1. Filling in Some More Background
Lecture 10
The concept of energy

2. We have previously defined
Momentum (p)
The momentum of an object is the mass of the object multiplied by the velocity of the object
p=mv
Momentum is CONCERVED
The total momentum of two objects that collide is the same after a crash as it was before the crash.
The momentum of a particle can be changed by an impulsive force.
Impulse = Force x Time
When an object (like a molecule) crashed into a wall, its momentum changes DIRECTION and consequently the wall is pushing it around.
N3 says that the object does the same to the wall

3. Impulse - Momentum
WALL v0 vf
IMPULSE
IMPULSE
CHANGE IN MOMENTUM = 2v0

4. PRESSURE
Recall that it was the impulse-momentum relationship that created pressure.
IMPULSE = CHANGE OF MOMENTUM
IMPULSE = (F) X (t)
Chg of Momentum (p) = m(vf-vi)
In an elastic collision with a wall, the change of momentum was TWICE the original momentum.
DO you remember???

5. Push an Object with a FORCEd
WORK = FORCE X DISTANCE THROUGH WHICH IT ACTS
W = Fx

6. BOLEMON – CHAPTER 6 Omit the angle stuff. WORK
The unit of work is the JOULE.
A force of one Newton pushing a distance of 1 meter is one Joule.
Work is good for you!
BOLEMON – CHAPTER 6
Omit the angle stuff.

7. Some random thoughts
Last night you “worked” hard studying for this class.
No … you studied but did not do any work.
If I hold a two ton bolder over my head I get tired very quickly so I must be doing work.
Your muscles require “energy” to operate but you are not doing work in the physics sense.

8. OK … I have to push through a DISTANCE. Then I work.
Where does the work go??
Lets consider a mass that starts from rest and is pushed by a force F over a distance d.
We know it will accelerate and go faster.

9. KINETIC ENERGY

10. Some comments NOISE ??? An object at rest has no kinetic energy.
Energy must also be the ability to do work.
Example- A speeding truck hits a heavy garbage can.
The can is accelerated by the truck and hence the truck is doing work on the can.
The truck loses energy in the crash (although probably not all of it).
The can gains energy.
There is lots of noise!!!
NOISE ???

11. BUT WAIT …. Noise is sound.
Sound somehow gets to our ear just like music does and pushes our eardrum (tympanic membrane) through a distance.
Although the distance was very small … work is done on our ear so that some of the energy must have traveled through the air.
Strange.

12. So?? Work and energy must somehow be equivalent.
Sound is energy flying through the air as a pressure wave.
Let’s look at this process….

13. Back to the truck and the garbage
All is quiet
The truck hits the garbage can which flies off to the
Right.
The can gains kinetic energy.
The truck loses a bit.

14. An Energy Balance ……
BEFORE AFTER
KE=E0

15. ENERGY IS CONSERVED
No Energy is Lost
JUST SOMETIMES MISPLACED!

16. A Conclusion Sound must be energy in motion!
It still doesn’t exist without a brain translator.

17. Question A BIG Truck is speeding down the road.
It has kinetic energy
The traffic light turns RED.
The truck comes to a stop.
Now it doesn’t have kinetic energy because v=0.
Where did the energy go???

18. The missing energy FRICTION Turns the KE of the Truck into HEAT!
HEAT MUST BE SOME FORM OF ENERGY TOO !

19. Kinds of Energy so far … Kinetic Energy Heat Sound
The energy of “motion”
Heat
Internal molecular motion (Temperature related)
Sound
Very important for the physics of music
Chemical, Nuclear, any others???
Let’s look at another kind of energy…

20. Hold a block of something in the air: Rhetorical Questions
ANY ENERGY?? m h
Energy Here??

21. Let’s look WORK = F x distance or WORK = F x h = mgh It takes WORK to
raise the block to
the top. h
F=mg
WORK = F x distance or
WORK = F x h = mgh

22. This energy of POSITION is called POTENTIAL ENERGY
The work we do to bring a mass to a particular height (frem a reference level) is the potential energy.
This is energy that we can recover!
If we drop the weight we get it all back.

23. Example m
Potential Energy = mgh h
Kinetic Energy = (1/2) mv2 v

24. Conservation of Energy
SO …
The same as before!
The same as before!
The same as before!
The same as before!
The same as before!
The same as before!
The same as before!
The same as before!
The same as before!

25. A thinker Which one is traveling FASTER when it hits the bottom? h
NO FRICTION

26. Answer?? Both START with the same potential energy.
Both must FINISH with the same potential energy
Both will have the SAME (1/2)mv2
Both will have the SAME v.
Both traveling at the same speed at the bottom! (Surprised?)

27. ANOTHER PUZZLE

28. ANOTHER PUZZLE
BOTH WIND UP WITH THE SAME FINAL SPEED CONSERVATION OF ENERGY
BUT THE GREEN CAR SPENDS MORE TIME GOING
FASTER SO IT WINS THE RACE!

29. Energy Relationships for Us

30. During Oscillation (Important for Music)

31. OCTAVE