Work and Machines What Is Work? How Machines Do Work Simple Machines Table of Contents
Work and Machines What Is Work? How Machines Do Work Simple Machines Table of Contents.
Apr 01, 2015
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Work and Machines
What Is Work?
How Machines Do Work
Simple Machines
Table of Contents
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.
•To do work on an object:
•The object must move when force is applied.
•The object must move in the same direction as the force.
Work and Machines
Calculating Work and Power
- What Is Work?
•Force is measured in Newtons (N).
•Formula for Work:
• Work = Force x Distance
•The SI unit of measure for work is joule (J), in honor of James Prescott Joule, who studied work in the mid-1800s.
•Formula for Power:
• Power = Work OR Force x Distance Time Time
•The SI unit of measure for power is Watts (W), in honor of James Watt who greatly improved the steam engine.
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?
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?
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 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?
Work and Machines
Question Answer
Asking QuestionsBefore 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.
What is work? Work is done when an object moves in the same direction in which the force is exerted.
How can you calculate work?
Work = Force X Distance
What is power? Power is the rate at which work is done.
- What Is Work?
Work and Machines - How Machines Do Work
Input and Output WorkThe amount of input work done by the gardener equals the amount of output work done by the shovel.
Input force – effort you put into the machine.
Output force – effort the machine puts into an object.
Work and Machines - How Machines Do Work
What Is a Machine?A machine makes work easier by changing at least one of three factors:
•The amount of force you exert
•The distance over which you exert your force
•The direction in which you exert your force.
Ex. A ramp or faucet
Ex. Hockey stick, chopsticks, riding a bike
Ex. A weight machine with pulleys
Work and Machines
Efficiency
Efficiency determines how much work was wasted due to friction.
Efficiency = Output Work x 100%
Input Work
The higher the percentage, the more efficient the machine (wastes little work).
An ideal machine would have 100% efficiency.
- How Machines Do Work
Work and Machines
Mechanical Advantage
The input force and output force for three different ramps are shown in the graph.
- How Machines Do Work
Mechanical advantage = Output force Input force
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
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 - Simple Machines
Inclined PlaneAn inclined plane is a flat, sloped surface.
Ideal mechanical advantage = Length of incline
Height of incline
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.
Ideal Mechanical Advantage = Length of Wedge
Width of Wedge
Work and Machines - Simple Machines
ScrewsA screw can be thought of as an inclined plane wrapped around a cylinder.
I.M.A. = Length around the threads
Length of the screw
Work and Machines - Simple Machines
Levers
A lever is a ridged bar that is free to pivot, or rotate, on a fixed point (fulcrum).
I.M.A. = Distance from fulcrum to input force
Distance from fulcrum to output force
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 AxleA wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.
I.M.A. = Radius of wheel
Radius of axle
Work and Machines - Simple Machines
PulleyA pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.
I.M.A. = Number of sections of rope that support the object
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. Your teeth are wedges.
Work and Machines - Simple Machines
Compound Machines
A compound machine is a machine that utilizes two or more simple machines.
I.M.A. = The product of the individual I.M.A.s of the simple machines that make it up.
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
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