Chapter 9: Work

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Chapter 9 Work Energy
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Work

• Is only done when a force causes a change in
  motion of an object.
• Work Equation work force x distance
• W Fd
• SI units Joules (J) or Nm or kgm2/s2

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Work Example

• Imagine a father playing with his daughter by
  lifting her repeatedly in the air. How much work
  does he do with each lift, assuming he lifts her
  2.0 m and exerts an average force of 190 N?
• WFd
• W190 N x 2.0 m 380Nm or 380 J

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Remember

• To convert from mass to force (weight) use
• wmg g 9.81 m/s2
• Plug the value for weight into the work equation
  for force.

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Power

• The rate at which the work is done.
• Power Equation power work or PW
• 
  time t
• SI units watts (W) or J/s

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Power Example

• It takes 100kJ of work to lift an elevator 18 m.
  If this is done in 20 s, what is the average
  power of the elevator during the process?
• PW
• t
• P 1x105 J 5000 J/s or 5000 W or 5kW
• 20 s

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Machines

• They redistribute the work put into them.
• Example Car jack
• Does the amount of work change?
• No, but it makes the work easier!
• They multiply and redirect forces.

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Mechanical Advantage

• Different forces can do the same amount of work.
• Mechanical advantage measures how much a machine
  multiplies force or increases distance.
• MA greater than 1 multiplies force
• MA less than 1 increases distance or speed

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M.A. Equation

• M.A. output force input distance
• input force output distance
• NO UNITS! Yea!

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Simple Machines

• 2 families of basic machines
• 1. Levers 2. Inclined planes
• - lever - inclined plane
• - pulley - wedge
• - wheel axle - screw

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Levers

• Lever parts

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Levers cont.

• 3 types of levers
• First-class
• crowbar
• seesaw
• scissors
• Second-class
• wheelbarrow
• Third-class
• tweezers
• tongs

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Lever M.A.

• M.A. Effort Arm Force Effort Distance
• Resistance Arm Force Resistance
  Distance
• ACTUAL
  THEORETICAL
• Effort Force x Effort Distance
  
• Resistance Force x
  Resistance Distance

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Interactives

• Lever Example 1
• Lever Example 2

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Inclined Planes

• This is your basic ramp.
• Spreads work over a large distance.
• Long gradual ? easier to push object.
• Short steep ? harder to push object.

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Inclined plane M.A.

• M.A. Run Object wt.
• Rise Force
• Example Determine the force needed to lift the
  block.

10 m 20 kg (cross multiply) 5 m F (10
m)F 20 kg x 5 m F 10 kg
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Pulleys

• Modified levers.
• Center represents fulcrum.

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Pulley M.A.

• M.A. Objects Wt. Force Distance
• Force Object Distance
• or of upward forces.
• Single fixed pulleys ALWAYS
  have a MA of

ONE!
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MA
4
MA
2
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Wheel Axle M.A.

• M.A. Wheel Circumference
• Axle Circumference
• Example 30 cm 6
• 5 cm
• Easy or Hard?

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Compound machines

• Made up of more than one simple machine.
• Examples

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What is Energy?

• The ability to do work.
• Measured in Joules
• 2 Types
• Potential
• Kinetic

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Potential Energy

• Stored energy.
• Example Rubberband
• Depends on MASS and HEIGHT
• Equation
• Grav. PE mass x gravity x height
• PE mgh

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P.E. Example

• A 65 kg rock climber ascends a cliff. What is
  the climbers gravitational potential energy at a
  point 35 m above the base of the cliff?
• PE mgh
• PE 65 kg (9.81 m/s2) (35m)
• PE 22000 kgm2/s2 or J

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Kinetic Energy

• Energy due to motion.
• Depends on MASS and SPEED.
• More on speed than mass.
• Equation
• Kinetic energy ½ x mass x speed squared
• KE ½ m?2

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K.E. Example

• What is the kinetic energy of a 44 kg cheetah
  running at 31 m/s?
• KE ½ m?2
• KE ½(44 kg)(31m/s)2
• KE 21000 kgm2/s2 or J

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Other Forms of Energy

• Mechanical energy sum of potential and kinetic
  energy.
• PE KE or mgh ½ m?2

• Potential
• Chemical energy
• Sugars/organic molecules stored in tissue
• Nuclear energy

• Kinetic
• Thermal
• Photosynthesis
• Digestion
• Respiration
• Electricity
• Light

Animation
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Energy Transformations

• Potential can become kinetic.
• Kinetic can become potential.
• Examples
• Roller Coaster
• Flight of a Ball
• Pendulum

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Energy Conservation

• Law of Conservation of Energy
• Energy cannot be created or destroyed.
• Energy doesnt appear out of nowhere.
• Energy doesnt disappear.
• Systems may be open or closed.

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Efficiency

• Not all work done by a machine is useful work.
• The other work may be transferred to heat.
• NO machine is 100 efficient. Why?
• Work output never equals and cannot exceed work
  input!

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Flash Clip
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References

• Lever Pic www.sciencebyjones.com/first_class_lev
  ers.htm
• Lever Types http//www.tiscali.co.uk/reference/e
  ncyclopaedia/hutchinson/m0017416.html
• Pulley Pics http//en.wikipedia.org/wiki/Pulley
  
• BlockTackle Pic http//content.answers.com/main
  /content/wp/en-commons/thumb/b/bb/180px-Polispasto
  4.jpg
• Screwdriver Pic www.logocommunications.com/ratch
  et.htm
• Bike Pic www.wildhorizons.com.au/bike/index.html
  
• Roller Coaster Physics. Discovery Channel
  School(2004). Retrieved September 18, 2006,
  fromunitedstreaming http//www.unitedstreaming.c
  om/
• PE KE Pics http//www.solpass.org/6s/6_science
  .htm
• Pendulum Pic http//www.physicsclassroom.com/mme
  dia/energy/pe.html
• RollerCoaster (bottom) PE/KE http//www.physicsc
  lassroom.com/mmedia/energy/ce.html
• RollerCoaster Pic http//www.glenbrook.k12.il.us
  /gbssci/phys/Class/energy/u5l2bb.html
• Bouncing Ball Pendulum Pic http//www.bbc.co.u
  k/schools/gcsebitesize/physics/energy/energy_work_
  and_powerrev4.shtml