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Table of Contents

Table of Contents. What Is Work? How Machines Do Work Simple Machines. - 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. Work and Power.

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Table of Contents

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  1. Table of Contents • What Is Work? • How Machines Do Work • Simple Machines

  2. - 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.

  3. Work and Power • The amount of work performed on an object can be determined by multiplying force times distance. • Work = Force x Distance • *The SI unit of work is the Joule (J = Newton x meter) • Power is the rate at which work gets completed or the amount of work done on an object in a given amount of time. • Power = work / time • Or • Power = Force x Distance / Time • *The SI unit for power is measured in Joules/second or Watts (W) • 1 J/s = 1 W • A watt is a small unit of power so large amounts are measured in larger units called kilowatts (1,000 watts). 746 watts = 1horsepower (non SI unit)

  4. So why is measuring power useful?

  5. Horsepower looks much different today!

  6. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Read and Understand What information have you been given? Force of the tow truck (F) = 11,000 N Distance (d) = 5.0 m Time (t) = 25 s - What Is Work? Calculating Power

  7. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Plan and Solve What quantity are you trying to calculate? The Power (P) the tow truck uses to pull the car = __ What formula contains the given quantities and the unknown quantity? Power = (Force X Distance)/Time Perform the calculation. Power = (11,000 N X 5.0 m)/25 s Power = (55,000 N•m)/25 sor 55,000 J/25 s Power = 2,200 J/s = 2,200 W - What Is Work? Calculating Power

  8. A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Look Back and Check Does your answer make sense? The answer tells you that the tow truck used 2,200 W to pull the car. This value is about the same power that three horses would exert, so the answer is reasonable. - What Is Work? Calculating Power

  9. Practice Problem A motor exerts a force of 12,000 N to lift an elevator 8.0 m in 6.0 seconds. What is the power produced by the motor? 16,000 W or 16 kW - What Is Work? Calculating Power

  10. Practice Problem A crane lifts an 8,000-N beam 75 m to the top of a building in 30 seconds. What is the crane’s power? 20,000 W or 20 kW - What Is Work? Calculating Power

  11. Question Answer - What Is Work? Asking Questions • Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. 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 What is power? Power is the rate at which work is done.

  12. End of Section:What Is Work?

  13. - How Machines Do Work Input and Output Work • The amount of input work done by the gardener equals the amount of output work done by the shovel.

  14. Machines and Work • A machine is a device that allows you to do work in a way that is easier. • Machines can be simple or complex. • Input force is the force you exert on the machine. • Output force is the force the machine exerts on an object. • The input force x input distance = input work • The output force x output distance = output work • *The amount of output work can never be greater than the amount of input work when using a simple machine!

  15. - How Machines Do Work 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. • A machine may change the distance over which you exert your force. • A machine may change the direction in which you exert your force. • *In each case, the amount of work stays the same!

  16. Mechanical Advantage • A machines mechanical advantage is the number of times a machine increases a force exerted on it. • Mechanical advantage = Output force / Input force • When output force is greater than input force, the mechanical advantage of a machine is greater than 1 and this increasesthe force. • If the mechanical advantage is less than 1, then distance over which the force is exerted will increase. • If the mechanical advantage is equal to 1, then direction has changed but output and input force will remain the same.

  17. - How Machines Do Work Mechanical Advantage • The input force and output force for three different ramps are shown in the graph.

  18. Input force Reading Graphs: What variable is plotted on the horizontal axis? - How Machines Do Work Mechanical Advantage

  19. 400 N Interpreting Data: If an 80-N input force is exerted on Ramp 2, what is the output force? - How Machines Do Work Mechanical Advantage

  20. Ramp 1: 10; Ramp 2: 5; Ramp 3: 2 Interpreting Data: Find the slope of the line for each ramp. - How Machines Do Work Mechanical Advantage

  21. The slope of each ramp’s graph equals the change in output force divided by the change in input force. This is the formula for mechanical advantage. Ramp 1 has the greatest mechanical advantage. Drawing Conclusions: Why does the slope represent each ramp’s mechanical advantage? Which ramp has the greatest mechanical advantage? - How Machines Do Work Mechanical Advantage

  22. Efficiency of Machines • The work you put into a machine is equals the work done by the machine (in an ideal situation). • However, output work is always less than the input work for any machine in reality! • Efficiency is a comparison of output work to input work and is expressed as a percentage (%). • The higher the percentage, the greater the efficiency and vice versa. • 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% • Ideally all machines strive towards 100% efficiency • *No machine is 100% efficient due to friction! • A machine’s measured mechanical advantage is called actual mechanical advantage

  23. 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? Read and Understand What information have you been given? Input Work (Winput) = 250,000 J Output Work (Woutput) = 200,000 J - How Machines Do Work Calculating Efficiency

  24. 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. - How Machines Do Work Calculating Efficiency

  25. 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? Look Back and Check Does your answer make sense? An efficiency of 80 percent means that 80 out of every 100 J of work went into cutting the lawn. This answer makes sense because most of the input work is converted to output work. - How Machines Do Work Calculating Efficiency

  26. Practice Problem You do 20 J of work while using a hammer. The hammer does 18 J of work on a nail. What is the efficiency of the hammer? 90% - How Machines Do Work Calculating Efficiency

  27. Practice Problem Suppose you left your lawn mower outdoors all winter. Now it’s rusty. Of your 250,000 J of work, only 100,000 J go to cutting the lawn. What is the efficiency of the lawn mower now? 40% - How Machines Do Work Calculating Efficiency

  28. - How Machines Do Work Identifying Main Ideas • As you read the section “What Is a Machine?” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea. Main Idea The mechanical advantage of a machine helps by… Detail Detail Detail changing the amount of force you exert changing the distance over which you exert your force changing the direction of the force

  29. End of Section:How Machines Do Work

  30. - Simple Machines Inclined Plane • An inclined plane is a flat, sloped surface.

  31. - Simple Machines Wedge • A wedge is a device that is thick at one end and tapers to a thin edge at the other end.

  32. - Simple Machines Screws • A screw can be thought of as an inclined plane wrapped around a cylinder.

  33. - Simple Machines Levers • A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.

  34. - Simple Machines Levers • Levers are classified according to the location of the fulcrum relative to the input and output forces.

  35. - Simple Machines Wheel and Axle • A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.

  36. - Simple Machines Wheel and Axle • You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle.

  37. - Simple Machines Pulley • A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.

  38. - Simple Machines Simple Machines in the Body • Most of the machines in your body are levers that consist of bones and muscles.

  39. - Simple Machines Compound Machines • A compound machine is a machine that utilizes two or more simple machines.

  40. - Simple Machines Previewing Visuals • Before you read, preview Figure 17. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions. Three Classes of Levers Q. What are the three classes of levers? A. The three classes of levers are first-class levers, second-class levers, and third-class levers. Q. How do the three classes of levers differ? A. They differ in the position of the fulcrum, input force, and output force.

  41. End of Section:Simple Machines

  42. Graphic Organizer Mechanical Advantage Example Simple Machine Length of incline ÷ Height of incline Ramp Inclined plane Ax Wedge Length of wedge ÷ Width of wedge Length around threads ÷ Length of screw Screw Screw Distance from fulcrum to input force ÷ Distance from fulcrum to output force Seesaw Lever Radius of wheel ÷ Radius of axle Screwdriver Wheel and axle Pulley Flagpole Number of sections of supporting rope

  43. End of Section:Graphic Organizer

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