Chapter 14 Work, Power, and Machines

1. Chapter 14Work, Power, and Machines Physical Science

2. Resources CH 14 PPT slides w/ practice problems & assessment Quizlet VOCAB Flashcards

3. Work and Power 14.1 • Work – done when a force acts on an object in the direction the object moves • Requires Motion • Man is not actually doing work when holding barbell above his head • Force is applied to barbell • If no movement, no work done They do no work He does work

4. Work and Power 14.1 Work Depends on Direction • All force acts in same direction of motion = all force work. • Part applied force acts in the direction of motion =part force does work. • none of force in direction of the motion = force does no work.

5. Work and Power 14.1 Work Depends on Direction Continued… • All of the force does work on the suitcase. • The horizontal part of the force does work. • The force does no work on the suitcase. This force does work This force does no work Force Force Direction of motion Direction of motion Direction of motion Force and motion in the same direction Part of force in direction of motion Lifting force not in direction of motion

6. Calculating Work 14.1 • Work = Force x Distance • W = Fd • Force = mass x acceleration  F = ma or F = mg • Joule (J) = SI unit for work • Unit: J = N(m) • Named after James Prescott Joule (1818 – 1889) • Research work and heat

7. What is Power? 14.1 • Rate of doing work • More power = work at a faster rate • Size of engine often indicates power • Can work at a faster rate • Power = Work/Time • P= W/t • Watt (W) = SI unit for Power • Units: W = J/s

8. What is Power? 14.1 Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does.

9. Math Practice – Page 415 # 1 - 3

10. James Watt and Horsepower 14.1 • Horsepower (hp) = another unit for power • Equals ~746 watts • Defined by James Watt (1736- 1819) • Trying to describe power outputs of steam engines • Horses were most common used source of power in 1700s

11. James Watt and Horsepower The horse-drawn plow and the gasoline-powered engine are both capable of doing work at a rate of four horsepower.

12. Assessment Questions • In which of the following cases is work being done on an object? • pushing against a locked door • suspending a heavy weight with a strong chain • pulling a trailer up a hill • carrying a box down a corridor • 2. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer? • 2,500 J • 4,000 J • 20,000 J • 160,000 J • A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? • 5000 W • 6000 W • 50 W • 1000 W

13. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer?

14. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer? • 2,500 J • 4,000 J • 20,000 J • 160,000 J

16. Work and Machines 14.2

17. Machines Do Work 14.2 • Machine – device that change force • Car jack • You apply force  jack changes force  applies much stronger force to lift car • Jack increase force you exerted • Make work easier • Change size of force needed, direction of force, and distance over which force acts

18. Machines Do Work 14.2 • Increasing Force • Small force exerted over a large distance = large force over short distance • Like picking books up one at a time to move them --- trade off = more distance but less force

19. Machines Do Work 14.2 • Increasing Distance • Decreases distance for force exerted and increases amount of force required • Tradeoff = increased distance = greater force exerted • Changing Direction Boat moves in this direction. Input force Input distance Output force Output distance

20. Work Input and Work Output 14.2 • Work input to a Machine • Input Force – Force you exert on a machine • Oar = force exerted on handle • Input Distance – Distance the input force act thru • How far handle moves • Work Input – work done by the input force • F x d • Work Output of a Machine • Output Force- force exerted by machine • Output Distance – distance moved • Work output – F x d • Less than input work b/c of friction • All machines use some input work to overcome

22. Work and Simple Machines

23. What is work? • In science, the word work has a different meaning than you may be familiar with. • The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

24. Work or Not? • According to the scientific definition, what is work and what is not? • a teacher lecturing to her class • a mouse pushing a piece of cheese with its nose across the floor

25. Work or Not? • According to the scientific definition, what is work and what is not? • a teacher lecturing to her class • a mouse pushing a piece of cheese with its nose across the floor

27. SS -- What’s work? *For # 1-5 explain why each is work or not work • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?

28. What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 km grocery bag to her car? No

29. Formula for work Work = Force x Distance • The unit of force is newtons • The unit of distance is meters • The unit of work is newton-meters • One newton-meter is equal to one joule • So, the unit of work is a joule

30. W=FD Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

31. W=FD Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules (W = 20N x 10m)

32. Power • Power is the rate at which work is done. • Power = Work*/Time *(force x distance) • The unit of power is the watt.

33. Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your answers.

34. Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time. Power and time are inversely proportional.

35. 2. 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 Work=Force x Distance Power = Work/Time

36. 2. 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

37. History of Work Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

38. Simple Machines Ancient people invented simple machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

39. Simple Machines • The six simple machines are: • Lever • Wheel and Axle • Pulley • Inclined Plane • Wedge • Screw

40. Simple Machines • A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: • transferring a force from one place to another, • changing the direction of a force, • increasing the magnitude of a force, or • increasing the distance or speed of a force.

41. Mechanical Advantage • It is useful to think about a machine in terms of the input force (the force you apply) and the outputforce (force which is applied to the task). • When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

42. Mechanical Advantage • Mechanical advantage is the ratio of output force divided by input force. If the output force is bigger than the input force, a machine has a mechanical advantage greater than one. • If a machine increases an input force of 10 pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10. • In machines that increase distance instead of force, the MA is the ratio of the output distance and input distance. • MA = output/input

43. No machine can increase both the magnitude and the distance of a force at the same time.

44. The Lever • A lever is a rigid bar that rotates around a fixed point called the fulcrum. • The bar may be either straight or curved. • In use, a lever has both an effort (or applied) force and a load (resistant force).

45. The 3 Classes of Levers • The class of a lever is determined by the location of the effort force and the load relative to the fulcrum.

47. To find the MA of a lever, divide the output force by the input force, or divide the length of the resistance arm by the length of the effort arm.

48. First Class Lever • In a first-class lever the fulcrum is located at some point between the effort and resistance forces. • Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws. • A first-class lever always changes the direction of force (I.e. a downward effort force on the lever results in an upward movement of the resistance force).

49. Fulcrum is between EF (effort) and RF (load)Effort moves farther than Resistance. Multiplies EF and changes its direction

50. Second Class Lever • With a second-class lever, the load is located between the fulcrum and the effort force. • Common examples of second-class levers include nut crackers, wheel barrows, doors, and bottle openers. • A second-class lever does not change the direction of force. When the fulcrum is located closer to the load than to the effort force, an increase in force (mechanical advantage) results.