1. Work & Machines

2. Topics Work and Power –Definition, Calculation, and Measurement Using Machines –Nature of Machines –Mechanical Advantage –Efficiency of Machines Simple Machines

3. Work Definition: Work is the application of a force to an object that results in the movement of the object over a certain distance. –For work to occur, object must move –The motion of the object must be in the same direction as the applied force on the object Work and energy are related. Energy is the ability to do work. Motion results when work is being done.

4. ExampleIs Work Being Done? The teacher pushes on the wall until she is tired. No. The wall did not move. A book falls off the table and hits the floor. Yes. Gravity applied a force and moved the book in the direction of the floor. The waiter carries a tray of food No. The force to hold the tray is not applied in the direction of the motion. A rocket acclerates through space Yes. The force of the rocket thrust is causing the rocket to move.

5. Calculating Work Work equals the product of a force (F) and the distance (d) over which the force is applied. Work (W) = Force (F) x Distance (d) SI Unit: Joules = kg x m 2 /s 2

6. Example A mover pushes a box for 5 meters across the floor. He applies a force of 100N directly to the floor. How much work does he do?

7. Power Definition: Amount of work done in a certain amount of time; or Rate at which work is done. Calculating Power: Measured in watts (w) Power = Work (Energy) time

9. Using Machines What is Machine Mechanical Advantage Efficiency of Machine

10. Using Machines Definition of Machine – Device that makes work easier to do. Machines increase applied force (input force) and/or change direction of applied force to make work easier. Because W = Fd, therefore, increasing distance reduces the amount of force needed to do the work.

11. Mechanical Advantage Mechanical Advantage (MA) is the number of times a machine multiplies the input FORCE (NOT WORK). Input Force (F I ) – force applied to machine, aka, Effort Force Output Force (F O ) – force applied by machine to do the task or overcome resistance, aka, Resistance Force When a machine takes a small input force and increases the magnitude (strength) of output force, a mechanical advantage is produced.

13. Calculating MA Example, if a machine increases an input force of 10N to an output force of 100N, the machine has a mechanical advantage of ( )??

14. Efficiency in Machines In machines, when talking about efficiency, it is WORK (NOT FORCE) There are two types of WORK Work Input (W in ) = Work done by you on a machine. Work Output (W out ) = Work done by the machine

15. Ideal Machine W in = W out But according to Conservation of Energy, no energy is created or lost. Therefore machines cannot create energy. W out is never greater than W in. Some energy is transferred in the form of heat due to friction. Lubricant can make machines more efficient by reducing friction

16. Efficiency Because the amount of work put into a machine is always greater than the amount of work done by the machine, machines are never 100 percent efficient. Definition: Measure of how much of the work put into (W in ) a machine is changed into useful output (W out ) by the machines

17. Calculating Efficiency Efficiency is a comparison between the work output and the work input of a machine. Efficiency Work = X 100% Work Output Work Input

19. Simple Machines Definition: A machine that does work with only one movement. To make work easier, simple machines change forces by: –Change the direction –Change the strength (magnitude) –Change both

20. Types of Simple Machines Lever Inclined Plane –Wedge –Screw Wheel and Axle –Gear –Pulley

21. Lever Definition: A bar that is free to pivot about a fixed point called fulcrum. The bar may be straight or curved. Three Parts: –Effort arm (Force) – part of lever where effort force is applied –Resistance arm (Load) – part of lever where resistance force is applied –Fulcrum – Support or balance

22. Three Types of Lever Depending on the location of the resistance arm (resistance force) and the effort arm (effort force) relative to the fulcrum, there are three types of lever:

23. First-Class Lever Definition: Fulcrum is located between the effort and resistance forces; effort being further than resistance; multiples and changes direction of force.

24. Examples Crowbars, scissors, pliers, seesaws.

25. Second-class Lever Resistance force is located between the effort force and fulcrum; always multiples force. No change in direction of force. Greater MA is resulted when fulcrum is closer to the load

26. Examples Nutcrackers, wheel barrows, doors, and bottle openers

27. Third-class Lever Effort force is between resistance force and fulcrum; does NOT multiply force, but does increase distance over which force is applied. Always produce a gain in speed and distance and a corresponding decrease in force.

28. Examples Tweezers, hammers, brooms, shovel, your arm

29. Inclined Plane Definition: A sloping surface that reduces the amount of force required to do work. It is easier to move a weight from a lower to higher elevation

30. MA of inclined plane is equal to the length of the slope divided by the height of the slope. Less force is required if a ramp is longer and less steep

31. 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

32. Wedge Definition: Inclined plane with one or two sloping sides. If two sides, the planes are joined back to back. Wedges are used to split things. Examples: Axe, knife, your teeth, saw, doorstop

33. Screw Definitions: inclined plane wrapped in a spiral around a cylindrical post MA of an screw can be calculated by dividing the number of turns per inch.

34. Screw Three parts: head, shaft, and tip. Head: part where you exert force Shaft: has ridges called threads that wind around the screw. Tip is usually sharp. Uses: fasten things together. Examples: jar lid, screws, drill bids

35. Wheels and Axle Definition: machine with two wheels of different size rotating together. Consists of a large wheel rigidly secured to a smaller wheel or shaft called axle. Example: doorknob,

36. MA of Wheel and Axle The mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to the radius of the axle 5 1

37. Gear Modified form of wheel and axle. Each gear in a series reverses the direction of rotation of the previous gear. The smaller the gear will always turn faster than the larger. Example: Can opener, bicycle, egg beater

38. Pulley Definition: Grooved wheel with a rope, simple chain, belt or cable running along the groove is a pulley.

40. Types of Pulley Fixed pulley is attached to something that doesn’t move. Force is not multiplied, but direction is changed. Movable pulley has one end of the rope fixed and the wheel free to move; multiplies force. Block and tackle (compound) system of pulleys of fixed and movable pulleys.

41. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts