1. Work, Energy, & Power
(Power Point 05)
Images from Creative Commons

2. Student Learning Objectives
Describe the properties of mechanical energy
Compare and contrast mechanical work, energy, and power
Name examples of simple machines
Describe renewable energy sources

3. What is mechanical work?
Mechanical work is the use of energy to move an object a distance.
Depends on total displacement, not path
W = Fd

4. Practice Is mechanical work a vector quantity? Explain.
How much work would be done on a 150 lb (667 N) barbell?
When it is lifted 2 ft (0.61 m)?
While it is held overhead?
When it is lowered 2 ft (0.61 m)?

5. More Practice What is the work done to lift a 15 kg child 1 meter?
What is the work done to slide a 15 kg child 1 meter across the kitchen floor?

6. What is mechanical energy?
Mechanical energy is associated with objects that are in motion or may become in motion.
Objects in motion have ability to do mechanical work.

7. Kinetic Energy KE = (½)mv2
Kinetic energy is the energy an object has when it is in motion.
KE = (½)mv2
Stock Image

8. Practice Which has the most kinetic energy?
Which has the most momentum?
Which has the greatest inertia?
18-Wheel
Truck
10,000 lbs
(4535 kg)
Parked
(0 m/s)
Football
Player
300 lbs
(136 kg)
Running 10 mph (4.47 m/s)
Olympic
Sprinter
150 lbs
(68 kg)
Running 22 mph (9.83 m/s)
Small
Car
2,640 lbs
(1,200 kg)
Rolling
(1 m/s)

9. Potential Energy GPE = mgh
Gravitational potential energy is the energy an object has that may fall or travel downward from a height.
The work done by or against gravity = DGPE
GPE = mgh

10. Practice A 60 kg person stands on top of a 4 m tall ladder.
What is the gravitational potential energy of the person?
How much work would the person do on a lazy observer lying on the floor if the person falls from the ladder?

11. More Practice
Is it possible for an object to have both GPE and KE, at the same time.
Can one type of energy become a different type of energy? Explain.
Can mechanical energy become mechanical work? Explain.

13. How is conservation of mechanical energy used?
Energy cannot be created or destroyed.
Energy may change form, but there always be the same amount of it.
Mechanical energy and work are interchangeable.
Energy  Work

15. For Systems on a Hill
GPEtop = KEbottom

16. Practice (Always assume mechanical energy is conserved)
A 10 kg steel ball is dropped 12 meters from the roof of a building.
Calculate the initial energy of the ball.
What is the KE of the ball the instant before impact?
What happens to the energy?

17. More Practice
A 68 kg skier begins to ski at the top of a hill that is 20 meters high.
How much kinetic energy does the skier have 75% of the way down?
When are GPE and KE equal?
If the skier runs into Teresa at the bottom of the hill, how much mechanical work may be done on Teresa?
What is the skier's speed at the bottom of the hill?

18. Practice
Elliptical orbits are maintained by conservation of energy. Explain.

19. What is Mechanical Power?
Mechanical Power is the rate at which work is done or energy is used.
Car Engines & Light bulbs
P = W t
P = E

20. Practice
How much power is needed to lift a 150 lb (667 N) barbell 2 ft (0.61 m), in 0.25 seconds? How many horsepower is this? 1 hp = 746 Watts
What exactly is a kWh?
The energy used by a 60 Watt light bulb in 2 hours, is 432,000 Joules. How does this compare to a kWh?

21. Simple machines: Lever, Inclined Plane, Pulley
What does a machine do?
Machines transform energy into mechanical work.
Simple machines: Lever, Inclined Plane, Pulley

22. Torque
Torque is work that results in rotational motion. (lever)
Force acts through a distance
Torque is equal on each side of the fulcrum (pivot)
t = Fd

23. Practice
If a 667 N person sits 3 meters from the pivot, where would a 533 N person need to sit to balance a seesaw?

24. More Practice
If a person applies 300 N to the long side of the wrench, what is the output force at the other end of the wrench? What is the mechanical advantage?
12 in
2 in
Pivot

25. What is a renewable energy source?
Renewable energy sources can be replenished in a short period of time.
Wind & Water
Solar
Geothermal
Biofuel

26. Solar https://www.youtube.com/watch?v=0elhIcPVtKE