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Using Machines

Using Machines. S8P3 c: Students will demonstrate the effect of simple machines (lever, inclined plane, pulley, wedge, screw, and wheel and axle) on work. Key Ideas. Machines make work easier. Machines can change the size, distance or direction of a force.

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Using Machines

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

  2. 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)

  4. 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.

  5. 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.

  6. 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?

  7. 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.

  8. 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

  9. 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 machine’s 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 machine’s Mechanical Advantage is: Output Force / Input Force Or Input distance/output distance Ideal Situation: No Friction!

  10. 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? 16 2.5 133 N 4

  11. 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? 33%

  12. 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 it’s 25% efficient?

  13. 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

  14. 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

  15. 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.

  16. 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.”

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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.

  22. Online Resources: http://www.cosi.org/files/Flash/simpMach/sm1.swf http://juniorengineering.usu.edu/workshops/machines/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/LeosMysteriousMachinery.html http://www.harcourtschool.com/activity/machines/simple_machines.htm http://www.mos.org/sln/Leonardo/GadgetAnatomy.html

  23. Activities & Assignments • Read Chapter 8 • Mechanical Advantage Worksheets • Lab Activities: Quick Labs & Gizmos • Simple Machine Flipbook • Build a Rube Goldberg Machine

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