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Work and Machines

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Title: Work and Machines


1
Work and Machines
  • Chapter 5
  • Sections 1-3

2
C5- Work Machines
  • Section 1- Work slides 3-13
  • Section 2- Using Machines slides 14-30
  • Section 3- Simple Machines slides 31-50

3
Section 1- Work
  • What Youll Learn
  • What work is
  • How work energy are related
  • How to calculate work power

4
Work
  • Work is done when a force causes an object to
    move in the same direction that the force is
    applied.
  • When a force makes an object move, energy is
    transferred from one object to another.

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5
How do you know if you are doing work?
  • Two things must happen
  • A force must be applied to the object
  • The object must move in the same direction as the
    force

6
What does direction have to do with work?
  • When you pick up a stack of books, your arms
    apply a force upward.
  • You start walking with the books.
  • The direction of motion has changed your arms
    are no longer working, instead your legs supply
    the forward direction

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7
Work Energy
  • Energy is the ability to cause change or to do
    work.
  • When work is done energy is transferred from one
    object to another.

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8
Calculating Work
  • You multiply force times distance to calculate
    work.
  • Work (joules) force (N) X distance (m)
  • WFd
  • Calculate the amount of work a painter does when
    he lifts a can of paint weighing 40 newtons 2
    meters.
  • WFd(40N) (2m) 80 J

9
When is work done?
  • When a pitcher throws a ball to the catcher, he
    applies force to the ball only when it is in his
    hand.
  • The ball moves 10 m after it leaves his hand but
    work occurred only while it was in his hand for
    1m.

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10
Power
  • Power is the rate at which work is done.
  • Something has more power if it can do the same
    amount of work in less time.

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11
How do you calculate power?
  • To calculate power, divide the amount of work
    done by the time it takes to do it.
  • Power (in watts) work (in joules)/time(s)
  • PW/t The SI unit for power is the watt.
  • Find the power of the machine that can do 5,000
    joules of work in 20 seconds.
  • PW/t5000J/20 s250 watts. The power of the
    machine is 250 watts.

12
How is power calculated when energy is
transferred?
  • You can also calculate power by dividing the
    amount of energy transferred by the time it took
    to transfer it.
  • Power (in watts)energy transferred (j)/ time (s)
    or PE/t.

13
How is energy transferred when no work is done?
  • Suppose a light bulb changes electrical energy
    into light heat at a rate of 100 j /2 s. How
    many watts of power will the light bulb have?
  • PE/t 100j/2s 50 watts

14
Section 2- Using Machines
  • What Youll Learn
  • How machines make work easier
  • How to calculate mechanical advantage
  • How to calculate the efficiency of a machine

15
What is a machine?
  • A machine is a device that makes work easier.
  • Knives, scissors, and doorknobs are simple
    machines.
  • Engines are more complex machines

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16
Making Work Easier
  • There are 3 ways machines make work easier
  • Increasing force on an object
  • Increasing the distance over which a force is
    applied or
  • Changing the direction of an applied force.

17
How can force be increased using a machine?
  • If Work force X distance, then either force or
    distance increases.
  • If distance increases, then force decreases.
  • Machines such as a car jack, increase the
    distance a force is applied thus reducing the
    amount of force needed to do the same amount of
    work.

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/hydraulicus/gearrackandpinioncarjack1.gifgt
18
How does increasing distance decrease force?
  • A rake makes the work easier by increasing the
    distance over which you apply force.
  • Your hands move a small distance at the top of
    the handle while the handle moves across a wider
    distance.

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t/uploads/2006/11/raking-leaves.jpggt
19
Why do you want a machine that will change
direction?
  • Sometimes its easier to apply force in a
    different direction
  • To raise a flag up a pole, its easier to pull
    down on a rope than to climb to the top.
  • When you use an axe to split wood, a downward
    force on the handle changes into a sideways force
    by the blade.

20
The Work Done by Machines
  • Pushing down on the handle raising the lid.
  • You are doing work on the crowbar that opposes
    the friction of the nails in the lid and crate.

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80/23298082.jpggt
21
What are input forces output forces?
  • A machine changes the way you do the work making
    it easier.
  • The force you apply to the machine is called the
    input force or Fin.
  • The force applied by the machine is called the
    output force or Fout.

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22
What are input work output work?
  • When you use a machine there are 2 kinds of work
  • Input work or Win , done by you
  • Output work or Wout, done by machine

23
How do machines use conservation of energy?
  • When you do work on a machine, you transfer your
    energy to the machine which then transfers energy
    to the object.
  • So, Wout is never greater than Win.
  • However, a machine does not transfer all its
    energy to the object. Due to friction, some of
    the energy changes to heat.
  • This means that Wout is always smaller than Win.

24
What is an ideal machine?
  • If you could build perfect machine without any
    friction, the input work would equal the output
    work.
  • WinWout or FinX dinFoutX dout
  • If the machine could increase the input force,
    then work output would be greater than the work
    input.

25
Mechanical Advantage
  • Some machines make work easier by making the
    output force greater than the input force. The
    number of times the applied force is increased by
    a machine is called the mechanical advantage (MA)
    of the machine. Mechanical advantage is the
    ratio of the output force to the input force. MA
    Fout/Fin

26
Mechanical Advantage
  • Using a pulley system you only need 300 N to lift
    a piano that weighs 1500 N. What is the MA?
  • MA Fout/Fin 1500 N/300 N 5
  • Notice this ratio cancels out units of newtons.

27
What is ideal mechanical advantage?
  • The MA of a machine without friction is called
    the ideal MA or IMA.
  • You can calculate this by dividing the input
    distance by the output distance.

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leygirl.jpggt
28
Efficiency
  • Some of the energy put into a real machine is
    changed into heat by friction.
  • So, the output work of a machine is always less
    than the work put into it.
  • Efficiency is the comparison of the amount of
    work put into a machine to the amount of work the
    machine puts out.
  • High-efficiency means less heat from friction!

29
How do you calculate efficiency?
  • Divide the output work by the input work to get a
    percentage.
  • Efficiency Wout/in X 100
  • To calculate the efficiency of a machine with Win
    of 50 joules a Wout of 40 joules 40/500.8, or
    80.
  • The efficiency of a real machine is always less
    than 100 due to friction.

30
How can machines be made more efficient?
  • Reducing friction by adding oil or grease to the
    surfaces that rub together fills the gaps between
    them so the surfaces slide across each other more
    easily.

31
Section 3- Simple Machines
  • What Youll Learn
  • Six types of simple machines
  • How simple machines make work easier
  • How to calculate the ideal mechanical advantage
    of simple machines

32
Types of Simple Machines
  • A simple machine is a machine that does work with
    only one movement of the machine.
  • There are six types lever, pulley, wheel axle,
    inclined plane, screw, and wedge.
  • The screw and wedge are different forms of the
    inclined plane.

33
Levers
  • A wheelbarrow, a rake a baseball bat are all
    examples of levers.
  • A lever is a bar that pivots, or turns around, a
    fixed point called the fulcrum.
  • The input arm is the distance from the fulcrum to
    the point where the input force is applied the
    output arm is the distance from the fulcrum to
    the point where the lever exerts the output force.

34
What are the 3 classes of levers?
  • The class of a lever is based on the location of
    the fulcrum, the input force and the output force.

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35
First-Class Lever
  • The top figure shows a first-class lever with the
    fulcrum located between the input and output
    forces.
  • The first-class lever always changes the
    direction of the force.
  • Examples include a crowbar, scissors, and a
    seesaw.

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36
Second-Class Lever
  • The middle figure represents a second-class lever
    with the output force between the input force and
    the fulcrum.
  • Both input and output forces move in the same
    direction.
  • The wheelbarrow is a classic example.

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37
Third-Class Lever
  • The bottom figure, a third-class lever, shows the
    output force is farther away from the fulcrum
    than the input force.
  • The output force is always less than the input
    force in a third-class lever such as a baseball
    bat, but the advantage is that it increases the
    distance over which the output force is applied.

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38
How is ideal mechanical advantage of a lever
calculated?
  • To calculate IMA of any machine, divide the input
    distance by the output distance.
  • For a lever, the input distance is the length of
    the input arm output distance is the length of
    the output arm.
  • IMA Lin/Lout

39
Pulleys
  • To raise a sail upward, a sailor pulls down on a
    rope wrapped around a pulley.
  • A pulley is a grooved wheel with a rope, chain or
    cable wrapped around it.
  • Pulleys may be fixed or movable or in systems.

40
What is a fixed pulley?
  • This modified first-class lever changes the
    direction of the input force like on a sail or a
    flagpole.
  • An elevator also uses a fixed pulley with a cable.

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/Large/L_Pulley_dia1_fixed.gifgt
41
What is a movable pulley?
  • A movable pulley has one end of the rope fixed
    the wheel is free to move.
  • The movable pulley doesnt change the direction
    of the force, but it does decrease the amount of
    input force needed to lift the object. MA2

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42
What is a block and tackle?
  • A block and tackle is a system of fixed movable
    pulleys used together.
  • The more sections of the rope a system uses to
    pull up an object, the greater the output force
    is. IMA of sections

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43
Wheel and Axle
  • Simple machine with an axle attached to the
    center of a larger wheel and both turn together.
  • Doorknobs and ferris wheels are examples.

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247.JPGgt
44
What is the IMA of the wheel axle?
  • A wheel and axle is a modified lever with the
    center of the axle as the fulcrum.
  • To calculate the IMA of a wheel and axle, use
    this equation
  • IMA radius of wheel (m)/radius of axle (m) or
    IMA rw/ra
  • To increase IMA, simply increase the radius of
    the wheel.

45
How do gears work?
  • A gear is a wheel and axle with teeth around the
    rim of the wheel.
  • One gear makes the other turn with the smaller
    gear turning more times than the larger one.
    Output force direction can be changed with a
    gear.

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-Gears.jpggt
46
Inclined Planes
  • An inclined plane is a sloping surface that
    reduces the amount of force it takes to do work.
  • Examples include ramps and stairways.

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47
How does an inclined plane make work easier?
  • You do the same work by lifting a box straight up
    or pushing it up a ramp.
  • As the inclined plane becomes longer, the force
    needed to move the object becomes less.
  • The input force is applied over a longer
    distance, so it takes less input force.
  • IMAlength of slope(m)/ height of slope(m)

48
The Screw
  • A screw is an inclined plane wrapped in a spiral
    around a post.
  • The inclined plane forms the threads on the
    screw.
  • Apply force by turning the screw friction holds
    it in place.
  • Examples jar lid, corkscrew, drill bit, light
    bulb.

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49
The Wedge
  • A wedge is an inclined plane with one or two
    sloping sides.
  • Like the screw, the inclined plane moves through
    the object. Knives are wedges.
  • The IMA increases as it gets longer thinner.

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oct03b.jpggt
50
Compound Machines
  • Some machines, like this can opener, are made of
    several simple machines.
  • Two or more working together are called a
    compound machine.
  • The handles are levers, a wedge pierces the can,
    a wheel axle turns to open the can.

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