Warmup:

1
Warm-up

• Which of the following takes more work?
• Pushing on a wall for 5 minutes
• Lifting a 5 pound book 1 meter off the table
• Why do you think so?

2
Work, Power and Energy
3
Work

• What is work?
• WORK is done only if a force F causes an object
  to move a distance d
• W F ? d
• The units for work are Newton ? meters (N?m) or
  Joules (J).
• 1 N?m 1 J

4
Power

• James Watt, an Englishman in the 1700s, needed a
  term to distinguish a good steam engine from a
  bad engine. The difference was the time
  required to do a certain amount of work.
• POWER is the rate of doing work
• P W/ t
• The units for power are J/s or watts (W)
• 1 J/s 1 W

5
Example Problem 1

• You exert a 100 N force to move a box 3 m in 4
  seconds. How much work did you do on the box?
  What was your Power Output?
• W F?d (100N)(3m) 300 J
• P W/t 300J / 4s 75 W

100 N Box 3 meters in 4 seconds

6
Example 2

• An electric pump lifts 5000 N of water from the
  bottom of a 6 m deep well in 1 minute. Find the
  work and power output.

• W F?d (5000N)(6m) 30,000 J
• P W/t 30,000J / 60s 500 W

7

• Volunteer
• Things to Ponder
• If you push on the wall for 5 hours, then do you
  do any work on the wall?
• If you drop a book, is work done on the book as
  it falls?
• Two Volunteers
• Who did more work?
• Who had the larger power output?

8
Harvard Step Test

• Teacher intro and safety demonstration (proper
  technique for stepping)
• Work done in 1 minute in the HST
• W F ? d
• Force weight in pounds converted to Newtons
• (use 1 lb 4.448 N)
• d convert of reps to total distance climbed
• Power output P W / (60 s)

9
Analysis Questions

• Calculate the work you performed.
• Determine your power output.
• How many 150 W light bulbs could you keep
  burning?
• How many horsepower are you worth? (1 hp
  745.7 W)
• How many people like you would it take to match
  the power output of a 100 horsepower engine?
• Electric energy (work) is selling for about .07
  per kilowatt-hour. How much was your activity
  worth?

10
Discussion Question

• Even 100 hp would make a pretty weak engine, so
  how do we get so much power from our cars? Where
  does this energy come from?
• Additional facts
• U-235 fission bomb (Hiroshima) 6.51016 kW
• Hydrogen fusion bomb 4.91019 kW
• the Sun 3.31023 kW

11
Energy

• Definition The capacity to do work or to cause
  changes to an object.
• Two types can be described with simple formulas
• Kinetic Energy
• Potential Energy
• Some other types of energy
• Thermal
• Field energy
• Internal

12
Kinetic Energy

• Definition Energy due to motion
• Equation
• KE (1/2)mv2
• m mass of the object in motion
• v velocity of the object
• Units are Joules!!! (in fact, all energy is
  measured in Joules)

13
10 m/s KE ??
1000 kg
Example 1
KE ½ mv2 ½ (1000kg)(10m/s)2 50,000 J
20 m/s KE ??
1000 kg
Example 2
KE ½ mv2 ½ (1000kg)(20m/s)2 200,000 J
14
Potential Energy

• Definition Energy stored due to position
• Types
• Gravitational energy PE mgh
• m mass of object
• g acceleration due to gravity (10 m/s2)
• h height of the object above some point
• Chemical energy an example is the energy in
  gasoline, which is due to the position of atoms
  in gasoline molecules.
• Mechanical PE like a compressed spring, or a
  stretched bow arrow
• Electromagnetic PE for example, magnetic
  attraction or static cling

15
1 kg

• Example

1 m
1 m
Relative to the table top PE mgh
(1kg)(10m/s2)(1m) 10 J Relative to the floor
PE mgh (1kg)(10m/s2)(2m) 20 J