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Work and Simple Machines

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Title: Work and Simple Machines Author: lewisv Last modified by: Windows User Created Date: 10/30/2004 2:17:16 AM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Work and Simple Machines


1
Work

Machines
2
What is work?
  • In science, the word work has a different meaning
    than you may be familiar with.
  • The scientific definition of work is using a
    force to move an object a distance (when both the
    force and the motion of the object are in the
    same direction.)

3
Work or Not?
  • According to the scientific definition, what
    is work and what is not?
  • A teacher lecturing to
  • her class?
  • A mouse pushing a piece of cheese with its nose
    across the floor?

4
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5
Whats work?
  • For work to be done on an object, the object
    must move in the same direction as the force.
  • So .2 things are required
  • Object must MOVE when force is applied
  • The direction of the motion must be the same as
    the force.

6
  • Imagine that you are late for school and are
    moving quickly to your locker carrying a heavy
    book bag. Because you are making the book bag
    move, are you doing work on it?
  • NO For work to be done on an object, the object
    must move in the SAME direction as the force.
  • You are applying a force to hold it up, but the
    bag is moving forward.

7
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8
Whats work?
  • A scientist delivers a speech to an audience of
    his peers.
  • A mother picks up her baby.
  • A mother carries her baby from room to room.
  • A body builder lifts 350 pounds above his head.
  • A woman carries a 20 kg grocery bag to her car?

9
Whats work?
  • A scientist delivers a speech to an audience of
    his peers. NO
  • A mother picks up her baby. Yes
  • A mother carries her baby from room to room. NO
  • A body builder lifts 350 pounds above his head.
    Yes
  • A woman carries a 20 kg grocery bag to her car?
    NO

10
Formula for Work
  • Work Force x Distance
  • (push or pull)
  • The unit of force is Newtons
  • The unit of distance is meters
  • The unit of work is Newton-meters
  • One Newton-meter is equal to one joule
  • So, the unit of work is a joule

11
WFD Work Force x Distance
  • Calculate
  • If a man pushes a concrete block 10 meters with a
    force of 20 N, how much work has he done?

12
WFD
  • Work Force x Distance
  • If a man pushes a concrete block 10 meters with
    a force of 20 N, how much work has he done?
  • W 20N x 10m
  • 200 joules

13
Power
  • Power is the rate ( speed/ how fast) at which
    work is done or that energy is transferred.
  • It measures how fast work happens or how
    quickly energy is transferred.
  • Power Work
  • Time (force
    x distance)
  • The unit of power is the watt (also
    joules/second).

14
Check for Understanding
  • Two physics students, Ben and Bonnie, are in the
    weightlifting room. Bonnie lifts the 50 kg
    barbell over her head (approximately .60 m) 10
    times in one minute Ben lifts the 50 kg barbell
    the same distance over his head 10 times in 10
    seconds.
  • Which student does the most work?
  • Which student delivers the most power?
  • Explain your answers.

15
  • Ben and Bonnie do the same amount of work they
    apply the same force to lift the same barbell the
    same distance above their heads.
  • Yet, Ben is the most powerful since he does
    the same work in less time.
  •  

16
What is a Machine?
  • Device or tool that makes work easier
  • A machine makes work easier by changing the size
    (magnitude) or direction of the force needed
  • What are some machines you
  • use everyday?

17
Input vs. Output
  • The WORK that you do on the machine is work
    input. (W F x D)
  • The FORCE you use on the machine is called input
    force.
  • The work done by the machine is called work
    output
  • The FORCE the machine applies is called output
    force.

18
Mechanical Advantage (MA)(Comparing Forces)
  • MA output force
  • input force
  • MA tells how many times a machine
  • multiplies force
  • If a machine can increase force more than
    others, work is generally easier and it has a
    greater mechanical advantage
  • gt1 MA -- can help move heavy objects
  • lt1 MA -- can increase the distance an
    object moves (like a hammer)

19
How Machines Help
  • They dont increase the amount of work done Work
    output cant be more than work input
  • But, machines allow the force to be applied over
    a greater distance which means less force is
    needed for the same amount of work.
  • Machines make work easier by changing the size or
    direction (or both) of the input force.

20
Mechanical Efficiency (ME)(Comparing Work Output
and Input)
  • ME Work output x 100
  • Work input
  • The work output of a machine is always less than
    the input.
  • Work has to overcome friction
  • The less work a machine has to do to overcome
    friction the greater the ME
  • We multiply Xs 100 because its expressed as a
    percentage.

21
  • Machines cant be 100 efficient because every
    machine has moving parts. Moving parts have to
    use some of the work input to
  • overcome friction.

22
Simple Machines
  • Ancient people invented simple machines that
    would help them overcome resistive forces and
    allow them to do the desired work against those
    forces.

23
Simple Machines
24
The Lever
  • A lever is a rigid bar or board
  • that rotates around a fixed
  • point called the fulcrum.
  • The bar may be either straight or curved.
  • In use, a lever has both an effort (or applied)
    force and a load (resistant force).

25
  • Levers can be used to exert a large force over
    a small distance at one end by exerting only a
    small force over a greater distance at the other.

26
The 3 Classes of Levers
  • The class of a lever is determined by the
    location of the effort force and the load
    relative to the fulcrum.

27
The 3 Classes of Levers
R is the resistance force which is the load.
E is the human effort force applied. The
fulcrum is the fixed point support on which a
lever pivots
28
First Class Lever
  • In a first-class lever the fulcrum is located
    at some point between the effort and resistance
    (load) forces.
  • Common examples of first-class levers include
    crowbars, scissors, pliers, and seesaws.
  • A first-class lever always changes the direction
    of force (I.e. a downward effort force on the
    lever results in an upward movement of the
    resistance force).

29
Fulcrum is between EF (effort) and RF
(load)Effort moves farther than Resistance.
Multiplies EF and changes its direction
30
Second Class Lever
  • With a second-class lever, the load (R) is
    located between the fulcrum and the effort force.
  • Common examples of second-class levers include
    nut crackers, wheel barrows, doors, and bottle
    openers.
  • A second-class lever does not change the
    direction of force. When the fulcrum is located
    closer to the load than to the
  • effort force, an increase in output force
    (mechanical advantage) results.

31
RF (load) is between fulcrum and EF Effort moves
farther than Resistance. Multiplies EF, but does
not change its direction
32
Third Class Lever
  • With a third-class lever, the effort force is
    applied between the fulcrum and the resistance
    force.
  • Examples of third-class levers include tweezers,
    hammers, shovels, fishing pole, and about any
    tool you swing (bat, club).
  • A third-class lever does not change the direction
    of force there is an increase distance with a
    corresponding decrease in force.

33
EF is between fulcrum and RF (load) Does not
multiply force Resistance moves farther than
Effort. Multiplies the distance the effort force
travels
34
Lever Mechanical Advantage
  • MA
  • length of effort arm length of resistance
    arm
  • If the effort arm distance (from effort to
    fulcrum) is greater than the resistance arm,
    then the effort required will be less than the
    load being moved. This is known as a 'positive
    mechanical advantage'.

35
Wheel and Axle
  • The wheel and axle is a simple machine consisting
    of a large wheel rigidly secured to a smaller
    wheel or shaft, called an axle.
  • When either the wheel or axle turns, the other
    part also turns. One full revolution of either
    part causes one full revolution of the other
    part.

36
Pulley
A pulley consists of a grooved wheel that turns
freely in a frame called a block.
  • A pulley can be
  • Fixed
  • Movable

37
Fixed Pulley
  • A pulley is said to be a fixed pulley if it
    does not rise or fall with the load being moved
    (only spins). A fixed pulley changes the
    direction of a force.

38
Moveable Pulley
  • A moveable pulley rises and falls with the
    load that is being moved. A single moveable
    pulley does not change the direction of a force.
  • Movable pulleys increase force and distance over
    which the input force must be exerted.

39
Block and Tackle(Compound Pulley)
  • Combines fixed and movable pulley sometimes
    more than one of each.

40
A Pulleys Mechanical Advantage
  • The Mechanical Advantage of a pulley equals the
    number of rope segments that support the load.
  • MA 4
  • (Dont count the rope you are pulling)

Pulley Video
41
Inclined Plane
  • An inclined plane is an even sloping surface.
    The inclined plane makes it easier to move a
    weight from a lower to higher elevation.

42
Inclined Plane
  • The mechanical advantage of an inclined plane is
    equal to the length of the slope divided by the
    height of the inclined plane.
  • While the inclined plane produces a mechanical
    advantage, it does so by increasing the distance
    through which the force must move.

43
Although it takes less force for car A to get to
the top of the ramp, all the cars do the same
amount of work.
A B
C
44
Inclined Plane
  • A wagon trail on a steep hill will often traverse
    back and forth to reduce the slope experienced by
    a team pulling a heavily loaded wagon.
  • This same technique is used today in modern
    freeways which travel winding paths through steep
    mountain passes.

45
Wedge
  • The wedge is a modification of the inclined
    plane. Wedges are used as either separating or
    holding devices.
  • A wedge can either be composed of one or two
    inclined planes. A double wedge can be thought of
    as two inclined planes joined together with their
    sloping surfaces outward.

46
The Screw
  • The screw is also a modified version of the
    inclined plane.
  • While this may be somewhat difficult to
    visualize, it may help to think of the threads of
    the screw as a type of circular ramp (or inclined
    plane).

47
A Screws MA
The longer and thinner the screw is the greater
the MASimilarly, the longer the spiral on the
screw and closer together the threads are, the
greater the MA.
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