1. Cliff Diver A 50kg cliff diver climbs to a
height of 50 meters above the water.
How much KE does he have before he lands in the water?
How much PE does he gain in his climb?
What is his speed as he reaches the water?

2. Aim: How do we describe internal energy?
HW: Finish Energy WS

3. PEtop = 25,000 J
(relative to the
water!)
KEtop = 0 J
50 m
PEmiddle = 12,500 J
KEmiddle = 12,500 J
25 m
0 m
PEbottom = 0 J
KEbottom = 25,000 J

4. Energy & Kinematics?
A box is pushed with a 40N force at a constant speed of 2 m/s for 6 seconds. What is the work done to move the box?
W = F x d
What is distance?
v = d/t
2 m/s = d/6s
d = 12m
W = 40N x 12 m
W = 480J

5. Activity I - A closer look at a pendulum
Where is KE max?
Where is PE max?
Explain how energy is conserved.

6. Where was Kinetic and Potential energy the greatest on this ride? Why?
Where do you think they were equal to each other?
Why do coasters never go as high as the initial drop?
Official Top Thrill Dragster
Note: Do Now is on WS which is handed out at start of class

7. Think about this
You start out on top of a cliff with 1000J of potential energy. When you hit the ground (having used a parachute, I hope…), you have 400J of kinetic energy.
Where did the energy go?
Did it disappear?
Energy was lost due to friction (air resistance in this case)…

8. Law of Conservation of Energy
The energy of a closed system will always remain constant – energy cannot be created or destroyed.
Q = “Work not conserved” -or- Heat (from friction) –or- Internal energy
PE+KE = “Total Mechanical Energy” (not always constant)

9. Skiier A skier starts at a height of 52 meters and slides
down the hill. Note how PE becomes KE, then how
KE is dissipated as friction does WORK on the skier!
How does energy conservation work in this system?

10. Energy Lost
A 5N horizontal force is applied to an object causing it to move 10m. If the object gains 40J of kinetic energy, determine the work done against friction, then the total energy.

11. WNC = ΔPE + ΔKE Alternate Conservation Equation (AP style)
WNC stands for “work not conserved”
Means same thing as Q
A 1,000 kg car is driving at 40 m/s…
The car brakes and slows to 20 m/s…
How much energy is lost as heat?

12. Exit Card
A 40 kg boy slides down a slide from a height of 9 meters. The boy has a kinetic energy of 80 Joules at the base.
Calculate Internal energy due to friction. (Show all work with equation)
What is the Mechanical energy of the system at the base?
Name a situation involving “Work not conserved” (Internal Energy). How
do you know internal energy is involved?