Presentation on theme: "The Meaning of Work - What Is Work?"— Presentation transcript:
1 The Meaning of Work - What Is Work? Work is done on an object when the object moves in the same direction in which the force is exerted.
2 WorkWork = Force x Distance A student drags a backpack 10 meters by applying a force of 50 Newtons. Calculate the work done by the student.
3 PowerPower = Work Time If the student in the previous problem drags the backpack over the period of 20 seconds, calculate the power required If the backpack is dragged over the period of 10 seconds, calculate the power reqiuired.
4 How Machines Do Work Machine A machine is a device that allows you to do work in an easier or more effective wayA machine makes work easier by changing one or more of three thingsThe force you exertThe distance over which you exert the forceThe direction over which you exert the forceInput Force: The force you exertOutput force: The force the machine exerts
5 Calculating Power - What Is Work? 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 SolveWhat 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)/TimePerform the calculation.Power = (11,000 N X 5.0 m)/25 sPower = (55,000 N•m)/25 s or 55,000 J/25 sPower = 2,200 J/s = 2,200 W
6 Calculating Power Practice Problem - What Is Work? 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
7 Input and Output Work - How Machines Do Work The amount of input work done by the gardener equals the amount of output work done by the shovel.
9 Mechanical AdvantageMechanical Advantage = Output Force Input Force See example pg. 416
10 Mechanical Advantage - How Machines Do Work The input force and output force for three different ramps are shown in the graph.
11 Mechanical Advantage - How Machines Do Work Reading Graphs: What variable is plotted on the horizontal axis?Input force
12 Mechanical Advantage - How Machines Do Work Interpreting Data: If an 80-N input force is exerted on Ramp 2, what is the output force?400 N
13 Mechanical Advantage - How Machines Do Work Drawing Conclusions: Why does the slope represent each ramp’s mechanical advantage? Which ramp has the greatest mechanical advantage?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.
14 Calculating Efficiency - How Machines Do WorkCalculating EfficiencyYou 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 UnderstandWhat information have you been given?Input Work (Winput) = 250,000 JOutput Work (Woutput) = 200,000 J
15 Calculating Efficiency - How Machines Do WorkCalculating EfficiencyYou 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 SolveWhat 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.
16 Calculating Efficiency - How Machines Do WorkCalculating EfficiencyYou 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 CheckDoes 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.
17 Calculating Efficiency - How Machines Do WorkCalculating EfficiencyPractice ProblemYou 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%
18 Calculating Efficiency - How Machines Do WorkCalculating EfficiencyPractice ProblemSuppose 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%
19 Inclined Plane - Simple Machines An inclined plane is a flat, sloped surface.
20 Wedge - Simple Machines A wedge is a device that is thick at one end and tapers to a thin edge at the other end.
21 Screws - Simple Machines A screw can be thought of as an inclined plane wrapped around a cylinder.
22 Levers - Simple Machines A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.
23 Levers - Simple Machines Levers are classified according to the location of the fulcrum relative to the input and output forces.
24 Wheel and Axle - Simple Machines A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.
25 Wheel and Axle - Simple Machines 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.
26 Pulley - Simple Machines A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.
27 Types of Pulleys Activity - Simple MachinesTypes of Pulleys ActivityClick the Active Art button to open a browser window and access Active Art about types of pulleys.
28 Simple Machines in the Body Most of the machines in your body are levers that consist of bones and muscles.
29 Compound Machines - Simple Machines A compound machine is a machine that utilizes two or more simple machines.
30 Identifying Main Ideas - How Machines Do WorkIdentifying Main IdeasAs 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 IdeaThe mechanical advantage of a machine helps by…DetailDetailDetailchanging the amount of force you exertchanging the distance over which you exert your forcechanging the direction of the force
31 Links on Mechanical Efficiency - How Machines Do WorkLinks on Mechanical EfficiencyClick the SciLinks button for links on mechanical efficiency.
33 Three Classes of Levers - Simple MachinesPreviewing VisualsBefore 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 LeversQ. 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.
34 Click the Video button to watch a movie about levers. - Simple MachinesLeversClick the Video button to watch a movie about levers.
35 Click the Video button to watch a movie about pulleys. - Simple MachinesPulleysClick the Video button to watch a movie about pulleys.
37 Graphic Organizer Simple Machine Mechanical Advantage Example Inclined planeLength of incline ÷ Height of inclineRampWedgeLength of wedge ÷ Width of wedgeAxLength around threads ÷ Length of screwScrewScrewDistance from fulcrum to input force ÷ Distance from fulcrum to output forceLeverSeesawWheel and axleRadius of wheel ÷ Radius of axleScrewdriverPulleyNumber of sections of supporting ropeFlagpole