Transcript 14.2 -- Using Machines

Using Machines
Chapter 14.2
Pgs 432-436
SPI 0707.11.2
Learning Objectives
• Explain how a machine makes work easier.
• Calculate mechanical advantages and
efficiency of a machine.
• Explain how friction reduces efficiency.
New Vocabulary
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Input force
Output force
Mechanical advantage
Efficiency
What is a machine?
• Machines make work easier for us
• Machines do not decrease the
amount of work we need to do
• We still exert a force over some
distance to use machines
– Raking leaves: you exert a force on a
rake to gather leaves
– Using a wheelbarrow: you exert a
force to lift the handles of the
wheelbarrow
Input & Output Forces
• The force that you apply to a machine is the input
force.
• The work you do on the machine is equal to the input
force times the distance over which your force moves
the machine
• The work you do on the machine is input work
• The machine also does work by using force to move an
object
• The force the machine applies is the output force.
• The work the machine does is output work.
Using a Machine
• When you use a machine, the output work
can never be greater than the input work
• The machine makes the work easier by:
– Changing the amount of force you need to exert
– Changing the distance over which force is
exerted, or
– Changing the direction in which you exert your
force
Changing Force
• Some machines make work easier by reducing
the force you apply to complete a task
• These machines increase input force so output
force is greater
• The number of times a machine increases the
input force is the mechanical advantage of
the machine
Calculating Mechanical Advantage
• Mechanical advantage is the ratio of output
force to input force
• Can be calculated using this equation:
• Force is in newtons
• Applying math pg 433 (1 &2)
• Mechanical advantage does not have any units
because it is a ratio with two numbers of the
same units
Changing Distance
• Changing Distance (figure 5 pg
434)
– Some machines allow you to exert
a force over a shorter distance
– The output force is less than the
input force
– Example: using a rake
– The mechanical advantage of this
type of machine is less than one
because output is less than the
input
Changing Direction
• Changing Direction (pg 434,
fig.5)
– Sometimes its easier to apply a
force in a certain direction
– Example: raising a flag
– Neither force nor distance is
changed
– The mechanical advantage of
this type of machine is 1
because both forces are equal
Efficiency
• Machines can make output force greater than input
force but they cannot make OUTPUT WORK greater
than INPUT WORK
• When you use a machine there is friction between
moving parts
• Friction converts some input work to thermal energy &
the rest is converted to output work
• Output work is always less than the input work
• Efficiency is the ratio of output work to input work
Friction
• Imagine pushing a heavy box up a ramp
• The surfaces of the two objects are in contact
• At points along each surface, atoms and
molecules stick and bond together
• These bonds remain until a force breaks them
• To keep the box moving, force must be applied
to break the bonds as they continue to form
Assess What You’ve Learned
• Identify three specific situations in which
machines make work easier.
• Why does the output force exerted on a rake
have to be less than the input force?
• How does the efficiency of an ideal machine
compare with the efficiency of a real machine?
• How does friction reduce the efficiency of
machines?
• Applying math pg 436, 6-7