Work and Machines

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					Work and Machines

     Chapter 8
    The Scientific Meaning of Work
                                      Answer: No mental
   Definition: work                  effort might seem like
    occurs when a force               a lot of work, but you
    causes an object to               are not using a force to
    move in the                       move an object.
    direction of the
   Use the definition
    above, is doing
    homework, work??
Working Hard, or Hardly Working?

   Applying a force
    doesn’t always result
    in work being done.
       For example, pushing a
        car that won’t budge.
        If you can’t get it to
        move you haven’t done
        any work!
Work continued
         Force and motion in the
          same direction:
             For work to be done, the
              object must move in the
              same direction as the
             For example if you are
              carrying a heavy suitcase
              through the airport it is
              not considered work
              because the direction of
              force is upward, and the
              direction of motion is
Work or Not Work?
                Calculating Work
   Work = force x distance
   W= F x d
   If a man applies a force
    of 500 N to push a truck
    100 m down the street.
    How much work does he
 Answer:
W = 500 N x 100 m
W = 50,000 J
The unit used to
  express work is the
  newton-meter, but is
  more commonly
  called the joule (J).
     Nerdy Science Joke Break…

   Did you hear about
    the criminals who
    never had to do
    any work?

Answer: They were
joule thieves.
 Power is the rate at which work is done.
 It is how fast work happens!

 Power = Work

 The unit used to express power is joules
  per second (J/s), which is more simply
  called the watt (W).
 For example if you do 50 J of work in 5
  seconds, your power is 10 J/s or 10 W.
                   Try it…
   Calculate Power:
   What is the power
    of a small motor
    that can do 500 J
    of work in 25
          Nerdy Science Joke…

Q: “What is the unit of
A: “Watt.”
Q: “I said, what is the unit
  of power?”
A: “WATT!”
Q: “I SAID…”
                       Quiz 8.1
1. In which situation do you do more work?
   a. You lift a 75 N bowling ball 2 m off the floor.
   b. You lift two 50 N bowling balls 1 m off the floor.
2. The rate at which work is done is:
     a. Joule             b. Time
     c. Power             d. Work
3. Power is:
   a. Work/time                   b. time/Work
   c. Force x distance            c. Work/Force
       Quiz 8.1 continued…

4. The unit used for Power is:
      a. newton-meter          b. Joule
      c. meter per second      d. Watt
5. You push a 100 N box 3 m across the
  floor. How much work has been done?
      a. 33.3 J      b. 300 J
      c. 33.3 W      c. 300 W
   A machine is a device that helps make
    work easier by changing the size or
    direction of a force.
   The work that you do on a machine is
    called work input.
       Called input force
   The work done by the machine is called
    work output.
       Called output force
    The force-distance trade off

W = 450 N x 1 m = 450 J   W = 150 N x 3 m = 450 J
        Mechanical Advantage

   Mechanical
    Advantage tells you
    how many times the
    machine multiplies
   MA = output force
           input force
             Finding Advantage

   You apply 200 N to a machine, and the
    machine applies 2,000 N to an object.
    What is the mechanical advantage?
   Remember MA = output force/input force

    Answer: MA = 2000 N = 10
                   200 N
            Mechanical Efficiency
   Is a comparison of a machine’s work output
    with the work input.
   Mechanical Efficiency = work output X 100
                            work input
Simple Machines
   A lever is a simple
    machine consisting of
    a bar that pivots at a
    fixed point, called a
   There are 3 types of
               First Class Levers
   The fulcrum is between
    the input force and the
           Second Class Levers
   The load is between
    the fulcrum and the
    input force.
            Third Class Levers
   The input force is
    between the fulcrum
    and the load.
                    Inclined Planes

   A simple machine
    that is a straight,
    slanted surface.
   An inclined plane
    allows you to apply a
    smaller force over a
    greater distance.
   A wedge is a double
    inclined plane that
   Wedges are often
    used to cut materials.
   They allow you to
    exert your force over
    an increased distance.
   An inclined plane that
    is wrapped in a spiral.
   When you turn a
    screw, you exert a
    small input force over
    a large turning
    distance, but the
    screw itself doesn’t
    move very far.
Mechanical Advantage of Screws
   The threads on the
    left screw are closer
    together and wrap
    more times around,
    so that screw has a
    greater mechanical
    advantage that the
    one on the right.
                    Wheel and Axle
   A simple machine
    consisting of two
    circular objects of
    different sizes.
   Examples include a
    door knob, a crank, a
    fishing reel…
   A simple machine
    consisting of a grooved
    wheel that holds a
    rope or a cable.
   A load is attached to
    one end of the rope,
    and input force is
    applied to the other
   Fixed Pulleys only
    change the direction
    of the force.

                              Movable Pulleys are
                               attached to the object
                               that is being moved.
         Compound Machines

   Machines that are
    made of two or
    more simple

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