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Work and Machines
Table of Contents
What Is Work?
How Machines Do Work
Simple Machines
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 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 or N x m)
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 = 1 horsepower (non SI unit)
Work and Machines - What Is Work?
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
Work and Machines - What Is Work?
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 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 s or 55,000 J/25 s
Power = 2,200 J/s = 2,200 W
Work and Machines - What Is Work?
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?
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.
Work and Machines - What Is Work?
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
Work and Machines - What Is Work?
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
Work and Machines - 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.
Question
What is work?
Answer
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.
Work and Machines
End of Section:
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
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!
Work and Machines - How Machines Do Work
What Is a Machine?
A machine makes work easier
by changing at least one of
three factors:
1. A machine may change the
amount of force you exert.
2. A machine may change the
distance over which you
exert your force.
3. A machine may change the
direction in which you
exert your force.
*In each case, the amount
of work stays the same!
Work and Machines
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 increases
the 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.
Work and Machines - How Machines Do Work
Mechanical Advantage
The input force and output force
for three different ramps are
shown in the graph.
Work and Machines - How Machines Do Work
Mechanical Advantage
Reading Graphs:
What variable is plotted on
the horizontal axis?
Input force
Work and Machines - How Machines Do Work
Mechanical Advantage
Interpreting Data:
If an 80-N input force is
exerted on Ramp 2, what is
the output force?
400 N
Work and Machines - How Machines Do Work
Mechanical Advantage
Interpreting Data:
Find the slope of the line for
each ramp.
Ramp 1: 10; Ramp 2: 5;
Ramp 3: 2
Work and Machines - How Machines Do Work
Mechanical Advantage
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.
Work and Machines
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
Work and Machines - How Machines Do Work
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
Work and Machines - How Machines Do Work
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 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.
Work and Machines - How Machines Do Work
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.
Work and Machines - How Machines Do Work
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%
Work and Machines - How Machines Do Work
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%
Work and Machines - 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
Work and Machines - How Machines Do Work
Links on Mechanical Efficiency
Click the SciLinks button for links on mechanical efficiency.
Work and Machines
End of Section:
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 - Simple Machines
Types of Pulleys Activity
Click the Active Art button to open a browser window and
access Active Art about types of pulleys.
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 - 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.
Work and Machines - Simple Machines
Levers
Click the Video button to watch a movie about levers.
Work and Machines - Simple Machines
Pulleys
Click the Video button to watch a movie about pulleys.
Work and Machines
End of Section:
Simple Machines
Work and Machines
Graphic Organizer
Simple Machine
Mechanical Advantage
Inclined plane
Length of incline ÷ Height of incline
Ramp
Wedge
Length of wedge ÷ Width of wedge
Ax
Screw
Length around threads ÷ Length of
screw
Screw
Lever
Distance from fulcrum to input force ÷
Distance from fulcrum to output force Seesaw
Wheel and axle
Radius of wheel ÷ Radius of axle
Pulley
Example
Screwdriver
Number of sections of supporting rope Flagpole
Work and Machines
End of Section:
Graphic Organizer