1. Work, Energy, & Power
(Chapter 4)

2. Student Learning Objectives
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 1) Is mechanical work a scalar or vector quantity? Explain.
2) 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)?
3) What is the work done to lift a 15 kg child up one meter?

5. What is mechanical energy?
Mechanical energy is associated with objects that are in motion or may become in motion.
Object has ability to do mechanical work.

6. Kinetic Energy KE = (½)mv2
Kinetic energy is the energy an object has when it is in motion.
The work done to change the motion of a moving object = DKE
KE = (½)mv2

7. 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
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)

8. 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

9. 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?

10. An object can have both GPE and KE, at the same time.
Energy may change.

11. Image Credit: Physics Classroom

12. How is conservation of mechanical energy used?
Energy cannot be created or destroyed. The total amount of energy in the universe is conserved.
Mechanical energy is conserved when no energy is lost or gained by the system.
Mechanical energy and work are interchangeable.
Energy  Work
For Systems on a Hill
GPEtop = KEbottom

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

15. More Practice
2) 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?

16. Equal Areas in Equal Time Intervals
Elliptical Orbits
Elliptical orbits are maintained by conservation of energy.
Planets move in elliptical orbits with the Sun at one focus of the ellipse.
Perihelion
Aphelion
Equal Areas in Equal Time Intervals

17. Which planet has the longer orbital period?
The orbital periods of the planets depend on the semi-major axis of the orbit.
P2 = a3
Orbits Practice
Which planet has the longer orbital period?
Saturn: a = 9.54 AU
Jupiter: a = 5.2 AU a

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

19. Practice 1 hp = 746 Watts
1) 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? 2) What exactly is a kWh? 3) The energy used by a 60 Watt light bulb in 2 hours, is 432,000 Joules. How does this compare to a kWh?

20. Lever Ramp (Inclined Plane) Pulley What does a machine do?
Machines transform mechanical energy into mechanical work.
Simple machines can multiply input force.
Lever
Ramp (Inclined Plane)
Pulley

22. t = Fd 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
1) 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
 2) A wrench measures 12 inches on the long side and 2 inches on the short side. If a person applies 300 N to the long side of the wrench, how many Newtons are exerted on the other end of the wrench?
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