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Work and Machines


									1. How would the effort exerted by a
backpacker over level ground compare to
the effort in climbing a steep hill?
2. How would the weight of the backpack
affect the amount of force needed to
move it?
Work and Machines

   Chapter 5
   Section 1
  Work—transfer of energy
    that occurs when a force
    makes an object move.
     (Force is a push or pull)
Doing Work

       To determine if work is being done 2
        conditions must be satisfied:
        The object has to move
        The motion of the object must be in the
         same direction as the applied force.
       Figure 1 and 2, page 126-127.
Work and energy are related

   When work is done, a transfer on energy
    always occurs.
   Energy is transferred from the object doing
    the work to the object on which the work is

      Box being

       Work is done on an object only when a
        force is being applied to the object and the
        object moves.
       The amount of work done depends on 2
        the amount of force exerted
        the distance over which the force is applied
Calculating Work

    Work = Force x distance
     W = F (N) x d (meters)

    Work is measured in Joules
Math skills activity
Practice problems, page 128

   Power—amount of work done in a certain
    amount of time.

   It is the rate at which work is done
How Do You Calculate Power?

       P = W/t (seconds)

       Power is measured in watts
        1 W = 1 J/s (very small unit)
        One kilowatt (kW) = 1000 W
Conversions of Power
Btu = 1,055 watts          Horsepower = 746
(for heating and cooling    watts
   units)                  (for motors and engines)
Since work and energy are related,
    power can also be calculated:
Math skills Activity—page 130

Practice problems, page 130

Section 1 Assessment-page 131
        Questions 1-6.
Who is more powerful?
   Sally and Pete do the same amount of
    work. Sally does the work in 2.3 hours
    and Pete does it in 2.5 hours. Who is
    more powerful? Explain. (Read page
    129 to help answer question)
Using Machines
Section 2
Using Machines
                      The car may weigh a
                      lot, but you don’t have
   A machine makes   to use nearly that
    doing work        much force to lift it
    easier.           with a jack.

   They may
    multiply the
    applied force.
Machines may increase the distance over
 which a force is applied.

In this case, the amount of force necessary to push the
chair up the ramp was decreased.
Machines may change the direction
 a force has to be applied.

     The nail comes up as the person
             pulls to the side.
   Since work is equal to force times distance
    (W = F/d), the same amount of work can be
    done by applying a small force over a long
    distance as can be done applying a large
    force over a short distance.

       Small force over ling   Large force over
             distance           short distance
Two forces are involved when a
  machine is used to do work:
   effort force              resistance force

    force applied to the       force applied by the
     machine                    machine to overcome
When prying a nail out of a piece of wood with
a claw hammer, you exert force on the handle
    of the hammer, and the claw exerts the
              resistance force
Amount of energy the machine
  transfers to the object cannot by
  greater than the amount of energy
  transferred to the machine.
   Win                    Wout

    work done by            work done by the
    you on a                machine is called
    machine is called       output work.
    input work.
   A machine cannot create energy so
    output work (W out) is never greater than
    input work (W in)

   Some energy transferred is changed to
    heat due to friction.

   An ideal machine is theoretical; it does
    not take friction into account.

   Ideal Machine:    Win = Wout
Mechanical advantage (MA)

   Mechanical                    To calculate:
    advantage is the
    number of times a
    machine multiplies
    the effort force

    Some machines don’t multiply the force that is
    applied to them. For example, mini blinds—the
    effort force equals the resistance force, the MA of
    the mini blinds is 1
   Efficiency measures of how much of the
    work put into a machine is changed into
    useful output work by the machine.

   To calculate:

   Efficiency of a machine is always less
    than 100%
   Friction changes the useful work of a
    machine into thermal energy.

   Adding lubricant such as oil and
    graphite to reduce friction can make
    a machine more efficient.

 Oil fills the space
between surfaces so
high spots don’t rub
against each other.
Section 2 Assessment
    Question 1-6
      Page 137
Simple Machines
     Section 3
Simple Machine

   A machine that does work with only
   one movement is a simple machine.
Two basic kinds of simple machines

   Lever                      Inclined plane
   Lever can be modified      Inclined plane can be
    into:                       modified into:
       Pulley                     Screw
       Wheel and axle             wedge

   Lever is a bar that is      Effort arm is part of
    free to pivot about a        the lever on which
    fixed point called the       effort force is applied.
    fulcrum.                    Resistance arm is
                                 part of the lever that
                                 exerts the resistance
Three classes of levers are
based on positions of effort
force, resistance force, and
First-class lever
   Fulcrum is located between the effort force and
    resistance forces
   Multiplies and changes direction of force.
Second class lever

   Resistance force is
    located between the
    effort force and
   Always multiplies
Third class lever

   Effort force is
    between the
    resistance force and
   Does not multiply
    force but does
    increase distance
    over which force is
Levers in human body

   First class   Third class   Second class
Calculating ideal mechanical
advantage (IMA) of a lever

   A pulley is a grooved wheel with
   a rope, chain, or cable running
   along the grove.
Types of pulleys

   A fixed pulley is
    attached to
    something that
    doesn’t move
   Force is not            4N   4N
    multiplied but
    direction is changed.
   IMA = 1
Types of pulleys

   A movable pulley has
    one end of the rope
    fixed and the wheel
    free to move
   Multiplies force
   IMA = 2
Types of pulleys

   Block and tackle—
    system of pulley
    consisting of fixed
    and movable pulleys
   IMA = number of
    ropes supporting
    resistance weight.

The 100 N weight is divided
  equally among the four
  rope segments, so you
  only have to use a 25 N
    force to lift weight.
Wheel and Axle

   Wheel and axle is a
    machine with two wheels
    of different sizes rotating
   Modified lever form
   IMA = radius of wheel
    divided by the radius of

                                  Effort distance
   Gears are a modified form
    of a wheel and axle.
Inclined Plane
   Inclined plane—
    sloping surface that
    reduces the amount
    of force required to
    do work.

   Less force is required
    if a ramp is longer
    and less steep.

   A screw is an inclined      Wedge is an inclined
    plane wrapped in a           plane with one or two
    spiral around a              sloping sides.
    cylindrical post.
Compound Machine
   A compound machine uses a combination of
    two or more simple machines.

                            1    Lever

                            2    Wheel and axle

                            3    Inclined plane

                            4    Lever

                            55   Pulley

                            6    Lever

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