Using Machines

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Using Machines

• S8P3c Students will demonstrate the effect of
  simple machines (lever, inclined plane, pulley,
  wedge, screw, and wheel and axle) on work.

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Key Ideas

• Machines make work easier.
• Machines can change the size, distance or
  direction of a force.
• Mechanical advantage is the number of times a
  machine multiplies the effort force.
• Six types of simple machines make work easier.
• Simple machines can be combined to make complex
  machines.

3
Review

• Work happens when a force moves an object
• W F x d
• Work is measured in Joules (Newtons x meters)
• Power is the rate at which work is done
• Power Work / time
• Power is measured in Watts (joules/second)

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Machines Make Work Easier

• Remember that Work is what happens when a force
  is used to move (displace) an object.
• Machines do not reduce the amount of work done.
• Machines can be powered by different kinds of
  energy.

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Machines Can Change the Size of the Force

• The work you put in is equal to the work you get
  out.
• But you can change the size of the force to make
  work easier.
• By changing the distance you put that work in
  for, you can alter the amount of force required
  to do the same amount of work.
• When you reduce the amount of force, you increase
  the distance.

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Machines Can Change the Distance of the Force

• Since
• Work Force x Distance
• When Machines change the SIZE of a force, the
  DISTANCE must change inversely.
• If the amount of force is reduced, the distance
  will be increased.
• If the distance is reduced, the amount of force
  required will be greater.

What happens to the distance as the angle
increases? What happens to the force needed to
move an object to the top of the ramp?
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Machines Can Change the Direction of a Force

• Some machines change the direction of a force
• Push shovel down, dirt goes up
• Push down on car jack, car goes up
• Swing ax downward, wood splits apart.

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Input Output Forces

• Two forces involved when machines are used to do
  work
• Input force applied TO machine (Fin)
• Output force applied BY machine (Fout)
• Likewise, two distances are involved, input
  distance and output distance
• Work put INTO a machine is greater than the work
  DONE by the machine because of friction

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Mechanical Advantage

• The benefit of doing work with a machine means
  less force is needed to do work
• The number of times a machine multiples the input
  force is called the machines MECHANICAL
  ADVANTAGE
• Machines that allow less force over greater
  distance (ramp) have a MA of greater than 1
• Machines that allow more force over shorter
  distance (rake) have a MA of less than one
• Machines that change the direction of the force
  (crowbar), but not the amount, have a MA of one

The formula to find a machines Mechanical
Advantage is Output Force / Input Force Or
Input distance/output distance
Ideal Situation No Friction!
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Calculating Mechanical Advantage

• What is the mechanical advantage of a hammer if
  the input force is 125 N and the output force is
  2,000 N?
• If you apply 100 N of force to a crowbar to lift
  a 250 N rock, what is the MA?
• What force is needed to lift a 2,000 N weight
  using a machine with a mechanical advantage of
  15?
• What is the MA of a ramp that is 6.0 m long and
  1.5 m high?

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2.5
133 N
4
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Efficiency

• For real machines, some of the energy put into a
  machine is always less than the work put out due
  to friction.
• Efficiency is a measure of how much of the work
  put into the machine is changed into useful
  output work.
• When mechanical energy is lost, efficiency is
  reduced.
• To calculate Efficiency
• Efficiency ()
• Output (j) / Input (j) X 100

What is the efficiency of a machine that does 800
J of work if the input work is 2400 J?
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Efficiency

• A sailor uses a rope and an old, squeaky pulley
  to raise a sail that weighs 140 N. He finds that
  he must do 180 J of work on the rope in order to
  raise the sail by 1 m (doing 140 J of work on the
  sail). What is the efficiency of the pulley?
• It takes 1200 J of work to lift a car high enough
  to change a tire. How much work must be done by
  the person perating the jack if its 25
  efficient?

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Six Types of Simple Machines

• A Simple Machine is one which does work with
  only one movement of the machine.
• All other machines are either modifications or
  combinations of these.
• These are divided into two families
• Levers Inclined Planes

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Levers

• A lever is a bar that is free to turn around a
  fixed point.
• The fixed point is called a FULCRUM
• The input arm is the distance of the EFFORT arm
• The output arm is the distance of the RESISTANCE
  arm
• There are three classes of levers, depending on
  the position of each of these.

MA Length of Input Arm Length of Output Arm
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Wheel Axle

• A wheel and axle is a lever that rotates in a
  circle around a center point (axle) which is the
  fulcrum.
• A wheel is a lever that can turn 360 degrees and
  can have an effort or resistance applied anywhere
  on that surface. The mechanical advantage of the
  wheel axle is the ratio of the radius of the
  wheel to the ratio of the radius of the axis.
• Wheels can also have a solid shaft with the
  center core as the axle such as a screwdriver or
  drill bit or the log in a log rolling contest.

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Pulleys

• A pulley is a wheel with a grooved rim and a rope
  or cable that rides in the groove.
• The mechanical advantage of a pulley system is
  approximately equal to the amount of supporting
  ropes or strands. A single, fixed pulley has MA
  1.
• Using more than one pulley can also increase the
  mechanical advantage.
• Multiple pulleys can be combined into a single
  unit called a block tackle.

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Inclined Planes

• An inclined plane is a slanted surface used to
  raise an object.
• When an object is moved up an inclined plane,
  less effort is needed than if you were to lift it
  straight up, but, you must move the object over a
  greater distance.
• They make work easier because they support part
  of the weight of the object while it is being
  moved.

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Wedge

• A wedge is an inclined plane which moves. Most
  wedges (but not all) are combinations of two
  inclined planes.
• The mechanical advantage is determined by
  dividing the length of the slope by the thickness
  of the widest end.
• Generally it can be anything that splits, cuts,
  or divides another object including air and
  water.
• The angle of the cutting edge determines how easy
  it can cut through an object.

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Screw

• A screw, like a wedge, is another form of an
  inclined plane. A screw is an inclined plane
  wrapped around a cylinder to form a spiral.
• The advantage of using a screw is the large
  amount of friction that keeps it from turning and
  becoming loose.

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Compound Machines

• Compound machines are two or more simple machines
  working together. A wheelbarrow is an example of
  a complex machine that uses a lever and a wheel
  and axle.

A Rube-Goldberg device is an invention that uses
simple machines to make complex devices that
perform simple tasks in indirect, convoluted
ways.
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Check for Understanding

• Using a single fixed pulley, how heavy a load
  could you lift?
• Give an example of a machine in which friction is
  both an advantage and a disadvantage.
• Why is it not possible to have a machine with
  100 efficiency?
• What is effort force? What is work input? Explain
  the relationship between effort force, effort
  distance, and work input.

Since a fixed pulley has a mechanical advantage
of one, it will only change the direction of the
force.
One answer might be the use of a car jack.
Advantage of friction It allows a car to be
raised to a desired height without slipping.
Disadvantage of friction It reduces efficiency.
Friction lowers the efficiency of a machine. Work
output is always less than work input, so an
actual machine cannot be 100 efficient.
The effort force is the force applied to a
machine. Work input is the work done on a
machine. The work input of a machine is equal to
the effort force times the distance over which
the effort force is exerted.
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Online Resources

• http//www.cosi.org/files/Flash/simpMach/sm1.swf
• http//juniorengineering.usu.edu/workshops/machine
  s/machines.php
• http//42explore.com/smplmac.htm
• http//www.tooter4kids.com/Simple_Machines/
• http//teacher.scholastic.com/dirtrep/simple/index
  .htm
• http//www.mos.org/sln/Leonardo/LeosMysteriousMach
  inery.html
• http//www.harcourtschool.com/activity/machines/si
  mple_machines.htm
• http//www.mos.org/sln/Leonardo/GadgetAnatomy.html
  

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Activities Assignments

• Read Chapter 8
• Mechanical Advantage Worksheets
• Lab Activities Quick Labs Gizmos
• Simple Machine Flipbook
• Build a Rube Goldberg Machine