work and machines - what is work? the meaning of work work is done on an object when the object...
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Work and 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 Machines
Work
Work = Force x Distance
A student drags a backpack 10 meters by applying a force of 50 Newtons. Calculate the work done by the student.
Work and Machines
Power
Power = 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.
Work and Machines
How Machines Do Work
Machine
• A machine is a device that allows you to do work in an easier or more effective way
• A machine makes work easier by changing one or more of three things
1.The force you exert
2.The distance over which you exert the force
3.The direction over which you exert the force
• Input Force: The force you exert
• Output force: The force the machine exerts
Work and Machines
Calculating Power
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)/Time
Perform the calculation.
Power = (11,000 N X 5.0 m)/25 s
Power = (55,000 N•m)/25 s or 55,000 J/25 s
Power = 2,200 J/s = 2,200 W
- What Is Work?
Work and Machines
Calculating Power
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?
Work and Machines - 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.
Work and Machines - How Machines Do Work
Screwdriver or Faucet
Work and Machines
Mechanical Advantage
Mechanical Advantage = Output Force
Input Force
See example pg. 416
Work and Machines
Mechanical Advantage
The input force and output force for three different ramps are shown in the graph.
- How Machines Do Work
Work and Machines
Mechanical Advantage
Input force
Reading Graphs:
What variable is plotted on the horizontal axis?
- How Machines Do Work
Work and Machines
Mechanical Advantage
400 N
Interpreting Data:
If an 80-N input force is exerted on Ramp 2, what is the output force?
- How Machines Do Work
Work and Machines
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.
Drawing Conclusions:
Why does the slope represent each ramp’s mechanical advantage? Which ramp has the greatest mechanical advantage?
- How Machines Do Work
Work and Machines
Calculating Efficiency
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
Work and Machines
Calculating Efficiency
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 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.
- How Machines Do Work
Work and Machines
Calculating Efficiency
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
Work and Machines
Calculating Efficiency
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
Work and Machines
Calculating Efficiency
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
Work and Machines - Simple Machines
Inclined Plane
An inclined plane is a flat, sloped surface.
Work and Machines - Simple Machines
Wedge
A wedge is a device that is thick at one end and tapers to a thin edge at the other end.
Work and Machines - Simple Machines
Screws
A screw can be thought of as an inclined plane wrapped around a cylinder.
Work and Machines - Simple Machines
Levers
A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.
Work and Machines - Simple Machines
Levers
Levers are classified according to the location of the fulcrum relative to the input and output forces.
Work and Machines - 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.
Work and Machines - 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.
Work and Machines - Simple Machines
Pulley
A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.
Work and Machines
Types of Pulleys Activity
Click the Active Art button to open a browser window and access Active Art about types of pulleys.
- Simple Machines
Work and Machines - Simple Machines
Simple Machines in the Body
Most of the machines in your body are levers that consist of bones and muscles.
Work and Machines - Simple Machines
Compound Machines
A compound machine is a machine that utilizes two or more simple machines.
Work and Machines
Main Idea
Detail Detail Detail
Identifying 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.
The mechanical advantage of a machine helps by…
changing the amount of force you exert
changing the distance over which you exert your force
changing the direction of the force
- How Machines Do Work
Work and Machines
Links on Mechanical Efficiency
Click the SciLinks button for links on mechanical efficiency.
- How Machines Do Work
Work and Machines
End of Section:How Machines
Do Work
Work and Machines
Previewing 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 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.
- Simple Machines
Work and Machines
Levers
Click the Video button to watch a movie about levers.
- Simple Machines
Work and Machines
Pulleys
Click the Video button to watch a movie about pulleys.
- Simple Machines
Work and Machines
End of Section:Simple Machines
Work and Machines
Graphic Organizer
Wheel and axle
Simple Machine Mechanical Advantage Example
Inclined plane Length of incline ÷ Height of incline Ramp
Radius of wheel ÷ Radius of axle Screwdriver
Wedge Length of wedge ÷ Width of wedge Ax
ScrewLength around threads ÷ Length of screw Screw
Lever Distance from fulcrum to input force ÷ Distance from fulcrum to output force Seesaw
Pulley Number of sections of supporting rope Flagpole
Work and Machines
End of Section:Graphic Organizer