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Work & Simple Machines

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Work = force x distance

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Work & Simple Machines

  1. 1. Work &Work & SimpleSimple MachinesMachines
  2. 2. Work  Work: the transfer of energy that occurs when a force moves an object.  a force must act on the object the entire distance  the object must move in the same direction that the force is applied  Pulling up on the bar while the weight is on the ground (no movement) is NOT work.  Holding the barbell overhead is NOT work.  Lifting the weight up IS work.
  3. 3. NOT WorkNOT Work (Examples)(Examples)
  4. 4. Examples of Work
  5. 5.  If 10 N of force is applied to this cart, moving it 20 m, how much work is done?  W = F x d  W = 10 N  20 m  W = 200 Nm  1 Joule = 1 N force that moves an object 1 m  W = 200 J
  6. 6. Mechanical Energy  Mechanical energy =  The ability to do work or  The amount of work done  Has same units as work (Joules)
  7. 7. Intro to Machines  A machine is a device that makes work easier.
  8. 8. Work & Machines  A machine makes work easier by changing the direction or size of the force needed to move an object. (They cannot save or reduce work) W = Force x distance
  9. 9. Simple Machines  All machines are made up of one or more of these 6 basic tools  Each one reduces the amount of force needed to accomplish work.
  10. 10. Mechanical Advantage M.A. = resistance force effort force  Multiplied force:  A machine allows you to lift an 8N block with 4N of force  Mechanical Advantage = 8N/4N = 2  M.A. of 2 means your effort force is doubled  Change of direction:  A machine lets you lift a 10 N block with 10 N of force  Mechanical Advantage = 10N/10N = 1  M.A. of 1 means the direction of your effort force is changed
  11. 11. Experiment 31 Measuring Work Question/Observation How is work done? Measured? Work is force applied through a distance. The direction of force and motion must be parallel for work to be done.  W = F x d = Energy How do simple machines make work easier? Less force over longer distance, or direction of force is changed (amount of work stays the same) Hypothesis How will using a lever, pulley, or inclined plane affect the work of lifting a bag of pennies? Experiment •Fill a bag with pennies (make sure it is within the measurable range of your spring scale). •Measure the height and length (distance) of your “ramp” and record it in the data table. •Slowly & steadily pull the bag of pennies to the top of the ramp with a spring scale. •Watch the spring scale and note the force required to move the pennies up the ramp (in newtons). •With a spring scale, lift the pennies straight up to the same height as the top of the ramp; record the force. •Repeat steps 2-5, using other simple machines to lift the same weight the same distance.
  12. 12. Inclined Plane  Inclined plane: any flat, slanted surface  A ramp is the most common type of inclined plane.  higher at one end than the other  the longer the ramp, the less force is needed to do the work  Inclined planes help by pushing up against gravity's pull on the load (normal force).  The load (resistance) moves in the same direction as the effort force.
  13. 13. Inclined Plane Examples
  14. 14. Wedge  Wedge: two inclined planes put together to form a V-shape  used to lift or pry apart heavy objects  can also stop an object from moving  A wedge works as the two inclined planes push at perpendicular angles on the load.  The load moves in a different direction than the effort force. In this picture, as the force moves the ax down, the load (wood) breaks apart and falls to the sides.
  15. 15. Wedge Examples
  16. 16. Screw  Screw: an inclined plane wrapped around a center post  longer inclined planes (more/closer threads) require less force to move the load  has two parts: the inclined plane and the center post  As the force rotates the screw, it goes down into the wood.  The force and the load move in the same direction.  Threads of screw increase surface area friction which allows screws to objects together better than nails  Wedge-shaped post allows screw to go in wood easier
  17. 17. Screw Examples
  18. 18. Lever  Lever: a straight bar that moves on a fixed point called a fulcrum.  the longer the distance between effort force and the fulcrum, the less force will be needed to move the load  has two parts: the bar and the fulcrum  A lever works to change the direction of effort force and/or the distance the force acts throughout.  Load & force move in opposite directions (1st class lever)  use gravity's pull on your own weight as the downward force  Moving the fulcrum changes the distance needed to push down  pushing the lever two feet down results in moving the rock up 1 foot
  19. 19. Three Classes of Levers
  20. 20. Levers  First Class Lever:  Fulcrum in middle, load & force on opposite ends  Load & force move in opposite directions.  Second Class Lever  Load is in middle, with effort on end.  Load & force move in same direction.  Third Class Lever  Bar is attached to fulcrum on one end.  Load is on end, with effort in middle.  Load & force move in same direction.
  21. 21. Lever Examples
  22. 22. Wheel & Axle  Wheel and axle: a circular disc locked to a center post.  The larger the wheel, the less force needed to move the load.  two parts: the wheel and the center post (axle)  When the wheel turns, it forces the axle to turn; or if the axle is turned, the wheel also turns.  One full turn of the large wheel results in one full turn of the smaller one.  The force moves in the same direction as the load.
  23. 23. Wheel & Axle Examples
  24. 24. Pulley  Pulley: a grooved wheel with a rope around it.  The larger the wheel, the less force will be needed to move the load.  two parts: the wheel and the rope  Pulleys can change the direction and/or amount of force needed to move the load.  Load & force move in opposite or same directions, depending on how pulley is attached. As the rope is pulled down, the flag goes up.
  25. 25. Pulley Systems  When used in combination, pulleys increase the mechanical advantage.  To calculate the M.A., count the # of pulley ropes pulling on the load
  26. 26. Pulley Examples
  27. 27. Compound Machines  Some tools have two or more simple machines working together.  scissors  2 levers joined at fulcrum  blades are wedges  bicycle  two wheel and axel machines  screws (hold frame together)  levers (pedals and hand brakes)  wheel barrow:  two levers  wheel & axle

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