Sct. 1 Work and Power

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Sct. 1 Work and Power

• Work
• Is done when a force causes an object to move.
• Carrying books
• Pushing grocery cart
• Lifting weights
• Dodge truck pulling Ford out of mud.

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Applying force to do work.

• Work is only done in direction of applied force
  and when object is moving.
• Lifting books off of desk work by arms
• Carrying books to class not work by arms but by
  legs

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Calculating Work

• Work Force x distance
• W F x d
• Joules N x meters
• What is the total amount of work done when a
  football player carries the ball with a force of
  25 N for 50 meters?
• HW Jot down a note about work that you did
  between now and tomorrow in class. Explain how
  having more power would have affected the work.

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• W F x d
• W 25 N x 50 m
• W 1250 N x m
• W 1250 J

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Power

• How quickly work is accomplished.
• Who is more powerful?
• A man who can lift 50 N of weight in 10 seconds
  or a woman who can lift 50 N of weight in 8
  seconds?
• Woman. Same weight and work accomplished, less
  time.

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Calculating Power

• Power work/time
• P w/t
• Watts Joules/second
• What is your power if you do 200 Joules of work
  in 20 seconds?

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• P W/t
• P 200 Joules/20 seconds
• P 10 Joules/sec.
• P 10 Watts

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Work and Energy

• When you do work on an object, because it has
  moved, you increase its potential or kinetic
  energy.
• Potential Energy energy of position/stored
  energy
• Height above earth
• Stretched rubber band
• Kinetic Energy energy of motion
• All moving objects have kinetic energy
• Work objects in motion

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Work and Power HomeWORK.

• How much work must you do to move 100, 10 N rocks
  a distance of 50 meters?
• How far did you push the grocery cart if it
  required 500 Joules of work and needed 25 N of
  force?
• How much power is needed for a Dodge truck to
  pull a Chevy truck from the mud in 10 seconds if
  it requires 4000 Joules of work?
• Who has more power? Mr. Replogle lifts 15 pieces
  of cake (1 piece 1 J of work) to his mouth in
  60 seconds and Mr. Brode lifts 21 pieces of cake
  to his big mouth in 80 seconds.
• What kind of energy does the cake have as it is
  being swallowed to the depths of Mr. Brodes
  belly?
• What kind of energy did the cake have when it was
  being held high above his mouth ready to plummet
  to its dark grave?

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• wf x d
• w1000N x 50 m
• w 50,000 J
• wf x d
• 500J 25 N x d
• d 20 m
• Pw/t
• P 4000J/10 sec
• P 40 Watts
• Barton Power 15 J/1500 sec. .o1 Watts
• Brode Power 21 J/1740 sec. .012 Watts
• Brode more powerful!
• Kinetic Energy
• Potential Energy

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Section 2 Using Machines

• Machine
• A device that makes doing work easier
• Pencil
• Lawn mower
• Screw driver
• Car
• Computer
• pulley

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Machines contd.

• Change the way a person does work, not how much
  work is done.
• Have
• Input work- effort put in
• Output work- the work done by the machine
• No machine can create work
• Input work always greater than output work.
• Due to friction

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Mechanical Advantage

• The number of times a machine multiplies the
  input force.
• MA output force/input force
• EX A car jack can lift a 2000 N car when a
  person applies 50 N of force.
• MA 2000 N / 50 N 40

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Work in Work out

• In an ideal machine. Not a real machine
• Fin x din Fout x dout
• If a machine produces 50 N of force when 10 N are
  put in and the distance out is 2 m, how much
  distance must the machine have put in?

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• Fin x din gt Fout x dout
• 10 N x din gt 50 N x 2 m
• din gt 10 m

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Ways that machines make work easier.

• Exert a smaller force over a greater distance to
  increase output force.
• Car jack
• Exert a larger force over a smaller distance to
  increase output distance.
• Rake
• Allow you to change the direction of the input
  force.
• Pulley

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Efficiency

• Ability of a machine to use all of the work in.
• E output work/input work x 100
• Efficiency is reduced by friction.
• Therefore always less than 100

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Efficiency Calculation

• How efficient is a car that produces 100 Joules
  of work for every 300 Joules of fuel consumed?
• E work out/work in
• E 100 J / 300 J
• E .33 or 33

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Group Work

• What is the mechanical advantage (ma force
  out/force in) of a wrench that you put 50 N of
  force into and it loosens a bolt with a force of
  250 N?
• What is the mechanical advantage of a rake that
  you must use 100 N of force to rake leaves with
  and the leaves require 20 N of force to move?
• What is the efficiency of a car jack that lifts a
  car using 5000 J of work when you put 10000 J of
  work into the jack?
• What is the efficiency of a machine that produces
  500 J of work when 400 J of work is put in? Does
  this machine exist, and if so, what is it?

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Homework

• Fin x din Fout x dout
• At home pick up a hammer and 2 nails.
• Start nails in a board and have the same amount
  of nail sticking out of the board.
• First nail, hold the hammer as close to the head
  as possible.
• Second nail, hold the hammer as close to the end
  of the handle as possible.
• Compare the force out.