Download Section 2 What Is a Machine?

Chapter 21
Work and Machines
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Section 1 Work and Power
Section 2 What Is a Machine?
Section 3 Types of Machines
Concept Mapping
Chapter 21
Section 1 Work and Power
Bellringer
First, in your science journal, define what specific
kind of work is being done in each activity below.
Then, select the activities that require the least
amount of work.
• carrying heavy books home
• reading a 300-page novel
• skiing for 1 hour
• lifting a 45 kg mass
• holding a steel beam in place for 3 hours
• jacking up a car
Chapter 21
Section 1 Work and Power
Objectives
• Determine when work is being done on an object.
• Calculate the amount of work done on an object.
• Explain the difference between work and power.
Chapter 21
Section 1 Work and Power
What Is Work?
• Work is the transfer of energy to an object by using
a force that causes the object to move in the direction
of the force.
• Transfer of Energy One way you can tell that work
is being done is that energy is transferred.
Chapter 21
Section 1 Work and Power
What Is Work?, continued
• Difference Between Force and Work Applying a
force doesn’t always result in work being done.
• Force and Motion in the Same Direction For
work to be done on an object, the object must move
in the same direction as the force.
Chapter 21
Section 1 Work and Power
Chapter 21
Section 1 Work and Power
How Much Work?
• Same Work, Different Force Work depends on
distance as well as force.
Chapter 21
Section 1 Work and Power
How Much Work?, continued
• Calculating Work The amount of work (W) done
in moving an object can be calculated by
multiplying the force (F) applied to the object by the
distance (d) through which the force is applied:
WFd
• The unit used to express work is the newton-meter
(N  m), which is more simply called the joule.
Chapter 21
Section 1 Work and Power
Chapter 21
Section 1 Work and Power
Power: How Fast Work Is Done
• Calculating Power Power is the rate at which
energy is transferred. To calculate power (P), you
divide the amount of work done (W) by the time (t) it
takes to do that work:
W÷t=P
• The unit used to express power is joules per
second (J/s), also called the watt. One watt (W) is
equal to 1 J/s.
Chapter 21
Section 1 Work and Power
Chapter 21
Section 1 Work and Power
Power: How Fast Work Is Done, continued
• Increasing Power It may take you longer to sand
a wooden shelf by hand than by using an electric
sander, but the amount of energy needed is the
same either way. Only the power output is lower
when you sand the shelf by hand.
Chapter 21
Section 2 What Is a Machine?
Bellringer
Write a one-paragraph answer in your science
journal to the following question:
Why do we use machines?
Chapter 21
Section 2 What Is a Machine?
Objectives
• Explain how a machine makes work easier.
• Describe and give examples of the force-distance
trade-off that occurs when a machine is used.
• Calculate mechanical advantage.
• Explain why machines are not 100% efficient.
Chapter 21
Section 2 What Is a Machine?
Machines: Making Work Easier
• A machine is a device that makes work easier by
changing the size or direction of a force.
Chapter 21
Section 2 What Is a Machine?
Machines: Making Work Easier, continued
• Work In, Work Out The work that you do on a
machine is called work input. The work done by the
machine on an object is called work output.
• How Machines Help Machines allow force to be
applied over a greater distance, which means that
less force will be needed for the same amount of
work.
Chapter 21
Section 2 What Is a Machine?
Machines: Making Work Easier, continued
• Same Work, Different Force Machines make
work easier by changing the size or direction of the
input force.
• The Force-Distance Trade Off When a machine
changes the size of the force, the distance through
which the force is exerted must also change.
Chapter 21
Section 2 What Is a Machine?
Chapter 21
Section 2 What Is a Machine?
Mechanical Advantage
• What Is Mechanical Advantage? A machine’s
mechanical advantage is the number of times the
machine multiplies force.
• Calculating Mechanical Advantage You can find
mechanical advantage by using the following equation:
output force
mechanical advantage ( MA) 
input force
Chapter 21
Section 2 What Is a Machine?
Mechanical Efficiency
• The less work a machine has to do to overcome
friction, the more efficient the machine is. Mechanical
efficiency is a comparison of a machine’s work output
with the work input.
• Calculating Efficiency A machine’s mechanical
efficiency is calculated using the following equation:
work output
mechanical efficiency 
 100
work input
Chapter 21
Section 2 What Is a Machine?
Mechanical Efficiency, continued
• Perfect Efficiency? An ideal machine would be a
machine that had 100% mechanical efficiency.
• Ideal machines are impossible to build, because
every machine has moving parts. Moving parts
always use some of the work input to overcome
friction.
Chapter 21
Section 2 What Is a Machine?
Mechanical Efficiency
Click below to watch the Visual Concept.
Visual Concept
Chapter 21
Section 3 Types of Machines
Bellringer
What type of machine can be found on at least half
the students in this room right now? What kinds of
machines were common 50 years ago? 100 years
ago? Are any of the same machines around today that
were common in the 1800s? What has changed about
those same machines today?
Record your thoughts in your science journal.
Chapter 21
Section 3 Types of Machines
Objectives
• Identify and give examples of the six types of
simple machines.
• Analyze the mechanical advantage provided by
each simple machine.
• Identify the simple machines that make up a
compound machine.
Chapter 21
Section 3 Types of Machines
Levers
• A lever is a simple machine that has a bar that
pivots at a fixed point, called a fulcrum.
• First-Class Levers With a first-class lever, the
fulcrum is between the input force and the load.
Chapter 21
Section 3 Types of Machines
Levers, continued
• Second-Class Levers The load of a second-class
lever is between the fulcrum and the input force.
Chapter 21
Section 3 Types of Machines
Levers, continued
• Third-Class Levers The input force in a third-class
lever is between the fulcrum and the load.
Chapter 21
Section 3 Types of Machines
Pulleys
• A pulley is a simple machine that consists of a
wheel over which a rope, chain, or wire passes.
• Fixed Pulleys A fixed pulley is attached to
something that does not move.
Chapter 21
Section 3 Types of Machines
Pulleys, continued
• Movable Pulleys Unlike fixed pulleys, movable
pulleys are attached to the object being moved.
• Blocks and Tackles When a fixed pulley and a
movable pulley are used together, the pulley system
is called a block and tackle.
Chapter 21
Section 3 Types of Machines
Pulleys, continued
Chapter 21
Section 3 Types of Machines
Wheel and Axle
• What Is a Wheel
and Axle? A wheel
and axle is a simple
machine consisting
of two circular
objects of different
sizes.
Chapter 21
Section 3 Types of Machines
Wheel and Axle, continued
• Mechanical Advantage
of a Wheel and Axle
The mechanical
advantage of a wheel
and axle can be found by
dividing the radius (the
distance from the center
to the edge) of the wheel
by the radius of the axle.
Chapter 21
Section 3 Types of Machines
Inclined Planes
• An inclined plane is a simple machine that is a
straight, slanted surface.
• Mechanical Advantage of an Inclined Plane The
mechanical advantage (MA) of an inclined plane can
be calculated by dividing the length of the inclined
plane by the height to which the load is lifted.
Chapter 21
Section 3 Types of Machines
Chapter 21
Section 3 Types of Machines
Inclined Planes, continued
• Wedges A wedge is a pair of inclined planes that
move.
• Mechanical Advantage of Wedges can be found by
dividing the length of the wedge by its greatest
thickness.
Chapter 21
Section 3 Types of Machines
Inclined Planes, continued
• Screws A screw is an inclined
plane that is wrapped in a spiral
around a cylinder.
• Mechanical Advantage of
Screws The longer the spiral on
a screw is and the closer
together the threads are, the
greater the screw’s mechanical
advantage is.
Chapter 21
Section 3 Types of Machines
Compound Machines of Machines
• What Are Compound Machines? Compound
machines are machines that are made of two or
more simple machines.
• Mechanical Efficiency of Compound Machines
The mechanical efficiency of most compound
machines is low, because compound machines
have more moving parts than simple machines do.
Thus, there is more friction to overcome.
Chapter 21
Section 3 Types of Machines
Compound Machine of Machine
Click below to watch the Visual Concept.
Visual Concept
Chapter 21
Work and Machines
Concept Mapping
Use the terms below to complete the concept map on
the next slide.
work input
output force
work
lever
distance
input force
mechanical efficiency
mechanical advantage
Chapter 21
Work and Machines
Chapter 21
Work and Machines