Work and Simple Machines

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Work

Machines
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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.)

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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?

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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.

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• 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.

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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?

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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

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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

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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?

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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

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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).

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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.

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• 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.
•  

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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?

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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.

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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)

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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.

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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.

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

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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.

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Simple Machines
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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).

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• 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.

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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.

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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
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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).

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Fulcrum is between EF (effort) and RF
(load)Effort moves farther than Resistance.
Multiplies EF and changes its direction
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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.

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RF (load) is between fulcrum and EF Effort moves
farther than Resistance. Multiplies EF, but does
not change its direction
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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.

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EF is between fulcrum and RF (load) Does not
multiply force Resistance moves farther than
Effort. Multiplies the distance the effort force
travels
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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'.

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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.

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Pulley
A pulley consists of a grooved wheel that turns
freely in a frame called a block.

• A pulley can be
• Fixed
• Movable

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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.

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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.

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Block and Tackle(Compound Pulley)

• Combines fixed and movable pulley sometimes
  more than one of each.

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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
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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.

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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.

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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
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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.

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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.

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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).

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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.