Work and Machines

1. Work and Machines

2. Work • Work occurs when a force causes an object to move in the direction of the force. • Your dad just came home from a night out bowling with his friends. He had lots of fun and did lots of work. • Applying a force to the bowling ball to make it move through a distance is WORK.

3. Working Hard or Hardly Working? • Applying a force doesn’t always result in work being done. Work is done on an object if two things occur: • The object moves as a force is applied. • The direction of the objects motion is the same as the direction of the force applied.

4. Calculating Work Work is a force applied through a distance. The greater the distance through which you exert a given force, the more work you do. The greater force you exert through a given distance, the more work you do. The amount of work (W) done in moving an object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied. W = F • d Remember, force is expressed in newtons, and the meter is the basic SI unit for length or distance. Therefore, the unit used to express work is the newton-meter, which is more simply called the joule (J).

5. Power – How Fast Work Is Done • Power is the rate at which work is done. • To calculate power (P), divide the amount of work done (W) by the time (t) it takes to do the work. P = The unit used to express power is joules per second (J/s), or the watt (W).

6. Questions • Your neighbor asks you to help push his stalled car. You push and push, but the car doesn’t move. You are exhausted and sweaty. Have you done any work? • What is work? • You lift a chair that weighs 50 N to a height of 0.5m and carry it 10m across the room. How much work did you do on the chair? • Explain the work/power difference between driving nails with a hammer and using a nail gun. • How would it feel if you were “power” and your enemy was “work?”

7. Kinetic Energy • Kinetic Energy The energy of an object that is due to the object’s motion is called kinetic energy. • Kinetic energy depends on speed and mass. Jet Roller coaster

9. Potential Energy • Potential Energyis the energy associated with an object because of the position, shape, or condition of the object.

10. Potential Energy In raising a mass m to a height h, the work done by the external force is: Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies.

11. Machines – Making Work Easier • A machine is a device that helps make work easier by changing the size or direction of a force. • Examples: • Using a jack and tire iron to change a flat tire • Using a screwdriver to pry the lid off a paint can • Using a wheelbarrow to haul a load of rocks • Give 3 other examples of machines that make work easier.

12. Work Work input is the work you do on a machine. You apply a force, called the input force, to the machine to move it through a distance. Work output is the work done by the machine. The machine applies a force, called the output force, through a distance. Work output can never be greater than work input.

13. Machines do NOT increase the amount of work done. Climbing a Hill: If you wanted to get to point C on the illustration below you have two options: start at A and go straight up, or start at B and walk up the slope. Which is easier? Well, actually, they both take the same amount of work. The only thing you have control over is if you put forth a lot of effort over a short distance (situation A-C) or a little effort for a long distance (situation B-C). Lets exam what work really is (at least in scientific terms).

14. Machines DO NOT save work How is it that the path from A-C takes the same amount of work as path B-C? In A-C we have to exert a lot of force to get up the vertical side, but we don't have to go very far. Let's suppose that the force required is 100 Newtons and the distance is 4 meters, the amount of work done is 400 Joules: W=F • dW=100 N • 4 m W= 400 J Getting from B-C does not require near as much force as A-C, but we do have to move a considerably longer distance. In fact, the actual force and distance are 25 Newtons and 16 meters. Calculating the work gives: W= 25 N • 16 m W= 400 J

15. The Force-Distance Trade-off When a machine changes the size of the force, the distance through which the force is exerted must also change. Force or distance can increase, but not together. When one increases, the other must decrease. The work output is never greater than the work input.

16. Types of Machines • Lever • First Class Lever • Second Class Lever • Third Class Lever • Inclined Plane • Wedge • Screw • Wheel and Axle • Pulley

17. Levers A lever is a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum. Levers are used to apply force to a load. The three classes of levers are based on the locations of the fulcrum, the load, and the input force.

18. With a first class lever, the fulcrum is between the input force and the load. • 1st class levers change the direction of the input force. • Depending on the location of the fulcrum, 1st class levers can be used to increase force or increase distance. • When the fulcrum is closer to the load than to the input force, the mechanical advantage is greater than 1. • When the fulcrum is exactly in the middle, the MA is exactly 1. • When the fulcrum is closer to the input force than to the load, the MA is less than 1.

19. With a second class lever, the load is between the fulcrum and the input force. • 2nd class levers allow you to apply less force than the force exerted by the load. • Because the output force is greater than the input force, you must exert the input force over a greater distance. • A 2nd class lever has a mechanical advantage of greater than 1.

20. With a third class lever, the input force is between the fulcrum and the load. • 3rd class levers do not change the direction of the input force. • 3rd class levers do not increase the input force. • The output force is always less than the input force. • They increase the distance through which the output force is exerted. • A 3rd class lever has a mechanical advantage of less than 1.

21. Inclined Planes • An inclined plane is a simple machine that is a straight slanted surface – a ramp. • The mechanical advantage of an inclined plane can be calculated by dividing the length of the inclined plane by the height to which the load is lifted. • An inclined plane allows you to apply a smaller force over a greater distance.

22. Wedges • A wedge is a double-inclined plane that moves. • A wedge applies an output force that is greater than your input force. • The greater the distance you move the wedge, the greater the force it applies on the object. • The mechanical advantage of a wedge is determined by dividing the length of the wedge by its greatest thickness.

23. Screws • A screw is an inclined plane that is wrapped in a spiral. • When rotated, a small force is applied over the long distance along the inclined plane of the screw. • The screw applies a large force through the short distance it is pushed. • Screws are most commonly used as fasteners. • The longer the spiral on a screw is and the closer together the threads, the greater the screw’s mechanical advantage. Archimedes’ Hydraulic Screw

24. Wheel and Axle • The wheel and axle is a simple machine consisting of two circular objects of different sizes. • The axle is the smaller of the two circular objects. • As the wheel turns, so does the axle. Because the axle is smaller than the wheel, it rotates through a smaller distance, which makes the output force larger than the input force. • The mechanical advantage of the wheel and axle can be determined by dividing the radius of the wheel by the radius of the axle.

25. Pulleys • A pulley is a simple machine consisting of a grooved wheel that holds a rope or cable. • A load is attached to one end of the rope, and an input force is applied to the other end. • There are two kinds of pulleys: fixed and movable. Movable Fixed

26. Fixed Pulleys • This is a fixed pulley. It doesn't move when the rope is pulled. It is fixed to the upper bar. • You can pull down on the rope in order to lift the load up. • A fixed pulley only spins. Therefore, there’s a mechanical advantage of 1.

27. Movable Pulleys • This is a movable pulley. As the rope is pulled up, it can also move up. The weight is attached to this moveable pulley. Each side of the rope is supporting the weight, so each side carries only half the weight. • The force needed to hold up the pulley in this example is 1/2 the weight! • The mechanical advantage of this system is 2. It is the weight (output force) divided by 1/2 the weight (input force).

28. More on Pulleys… • When a fixed pulley and a movable pulley are used together, the pulley system is called a block and tackle. You can see that the weight is now suspended by two pulleys rather than one. That means the weight is split equally between the two pulleys, so each one holds only half the weight, or 50 pounds (22.7 kilograms). That means that if you want to hold the weight suspended in the air, you only have to apply 50 pounds of force (the ceiling exerts the other 50 pounds of force on the other end of the rope).

29. Compound Machines Compound machines are made of two or more simple machines. Can you identify simple machines within a compound machine?

30. Answers…

31. Mechanical Efficiency of Compound Machines In general, the more moving parts a machine has, the lower its mechanical efficiency. Most compound machines have low mechanical efficiency. An automobile is an example of a compound machine that involves MANY simple machines. Too much friction could cause heating and damage to the simple machines involved. It’s important that friction be reduced through use of lubrication.

33. Questions • You are one simple machine and your best friend is another simple machine. What are each of you and how do you work together? • The radius of the wheel of a wheel and axle is 4 times greater than the radius of the axle. What is the mechanical advantage of this machine? • A winding road is actually a series of inclined planes. Describe how a winding road makes it easier for vehicles to travel uphill. • What compound machine would you choose to be in order to assist hurricane victims? Explain. • Why do you think you would not want to reduce the friction involved in using a winding road?

34. Who is Rube Goldberg? • Reuben Lucius Goldberg • 1883-1970 • Born in San Francisco • Graduated with engineering degree from University of California Berkeley • Worked as an engineer for City of San Francisco Water and Sewer Department • Convinced his father he wanted to work as an artist • Got a job as an office boy in sports department of a San Francisco newspaper • Kept submitting cartoons until he was published • Moved to New York to draw daily cartoons for Evening Mail • Founding member of National Cartoonist Society • Pulitzer Prize winner • National figure, often-quoted television and radio personality • 60-year career Now, time for a little comic relief…

35. Invention for Opening the Garage Door Without Getting Out of the Car

36. How to Keep Shop Windows Clean Passing man (A) slips on banana peel (B) causing him to fall on rake (C). As handle of rake rises it throws horseshoe (D) onto rope (E) which sags, thereby tilting sprinkling can (F). Water (G) saturates mop (H). Pickle terrier (I) thinks it is raining, gets up to run into house and upsets sign (J) throwing it against non-tipping cigar ash receiver (K) which causes it to swing back and forth and swish the mop against window pane, wiping it clean. If man breaks his neck by fall move away before cop arrives. Rube Goldberg - extremely complicated inventions used to perform an ordinary, simple tasks

37. More Rube Goldberg…

38. Can you explain this one?

39. What’s happening here?

40. So, what? • Rube Goldberg machines are examples of complex machines. • All complex machines are made up of combinations of simple machines. • Rube Goldberg machines are usually a complicated combination of simple machines. • By studying the components of Rube Goldberg machines, we learn more about simple machines. • The cartoons symbolize “man’s capacity for exerting maximum effort to accomplish minimal results.” • The cartoons depict convoluted machines functioning in complex ways to perform simple tasks. • “Rube Goldberg” has become synonymous with any complex system achieving a basic task. Next week you’ll create your own “Rube Goldberg” machine!

41. Questions • How are machines an important part of our lives? • How might “machines” be important to animal survival? • How would your life change if inclined planes no longer existed? • Write a narrative poem about machines, work, power, and force.

42. Websites http://staff.harrisonburg.k12.va.us/~mwampole/1-resources/simple-machines/index.html http://www.beaconlearningcenter.com/Weblessons/SimpleMachines/machines002.htm http://www.generalpatton.org/education/lesson_plans/PM_lessonPlan_sm.pdf http://juniorengineering.usu.edu/workshops/machines/machines.php http://discover.edventures.com/functions/termlib.php?action=&termid=67&alpha=t&searchString= http://infao5501.ag5.mpi-sb.mpg.de:8080/topx/archive?link=Wikipedia-Lip6-2/38857.xml&style http://en.wikipedia.org/wiki/Hammer http://www.usoe.k12.ut.us/curr/science/sciber00/8th/machines/sciber/lever3.htm http://www.enchantedlearning.com/physics/machines/Levers.shtml http://www.usoe.k12.ut.us/curr/science/sciber00/8th/machines/sciber/machine2.htm http://www.americanwheelchairs.com/ramps.shtml http://www.rollaramp.co.uk/accessories.htm http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=doorstop%20picture http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=plow%20picture http://search.msn.com/images/results.aspx?q=axe+head&mkt=en-us http://en.wikipedia.org/wiki/Screw/Bolt http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=doorknob%20picture http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=%22ferris%20wheel%22%20picture http://search.msn.com/images/results.aspx?q=%22steering+wheel%22&mkt=en-us http://www.howstuffworks.com/pulley.htm

43. Bibliography Silver, H. F., Hanson, J. R., Strong, R. W., Schwartz, P. B. Teaching Styles and Strategies: Interventions to Enrich Instructional Decision Making. Trenton: Thoughtful Education Press, 1996. Holt Science & Technology: Physical Science. Austin: Holt, Rinehart and Winston, 2001.