Work, Power Simple Machines by Adela Sanders

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									Work, Power &
     Work- A force acting through a distance.

     The distance that the object moves must be in
      the same direction as the force applied to the
What force is
having to be
overcome by the                   Lifting Force
lifting force?

                                 Movement Distance
   Work- A force acting through a distance.

   The distance that the object moves must be in
    the same direction as the force applied to the

What force is
having to be
overcome by the
pushing force?                         Pushing Force

    Friction!                          Distance Moved
    Work is not done every time a force is applied.
    Work is done only when a force moves an object
     in the same direction as the applied force.

  Is the man doing
  work when he
  lifts the box?                            Lifting Force


 Is the man doing                         Movement
 work when he                             Distance
 holds the box?
                               Is the man doing
                               work if he carries      No!
(But he IS applying a force)   the box?     (But he IS applying a force)
Think about pushing on wall that does not
  move. A force is applied but the wall has
  not moved a distance.

Is work


                        But I did expend
                        energy applying
                        a force!
            Calculating Work
 Work = force (N) x distance (m)
 W=FxD
 The unit for work is a Joule (J).
 If you lifted an object weighing 1N through a
  distance of 1m, you did 1 Joule of work:

           W = FD = 1N x 1m = 1 J

Think about how much force a Newton is…about
  ¼ of a pound.

Is a Joule of work…very much work?      No!
             Calculating Work
 Ifyou lifted an object weighing 200 N through a
  distance of 0.5m, how much work would you

Work = force x distance = 200 N x 0.5m = 100 J.
   Power is the rate at which work is done , or the
    amount of work per unit of time.

     Two men can move a lot of sand using shovels…

                               …but a front-end loader
                               can do it in less time…

     …because the front-end loader has more POWER.
 Power  is the rate at which work is done ,
 or the amount of work per unit of time.

 Power   = work / time or

 Power   = force x distance / time

    Because work = force x distance
 Theunit for power is watt (W). One watt is
 equal to 1 joule per second (1 J/sec).

 Largequantities of power are measured in
 kilowatts (kW).

 One                   1000
        kilowatt equals _____ watts.
       Machines : How do machines affect
 A machine is a device
    that makes work

   A machine is ANY
    device that helps you
    to do something.
Machines : How do machines affect
 What   are other examples of machines?
Machines : How do machines affect
Machines : How do machines affect work?
Machines : How do machines affect
             Machine Efficiency:
              Input and Output
   There are always two types of work involved in
    using a machine.

   Input work is the work that goes into the machine
    (like turning a pencil sharpener).

   Output work is the work that comes out of the
    machine (like the grinding of the pencil).

                   How hard was that??
Machine Efficiency: Input and Output
    The efficiency of a machine can be calculated:

     Efficiency = (work output / work input) x 100

 This is easy to remember…think about it…

 If you put 100 Joules of work into a pencil sharpener,
    but only got 80 Joules of work out, the pencil
    sharpener is 80% efficient:

(80 Joules / 100 Joules) x 100 = 80% efficiency
 Machine  efficiency can never be greater
  than or equal to 100% because the work
  output can never be greater than the work

 Inother words, there is no machine that
  has a 100% efficiency!

       Why not?
                        Think…it’s a force that opposes

        Friction makes every machine <100% efficient
         Machine Efficiency

 The friction in a machine “wastes” energy
 in the form of heat

 Machines   with the smallest amount of
 friction are the most efficient.
              Machine Efficiency
   The closer the work output is to work input, the
    more efficient the machine.

                                         Toyota hybrid
                                         “concept car”
Many household
appliances have energy
guides that tell the
consumer how efficient
the appliance is. The
more efficient the
appliance the more
money the consumer
will save.
You can also look for
  the “energy star” label
  on more efficient
   Machines make work easier because they
    change the size or the direction of the force put
    into the machine.

   Most machines make work easier by
    multiplying either force or distance.

                Force!                Distance!

        Which does which?
      What is multiplied, force or distance?

            Force                     Distance

Force when “prying” the dirt free, distance when lifting
it up out of the hole or into a truck or wheelbarrow
      Determining How Helpful a
             Machine Is

 Besidesthe efficiency of a machine we also
 can determine how helpful a machine is.
        Determining How Helpful a
               Machine Is
   What we mean by how helpful is how many times
    the machine multiples the effort force to overcome
    the resistance force
                                Effort Force

                            Resistance Force
      Determining How Helpful a
             Machine Is   ? Effort

Force   ?

Effort Force or
Resistance Force?
                    ?   Force
     Determining How Helpful a
            Machine Is
 The number of times a machine multiplies
 the effort force is called the mechanical

 Thistells you how much force is gained by
 using the machine. The more times the
 machine multiples the effort force, the
 easier it is to do the job.
       Quick quiz –open notes
 1. What is a machine?
 2. Describe relationship between friction
  and the efficiency of a machine.
    6 Kinds of Simple Machines

Inclined Plane   Wedge   Lever

Wheel & Axle     Screw   Pulley
             Inclined Plane
 Inclinedplane: A ramp is an example of an
 inclined plane. It is simply a flat slanted
 surface. It has no moving parts.
           Inclined Plane
 Aninclined plane decreases the size of
 the effort force needed to move an object.
 Wedge:    An inclined plane that moves.
 In a wedge, instead of an object moving
  along the inclined plane, the inclined plane
  itself moves to raise the object.
A wedge is usually a piece of wood or
 metal that is thinner at one end. A knife
 and axe are two examples.
 Wedge    : the longer and thinner a wedge
 is, the less the effort force required to
 overcome resistance. This also is true for
 an inclined plane.
 When  you sharpen a wedge, you are
  increasing its mechanical advantage by
  decreasing the effort force that must be
  applied in using it.
 A sharpened ax requires less force
  because the edge is thinner
A lock is another device that depends on
 the principle of the wedge. The edges of
 the key are a series of wedges.
A zipper is also a device that depends on
 the principle of a wedge.
 Lever : A lever is a rigid bar that is free to
 pivot , or move about a fixed point. The
 fixed point is called the fulcrum.
 When  a force is applied on a part of the
 bar by pushing or pulling it, the lever
 swings about the fulcrum and overcomes
 a resistance force.
 Leversare divided into three groups, or
 classes, depending on the location of the
 fulcrum and the forces.
 Examples of first class levers are
 crowbars, seesaw and pliers.
 The  fulcrum of a lever is not always
  between the effort force and the resistance
  force like the example of a crowbar.
  Sometimes it is at the end of the lever.
 A wheelbarrow is a second class lever.
 Where is the effort, resistance, and
 Because distance is decreased by the
 wheelbarrow, force must be increased.
 In second class levers, you multiply force
  but decrease distance.
 Also with second class levers, the
  direction in which you lift is the same as
  the direction in which the load moves.
 A 2nd class lever does not change the
  direction of the force applied to it.
 Ina third class lever, the fulcrum or fixed
  point, is at the end of the pole where you
  are holding it like a fishing pole.
 The effort force is applied by your other
 hand as you pull back on the pole. At the
 top of the pole is the resistance force. In
 this class, you need to move your effort
 force only a short distance to make the
 end of the pole move a greater distance.
A 3rd class lever, reduces the effort
 required but multiplies the distance
 through which the output force moves.
          Wheel and Axle
A wheel and axle is a simple machine
 made up of two circular objects of different
 sizes. The wheel is the larger object. It
 turns around a smaller object called the
           Wheel and Axle
A  force applied to the wheel is multiplied
 when it is transferred to the axle, which
 travels a shorter distance than the wheel
          Wheel and Axle
 The mechanical advantage depends on
 the radius of the wheel and of the axle.
          Wheel and Axle
 Examples  of wheels and axles are
 bicycles, Ferris wheels, gears, wrenches,
 door knobs, steering wheels, windmills
 and screwdrivers.
 Screw  : A screw is inclined plane wrapped
 around a central bar or cylinder to form a
 A screw rotates and with each turn moves a
  certain distance up or down. A screw multiplies
  an effort force by acting through a long distance.
 The closer together the threads, or ridges, of a
  screw, the longer the distance over which the
  force is exerted and the more the force is
  multiplied. Thus the mechanical advantage of a
  screw increases when the threads are closer
 Examples of a screw are the wood screw,
 corkscrew, nut and bolt, faucets, jar lids,
 and spark plugs.
A  pulley is a rope, belt, or chain wrapped
  around a grooved wheel.
 A pulley can function in two ways. It can
  change the direction of a force or the
  amount of force.
A pulley that is attached to a structure is
 called a fixed pulley. A fixed pulley does
 not multiply an effort force it only changes
 the direction of the effort force.
 Pulleyscan be made to multiply the force
 with which you pull on them. This is done
 by attaching a pulley to the object you are
 moving. This type of pulley is called a
 moveable pulley.
A greater mechanical advantage can be
 obtained by combining fixed and movable
 pulleys into a pulley system.
       Compound Machines
A  compound machine is a combination of
  two or more simple machines.
 Most of the machines you use everyday
  are compound machines: cars, bicycles,
  washing machine, VCR, blender,watches,
      Compound Machines
 Simple or compound machines cannot
 multiply work. You can get no more work
 out of a machine than you put into it.
 True or False (If false correct)
 1. Work equals force times time.
 2. The unit of work in the metric system is
  the newton-meter or joule.
 3. Power is work divided by force.
 4. The unit for power is newton.
 5. A pulley is an inclined plane that moves.
 6. An ax is an example of a wheel and

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