Understanding Work and Power in Physics

1. Chapter 4.1- Work & Power

2. What is Work? • For work to occur two things must happen: • Work is done when an object moves in the same direction in which the force is exerted. • The object has to move some distance as the result of your force. • The object has to move in the same direction as the applied force – only the force acting IN THE SAME DIRECTION as the object’s motion is performing work.

3. - What Is Work? The Meaning of Work • Work is done on an object when the object moves in the same direction in which the force is exerted. QUESTION: IF YOU PULL AN OBJECT HORIZONTALLY, WHAT PART OF YOUR FORCE DOES WORK? __All of your forces contributes to work

4. CalculatingWork • The amount of work done depends on BOTH the AMOUNT OF FORCE exerted and the DISTANCE THE OBJECT MOVES. • The amount of work can be determined by multiplying FORCE times DISTANCE: Formula: W = (F)(d) W = work F = Force d = Distance

5. Calculating Work • SI Unit of Work = • When force = NEWTONS (N) • When distance = METERS • Joule (N*m) • Named after James Prescott Joule a physicist from the mid 1800’s who studied work • 1 Joule =1 Newton of force to move an object 1 meter • Joule is abbreviated with a J

6. Practice: 1.How much work is performed if you move a 50 N object .5m? • What if the object was 100 N?

7. Work is UNAFFECTED BY TIME Power is the rate (time) at which work is done. how fast or slow work is done Power equals the amount of work done on an object in a unit of time. With more power you can do the same amount of work in less time. Power

8. Calculating Power • Formula to calculate power: Power = Work Time OR Power = force x distance Time Practice!!!!!!!!!

9. When work is measured in joules, and time in seconds: • SI Units for power: Joule/second (J/sec) • 1 Watt = Joule/Sec. (J/s) James Watt & His Steam Engine

10. Units of Power • One watt is a small amount of power • Power is more often expressed in KILOWATT • Kilowatt = 1000 watts • Horsepower: English Unit (not an SI unit) • 1 HP = 746 Watts Corvette

11. Question Answer Quick Summary: Work is done when an object moves in the same direction in which the force is exerted. What is work? How can you calculate work? Work = Force X Distance Power is the rate at which work is done. P = work / time What is power?

12. Chapter 4.2 How Machines Do Work?

13. What Is a Machine? i. A device that allows you to DO WORK IN A WAY THAT IS EASIER.

14. Input and Output Work • The amount of input work done by the gardener equals the amount of output work done by the shovel.

15. Input & Output work • Input force = the FORCE you exerton the machine. • Output force = the FORCE the machine exerts ON THE OBJECT

16. Calculating Input & Output work • Input work • Input force x Input distance • Output work • Output force x Output distance • When you use a machine, the amount of outputwork can never be greater than the amount of inputwork.

17. What Is a Machine? • A machine makes work easier by changing at least one of three factors. • A machine may change the amount of force you exert • the distance over which you exert your force • or the direction in which you exertyour force.

18. QUESTION: How does the cable system on a weight machine make raising the weights easier? The cable system enables you to raise the weights more conveniently by changing the direction to make work easier.

19. Mechanical Advantage • If you compare the input force to the output force, you can find the advantage of using a machine. • A machine’s mechanical advantage is the number of times a machine increases a forceexerted on it.

20. Calculating Mechanical advantage • Mechanical advantage =Output force input force Examples: A machine that increases force: output force is greater than input force. ex. a can opener

21. For a machine that increases distance the output force is less than the input force. • Ex. A wooden spoon. • For a machine that changes direction the input force will equal the output force • Ex. A weight pulley machine • The mechanical advantage of these types of machines will always be 1.

22. 400 N Interpreting Data: If an 80-N input force is exerted on Ramp 2, what is the output force? Mechanical Advantage

23. Efficiency of Machines • So far you learned that input work is equal to output work. This does not happen is real life. • WHY??? FRICTION In every machine, some work is wasted overcoming friction. Force must be applied to overcome the friction of the machine.

24. Efficiency of Machines Input Work is ALWAYS GREATER than Output Work • Force must be applied to OVERCOME THE FRICTION of the machine itself Expressed as a percentage • To calculate the efficiency of a machine, DIVIDE the OUTPUT WORK by the INPUT WORK and multiply the result by 100 percent. • Efficiency = Output Work / Input Work x 100

25. You do 250,000 J of workto cut a lawn with a hand mower. If the work done by the mower is 200,000 J,what is the efficiency of the lawn mower? Read and Understand What information have you been given? Input Work (Winput) = 250,000 J Output Work (Woutput) = 200,000 J Calculating Efficiency

26. You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Plan and Solve What quantity are you trying to calculate? The efficiency of the lawn mower = _?_ What formula contains the given quantities and the unknown quantity? Efficiency = Output work/Input work X 100% Perform the calculation. Efficiency = 200,000 J/250,000 J X 100% Efficiency = 0.8 X 100% = 80% The efficiency of the lawn mower is 80 percent. Calculating Efficiency

27. Real & Ideal Machines 1.Real machines • Efficiency is alwaysless than 100% 2.Ideal machines • Efficiency is always 100% ( these machines do not exist)

28. QUESTION: Why is output work always less than input work in real situations? Because friction exists in every machine and reduces the machines efficiency.

29. Inclined Plane Wedge Screw Wheel and Axle Lever Pulley Six Simple MachinesChapter 4 section 3

30. Inclined Plane • Also known as a ramp – a flat sloped surface. • How it works: • Allows you to exert your force over a longer distance. • Input force = force you use to push or pull the object • Output force = the force you would need to lift the object without the inclined plane • Output force is equal tothe objects weight

31. Inclined plane - Mechanical Advantage • IMA = length of incline Height of incline Example: if you are loading a truck that is 1 meter high using a ramp that is 3 meters long, the IMA = 3m / 1m = 3 Therefore: the inclined plane increases the force you exert on the object by three times. By increasing the length of the incline, the less input force needed to push or pull the object.

32. Wedge • A device that is thick at one end and tapers to thin edge at the other end. Input force wedge Output force Output force

33. Wedge – How it works • Instead of moving an object along an inclined plane, you move the inclined plane itself.

34. Screws • An inclined plane wrapped around a cylinder • The threads act like an inclined plane to increase the distance over which you exert an input force. • The threads exert an output force on the wood, pulling the screw into the wood. • The friction between the wood and the screw holds the screw in place. • The closer the threads are the greater the mechanical advantage.

35. Levers • A rigid bar that is free to pivot, or rotate, on a fixed point. • The fixed point that a lever pivots around is called a fulcrum • 3 classes, based on location of fulcrum, effort force (E) and resistance force (load) (R)

36. First class lever • Fulcrum is in betweenthe effort force and the resistance • Ex. Scissors, crowbar, car jack, prying the lid using a screw driver

37. Second class lever • Fulcrum is at one end, effort is at the other end, the resistance is in between Ex. Wheelbarrow, a Door, a Nutcracker,

38. Third Class levers • Fulcrum is at one end , effort force is close to fulcrum, resistance is at other end • Almost all sports equipment ex. • Golf club, baseball bat, lacrosse stick, fishing rod, rake, hockey stick etc.

39. Mechanical advantage • Ratio of the output force to the input force • IMA = ideal mechanical advantage, does not include friction • IMA Formula= Effort arm distance (cms) Resistance arm distance (cms)

40. Actual Mechanical advantage • Measures ratio of resistance force to effort force, includes friction • Formula =Resistance Force(g) Effort Force(g)

41. Mechanical Advantage • When MA is less than one the machine multiplies speed • When the MA is greater than one the machine multiplies your effort

42. Wheel and Axle • a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis • The object with the larger radius is the wheel • Example: screw driver • It works because the wheel is larger than the axle, the axle rotates and exerts a large output force. • The greater the ratio between the radius of the wheel and the axle the greater the mechanical advantage.

43. Pulley • A simple machine made of a small grooved wheel and a rope or cable wrapped around it • Pulley’s decrease the amount of input force needed to lift the object and it can change the direction of the input force. • Example: raising a flag pole. • Two basic types: A. fixed pulley-pulley that is attached to a structure B. movable pulley- pulley attached to a structure that can move

44. The ideal mechanical advantage is equal to the number of sections of rope that support the object

45. Compound Machines • Compound machine- utilizes two or more simple machines • Apple peeler, bicycle