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I. Work and Energy and Power.

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Title: I. Work and Energy and Power.


1
I. Work and Energy and Power.
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  • Work is done when a force causes an object to
    move. This meaning is different from the everyday
    meaning of Work.
  • Work Force x Distance.
  • W F x d
  • The most commonly used SI unit is joules (J).

3
Work done
  • Work is done only when a force causes a change in
    position or the motion of an object in the
    direction of the applied force.

4
Work done
  • If a force of 20 N is applied to move an object a
    distance of 5 m, the work done is
  • W force x distance
  • W 20 x 5
  • W 100 J

5
  • Power is the rate at which work is done.
  • The SI unit for Power is Watts

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  • Running up a flight of stairs doesnt require
    more work than walking up slowly does but its
    more exhausting.
  • The amount of time it takes to do work is an
    important factor

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II. Simple Machines
  • Machines help people by redistributing the work
    put into the machine . They can change either the
    size or the direction of the input force

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Simple Machines (contd)
  • The mechanical advantage of a machine describes
    how much the machine multiplies force or
    increases distance

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Simple Machines (contd)
  • The most basic machines are called simple
    machines. There are six types of simple machines
    in two families
  • A) The Lever family
  • B) The inclined plane family

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  • A) The Lever family
  • i) simple lever
  • ii) pulley
  • iii) wheel and axle
  • Levers have a rigid arm and a fulcrum

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  • There are three classes of levers

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  • Class 1 The fulcrum is located between the
    points of application of the input and the output
    forces.
  • Examples a) a pair of pliers is made of two
    first class levers joined together b)
    a trolley, the fulcrum is the wheel.

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Class 1 Lever
  • The workman uses a trolley to move the large
    packing case. The fulcrum is the wheel.

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  • Class 2 The fulcrum is at one end of the arm
    and the input force is applied to the other end.
  • Examples a) nutcrackers
  • b) hinged doors
  • c) a wheel barrow, the load is in the centre of
    the barrow

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Class 2 Lever
  • The gardener uses a wheel barrow to lift tools
    and garden waste. The load is in the centre of
    the barrow

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  • Class 3 The third class levers multiply
    distance rather than force.

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  • Examples a) the human forearm, the biceps
    muscles, which is attached to the bone near the
    elbow contracts a short distance to move the hand
    a large distance.
  • b) The fisherman catches the fish which becomes
    the load at the end of the lever.

21
Class 3 Lever
  • The fisherman catches the fish which becomes the
    load at the end of the lever.

22
Example of Levers
  • Give your own examples of the three classes of
    lever. Think in terms of examples that you have
    used at home, work or school.

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Pulleys
  • Pulleys Pulleys are modified levers. Using
    moving pulleys or more than one pulley at a time
    can increase the mechanical advantage. The point
    in the middle of the pulley is like the fulcrum.

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Pulleys
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  • The pulley to the left is suspended and as a
    consequence the mechanical advantage is
    increased. This happens because the rope on the
    left and right of the pulley are both lifting the
    LOAD, they each lift half its weight. The load is
    split into 2. The calculation is shown below.

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FORMULAS RELATING TO MECHANICAL ADVANTAGE
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FORMULAS RELATING TO VELOCITY RATIO
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Wheel and Axle
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  • B) The inclined plane family
  • i) simple inclined plane
  • ii) wedge
  • iii) screw
  • Compound machines are made of two or more simple
    machines

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The inclined plane
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Wedges
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The Screw
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  • Efficiency of a machine A machine cannot do more
    work than the work required to operate the
    machine. Because of friction, the work output of
    a machine is always somewhat less than the work
    input

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  • The efficiency of a machine is the ratio of the
    useful work performed by the machine to the work
    required to operate the machine

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III. Energy
  • Energy is the ability to do work.
  • Like Work, Energy is measured in Joules (J).

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  • Potential Energy The energy that an object has
    because of the position, shape or condition of
    the object.
  • Other forms of potential energy include elastic,
    chemical, electrical and magnetic

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  • Potential energy is stored Energy
  • Elastic potential energy is stored in any
    stretched or compressed elastic material.

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  • The Gravitational Potential energy of an object
    is determined by its mass, its height, and g,
    the free fall acceleration due to gravity.
  • PEmgh

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Gravitational Potential Energy
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Where would you have the greatest PE? Where would
you have the least PE?
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Kinetic Energy
  • An objects kinetic energy, or energy of motion
    is determined by its mass and speed.

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  • Potential energy and kinetic energy are forms of
    mechanical energy.

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  • In addition to mechanical energy, most systems
    contain non-mechanical energy.
  • Non-Mechanical energy does not usually affect
    systems on a large scale.
  • e.g. Internal Energy

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  • Internal Energy is the portion of energy
    absorbed by the atoms of an object, and accounts
    for the lost mechanical energy

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Atoms and Molecules
  • Atoms and Molecules are in constant motion
    therefore they have kinetic energy.

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  • Kinetic energy is transferred between particles
    through collisions.
  • (Bowling Ball hitting pins.)

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  • As the object gets hotter the particles move
    faster therefore K.E. is increased.
  • As the object cools down the particles slow down
    therefore K.E. is decreased.

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  • We Conclude that Kinetic Energy is related
    (proportional) to the objects temperature.
  • Increasing Temperature, increases the KE of the
    molecules of an object.
  • Decreasing temperature, decreases the KE.

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Energy Conservation
  • Energy readily changes from one form to another
  • In a mechanical system, potential energy can
    become kinetic energy, and kinetic energy can
    become potential energy.

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Conservation of Energy
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  • Mechanical energy can change to non-mechanical
    energy as a result of friction, air resistance,
    or other means.

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  • Energy cannot be created or destroyed , although
    it may change form.
  • This is called the law of conservation of energy.

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