
Potential energy – position
Kinetic energy - motion
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

17. Ch07 - Energy - Revised: 2/21/10
Potential Energy
Stored energy
potential for doing work
Example:
A stretched bow has stored energy that can do work on an arrow.
A stretched rubber band of a slingshot has stored energy and is capable of doing work.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

18. Potential Energy—Gravitational
Potential energy due to elevated position.
Example:
water in an elevated reservoir
Cat at the top of a tall tree
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

19. Potential Energy—Gravitational
Equal to the work done in lifting it
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

20. Ch07 - Energy - Revised: 2/21/10
Kinetic Energy
Energy of motion
Depends on the mass of the object and speed squared KE = ½mv2
If object speed is doubled  kinetic energy is quadrupled.
If you double the speed you drive you quadruple you stopping distance.
(slow down!!)
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

21. Law of Conservation of Energy
Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

22. Kinetic Energy and Momentum Compared
Kinetic energy is not momentum!!!!
Since we are not doing a unit on momentum, ask Grist or Dickson what the difference is if you are curious.
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

23. Ch07 - Energy - Revised: 2/21/10
Energy for Life
Body is a machine, so it needs energy.
Average people need 2000 Calories per day.
This is equals 2.2 kWh about 25 cents worth of energy from LPEA
MFMcGraw
Ch07 - Energy - Revised: 2/21/10

24. Ch07 - Energy - Revised: 2/21/10
Cool energy fact
Example:
Photovoltaic cells on rooftops catch the solar energy and convert it to electricity.
More energy from the Sun hits Earth in 1 hour than all of the energy consumed by humans in an entire year!
MFMcGraw
Ch07 - Energy - Revised: 2/21/10