1. Work
Work – a transfer of energy from one object to another by mechanical means
(mechanical - something physically makes the transfer)
Ex: If you lift a book above your head, you did work on the book
you lost some energy,
the book gains that energy, by being higher in the air
Eq’n: Work = Force * displacement
or W = F*d
Units: Joules = Newton * meter

2. 2 Most Common Reasons Work is Done
To change an object’s position, usually done against an existing force, so that the 2 forces balance…
like lifting something through gravity
Ex: a fireman climbing a ladder at a constant speed
like dragging something through friction
Ex: sliding a box across the floor at a constant speed
To more permanently change an object’s speed
Ex: riding in a vehicle or airplane
Ex: you push off the ground to get yourself going on a skateboard
Ex: a car’s brakes apply a force to slow the car to a stop

3. Consider a story about 2 students asked to help get books loaded into the cabinets. Both move equal numbers of books up to equal height cabinets, but one does it quickly and efficiently, while the other does not.
Who does more work?
Neither! Both apply same F thru = d
Who uses more power?
The quicker one
Power is the rate at which work gets done
Eq’n: Power = Work/time or P = W/t
units: Watts = Joule/sec (1 hp = 746 Watts)

4. The Energy Outline Energy - the ability to do work or
the ability to change an object or its surroundings
I. Mechanical NRG – energy due to a physical situation
Kinetic Energy (KE) – energy of motion
an object must have speed
Eq’ns: KE = ½ mv2
units: Joules = kgm2/s2
(recall from work: J = Nm = kgm2/s2)

5. B. Potential NRG (PE) – energy of position,
stored away, ready to set an object into motion
1. Gravitational PE (PEg) – object’s position is some height different than the reference level, where PEg = 0
Eq’ns: PEg = mgh = Fgh

6. 2. Elastic PE – object’s position is some shape different than normal

7. 3. Chemical PE – object’s atoms/molecules have the potential to rearrange
4. Electric PE – the charged particles within the object have the potential to move – create electricity!

8. II. Thermal NRG – energy of heat – the energy that is produced when it looks like energy isn’t being conserved…
Ex: heat from friction
sound
deformation

9. The Work – Energy Theorem
This is the connection between work and energy:
The amount of work done by* / on** an object will equal its change in energy
* “by” – object will lose NRG, so -ΔE
** “on” – object will gain NRG, so +ΔE
In effect, work is the act of changing an object’s NRG,
while energy means work can get done.
PE means it has the potential to get done.
KE means it is presently getting done.
Eq’n: W = ΔE or more specifically, W = ΔKE + ΔPE

10. Conservation of Energy
The Law of Conservation of Energy:
NRG can’t be created or destroyed, but it can be changed from one form to another.
Eq’n: Ei = Ef
Possible forms these NRG’s could be in?
in the ideal world: only KE & PE
in the real world: KE, PE & TE
Consider the double incline ramp:
Will the ball roll off the short side if released from the long side?
No, but why? Now we can answer that…

11. Consider the Ball on the Double Incline Ramp
2 forms of Mech NRG (ME) that the total amount of NRG fluctuates between:
max PEg at either end - #1 & #4
max KE at the bottom - #3
if ideal world: no loss of ME to thermal,
so h1 = h4
if real world: loss of ME to friction & sound as ball rubs on ramp & air resistance as it rolls along
so h1 > h4

12. Pendulum on a string
2 forms of ME that the total amount of NRG fluctuates between
max KE at equilibrium (vertical) position
max PEg at either end
if real world: loss of ME to friction where string rubs on support & air resistance
Mass on a spring – sketch it
3 forms of ME that the total amount of NRG fluctuates between
max KE at the equilibrium (midpoint) position
max PEe at the bottom and some at the top
max PEg at the top
if real world: loss of ME to friction inside spring & air resistance

13. Back to KE – what does it mean to have velocity2 in the equation?
Let’s consider how long it takes to stop a vehicle traveling at a particular speed:
At first you may think: if you’re traveling twice as fast,
it will take twice as far to stop,
But that’s not right… W = ΔKE
or F d = ½ m Δv2
So as v increases, both KE and the other side of the equation so goes up by the square of that increase,
Ex: for 2x’s as much v,
there’s 4x’s as much KE in the vehicle,
there’s 4x’s as much W needed to stop it,
and for a constant braking force,
that means 4x’s as much distance is required!

14. Multi-Color Tracks Which shape track wins? One with its dip 1st, wins
– gets to a faster speed earlier in its trip
Which shape track gets the ball going the fastest by the end?
Ideal: All have = PEg at start, so = KE at end,
so = v too
Real: the shortest distance (straight) track loses least KE to TE, so it’s going a little faster

15. I don’t use the following slide anymore…

16. Sketch the double incline ramp with 4 noted positions:
1: E1 = PEg Since there’s h, but no v
2: E2 = PEg2 + KE2 Since still some h, but also v
3: E3 = KE Since no more h, and all v
4: E4 = PEg Since no v, and back at original h – but is it?
L of C of E says: E1 = E2 = E3 = E4
So in the ideal world: PEg1 = PEg4
so mgh1 = mgh4
then h1 = h4
But in the real world: PEg1 = PEg4 + TE4
so mgh1 = mgh4 + TE4
then h1 > h4