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Chapter 5 Work and Machines

VIEWS: 24 PAGES: 34

  • pg 1
									Chapter 5 Work and Machines

         Page 126-131
                           Review
• Motion
  – Distance, time, speed (velocity), acceleration
  – Speed = rate of change of distance
  – Acceleration = rate of change of velocity
• Force
  – Force, mass, acceleration
  – A force is required to change the velocity of an object.
     • F = ma
  – Weight is the gravitational attraction on an object
     • W = mg (g= 9.8 m/s2 the acceleration of gravity)
              Review of Energy
• Energy is the ability to do work.
• Different forms of energy
• Different types of mechanical energy
  – KE = ½ mv2, PE (GPE)= mgh
• Energy is always conserved
• Energy may be conserved but it is often changed
  into a useless form (heat) by friction and air
  resistance.
• But Einstein said, E = mc2
                        Work
• Work makes something move
• Work is the transfer of energy when a force makes
  an object move.
• Two conditions
  – A force must make something move
  – The direction of motion must be in the direction of the
    force
  – Carrying books
• Work is a transfer of energy
• W= Fd (force in Newtons (kg m/s2, distance in m)
         Practice Problems P 128
            (turn in Monday)
• You push a refrigerator with a force of 100N.
  If you move the refrigerator a distance of 5m
  while you are pushing, how much work is
  done?
• Given:
• Asked:                      ?units
• Formula:
                      Power
•   Power is the rate of doing work.
•   How much work is done in 1 sec
•   P = W/t
•   W (joules) joules = kg m2/s2
•   Unit for power is joules/second called a Watt
•   A Watt is kg m2/s3
•   Since work is energy transferred
•   P= E/t
             Power Example
• You push a box with a force of 100 N across
  the floor 5 meters it takes you 45 seconds.
  Your friend pushes a similar box also with a
  100 N force also 5 meters across the floor but
  it only takes your friend 30 seconds.
  – How much work do you each do?

  – How much power do you each expend?
     Power Practice Problems P130
           (Turn in Tuesday)
• You do 900 J of work pushing a sofa. If it took
  5s to move the sofa, how much power did you
  use?
  – Given
  – Asked?               Units?
  – Formula
                   Review (P131)
•   Work is a transfer of energy
•   W= Fd
•   (force in Newtons (kg m/s2, distance in m)
•   N*m or Joules
•   Power is the rate of doing work.
•   How much work is done in 1 sec
•   P = W/t
•   W (joules) joules = kg m2/s2 , t= seconds
•   Joules/second = Watt
             Machines (P132)
• A machine is a device that makes work easier.
• Simple machines (not motorized)
• How machines make work easier
  – A machine can increase the force that can be
    applied to an object.- Car jack (p 132)
  – A machine can change the direction of the force-
    splitting wedge (p133)
        Work Done by Machines
• Figure 7 Page 133
• Choice – lift the chair straight up the height of
  the truck.
• W = Fg x height
• Or- slide the chair up the ramp
• Work will be the same
• Distance will be greater but force will be less
             Factors of Work
• Input force – the force applied to the machine
• Output force – the force applied by the
  machine
• Input distance – the distance the input force
  moves
• Output force – the distance the output force
  moves.
• work in = input force x input distance
• work out = output force x output distance
                Pulling a Nail
• work out = work in

•   Figure 10 page 135
•   hammer claw moves 1 cm to pull a nail
•   Handle moves 5 cm
•   Output force of 1,500 N
•   Input force = ?
             Ideal Machines
• Workin= Workout

Forcein X distancein= ForceoutX distanceout

• Fin X 5cm = 1500N X 1 cm
• Fin= ?
• distance units cancel out if the same unit
        Work, Distance and Force
   Machines don’t decrease the work, but they can
             decrease the force required.
               Ideal machine (p135)
                  Workin= Workout

Wout= 100 N * 3m =        Win = Fin *5cm , Wout= Win
300J                      W= 300J but d=5m so F= 60N

                         5m
          3m

100 N
                            4m
    Ideal Mechanical Advantage
              Workin= Workout
Forcein X distancein= ForceoutX distanceout
              Fin X din= FoutX dout

              F out d in
                   =
              Fin    dout

      IMA = F out = d in
             Fin      dout
           Real Machines
• Work out< Workin
• Some of the input work is always
  converted into heat by friction.
• Efficiency tells us how much of the
  input work is converted into output
  work.
             Workout x 100% < 100%
efficiency =
             Workin
                    Practice
• Page 137 – Applying math 5-6-7
• Wednesday –
    – AM Testing
    – PM Problems
• Thursday
    – Field Trip
•   Friday – Lab
•   Monday – Practice Problems
•   Tuesday – Chapter review
•   Wednesday- Chapter 5 Test
Simple Machines
   Section 3
   Page 138-146
      Types of Simple Machines
• Simple machine does work with only one
  movement.
• Lever
• Pulley
• Wheel and Axle
• Inclined Plane

• Compound Machines
                     Lever
• A bar that is free to pivot (turn) about a fixed
  Point
• The fixed point is called the fulcrum
• Input arm – distance from the input force to
  the fulcrum
• Output arm – The distance from the output
  force to the fulcrum.
• If output arm is shorter than the input arm
  then the output force is greater than the input
  force.
           Types of Levers (P138-139)
 First-class Lever
                                                                        Input Force
         Output Force        Output                  Input Distance
                             Distance




Second-class Lever      Output             Output Force
                        Distance

                                                Input Distance
                                                                      Input Force


                                          Output
Third-class Lever                         Distance                    Output Force
                            Input Distance


                                                     Input Force
        Ideal Mechanical Advantage
                of a Lever
               Force output
         IMA =
               Force input
Force output x Length output=Force input X Length input


              L input
        IMA =
              L output
           Pulley Types (P 141-142)
  Fixed Pulley

          Output Force 4N                Input Force 4N



                               4N




Movable Pulley                2N
                                           Input Force 2N



            Output Force 4N
                                    4N
             Types of Pulleys (cont)
Block and Tackle



                            1N
                                      1N
                                 1N
      Input Force 1N
                       1N




                                           Output Force 4N
                                 4N
   Ideal Mechanical Advantage
           of a Pulley
         Force output
   IMA =
         Force input




IMA =   Number of Stings Lifting the Load
        Wheel and Axel (P143)


Input Force
              rw   ra
                        Output Force




      Radius of wheel
IMA =
      Radius of axel
                   Inclined Plane
W= 100 N * 3m = 300J        W= 300J but d=5m so F= 60N
F= 100N, height = 3m



                                 5m
             3m

 100 N



               Length of Slope
         IMA =
               Height of Slope
          Types of Inclined Plane
•   Ramps, inclines, road grades
•   Screw
•   Wedge
•   IMA = length of slope/height of slope
•   Page 144
     Compound Machines
• Two or more simple machines
  that operate together.
   –Can opener
   –Automobile
   –Space shuttle
   Ideal Mechanical Advantage of Simple
                Machines
                               Force output > 1
   All Simple Machines IMA =
                               Force input
                         L input
        Lever    IMA =
                         L output
Inclined Plane              Length of Slope
                    IMA =
                            Height of Slope
Wheel and Axle         Radius of wheel
                 IMA =
                       Radius of axel
Pulley System
        IMA =    Number of Stings Lifting the Load
            Section Review
• Page 146 Applying Math
• 5-6-7
                Formulae

F = m*a , w= m*g    unit kg m/s2 = Newton

W= F*d units N*m = Joule


P= W/t   or   P= E/t       unit J/s = Watt

             Workout x 100% < 100%
efficiency =
             Workin
                      Assignment
•   Chapter 5 Review
•   Page 152-153
•   8,10,11,12,13,15,16,17,19,20,24-28
•   Due on Tuesday
•   Test on Chapter 5 Wednesday
    – Work and Power
    – Simple Machines
       • IMA
       • Efficiency

								
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