Energy and Environment, Class 3

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Energy and Environment, Class 3

• Horsepower exercise
• Introduce the idea of the difference between
  energy and power
• Designate a timer and note taker
• Take class to a long section of stair. Measure
  riser height and number of risers.
• Time each student as they run up the stair
  section.

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Mathematical Basis for Energy and Power

• Need to define the quantities used to describe
  energy use.
• Velocity, acceleration, mass, force, work, energy
  and power
• Also need to understand the metric units that we
  should be using and the alternate units that we
  will commonly see

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Personal Horsepower

• Calculate your personal horsepower based on the
  data we collected. How many watts is this?
• Assuming a candy bar has 300 Cal., calculate how
  many times that amount of energy would get you up
  the stairs (assuming you use the energy 100
  efficiently)

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Personal Horsepower Solutions

• 18 risers, 7 3/8 in. each 132.75 in 11.06 ft
• If a person weighs 180 lbs and climbs 11.06 ft.,
  they use 1990.8 ft.-lb. of energy.
• 1990.8 ft-lb x (1.356 J / 1 ft-lb)2699.5 J
• If it took them 3 seconds, then this is
• 2699.5 J / 3 sec 900 W,
• 900 W x (1 hp / 746 W) 1.21 hp

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Personal Horsepower Solutions

• Candy Bar 300 Cal 300,000 cal
• 300,000 cal x (4.184 J / 1 cal) 1.25 x 106 J
• So that candy bar would send you up

or 463 trips
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A Special Note on the Kilowatt Hour

• 1 kW 1000 W
• This is a unit of power, 1 kW 1000 J/s
• Since power energy/time, energypower x time
• Take the kW and multiply by the time, in hours,
  and you get a unit of energy

As goofy as this is, it happens to be a unit of
energy that is more useful than the joule (very
small).
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More on Graphs
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Alternative Presentation
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Notes

• The questions and problems assigned from chapter
  2 start on page 58. You dont need to read the
  section on Newtons laws of motion.