CH 8: Energy

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CH 8 Energy
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Last chapter

• How long a force acts makes a difference in an
  objects motion.
• F x t Impulse

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This chapter

• How long (how far) a force acts, which is known
  as work.

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• 1. Which takes more work lifting 1 book from the
  floor to your waist or lifting 10 books from the
  floor to your waist?
• Why?

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• 2. Which takes more work, lifting 1 book from
  the floor to your waist or lifting 1 book from
  the floor to over your head?
• Why?

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• 3. Which requires more work holding 100 pounds
  stationary over your head or holding 300 pounds
  stationary over your head?
• Why?

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Work

• A. product of a force acting in the direction
  of motion and the distance moved.
• B. Work force x distance
• W F x d
• C. Units for work
• Joules (J) N m

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Practice problem

• How much work is done on a 50 N skater that is
  lifted by her partner 1.5 m into the air?

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Another Problem

• Who does more work, a person who lifts 100 pounds
  2 m over their head very quickly or a person who
  struggles and takes a long time to lift the 100
  pounds 2 m over their head?
• Why?

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Power

• A. rate at which work is done
• B. Power work done
• time interval
• P W/ t
• C. Units for power
• watts (w) joules / t
• horsepower (hp)
• 1 hp 747.7 w

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Practice problem

• How much power is required to lift Bertha, (who
  weighs 70 N) 20 m in an elevator in 6 s?

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Energy

• A. ability to do work
• B. Units joules
• C. Mechanical energy

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1. Potential Energy

• a. stored energy
• b. energy due to position
• c. examples
• rubber band
• spring
• fuel - gas, batteries, food
• a book held high

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1. Potential Energy

• d. potential energy is obtained by work being
  done.
• e. PE mgh

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2. Kinetic energy

• a. energy of motion
• b. energy due to movement
• c. depends on mass and speed
• d. KE 1/2 mv2

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Conservation of Energy

• Energy can not be created or destroyed, only
  transformed from one form to another.
• Total amount of energy never changes.

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Conservation of Energy

• 3. Energy transforms from one form to another

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Sunlight

• Heat
• Chemical
• Mechanical
• Electrical

• Plants
• Warm air rises
• Solar cells

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IV. Machines

• A. device used to multiply forces or change the
  direction of forces
• B. basic idea is same as conservation of energy
• C. work in work out

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work in work out

• (F . d)in (F . d)out
• F . d f . d
• 10 N x 1 m 1 N x 10 m

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D. lever

• F . d f . d

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E. Mechanical Advantage

• MA output force
• input force
• pg. 112 80 N 8
• 10 N
• MA input distance
• output distance

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• F. Pulley (bonus question on the test)

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G. Efficiency

• 1. ideal machines have 100 efficiency
• Does not happen in the real world (some energy
  is ALWAYS lost as heat)

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G. Efficiency

• Work (out)
• Eff Work (in) X 100
• OR useful work output X 100
• total work input
• AMA
• Eff TMA X 100