Chapter 5
Lesson 2
What is a machine?
 A machine is a device that
makes doing work easier
 Machines can be simple.
 Some, like knives, scissors, and
doorknobs, are used everyday
to make doing work easier.
Making Work Easier
 Machines can make work easier
by increasing the force that can
be applied to an object.
 A second way that machines can
make work easier is by increasing
the distance over which a force
can be applied.
 Machines can also make work
easier by changing the direction
of an applied force.
Increasing Force
 A car jack is an example of a
machine that increases an
applied force.
 The upward force exerted by
the jack is greater than the
downward force you exert on
the handle.
Increasing Force
 However, the distance you push
the handle downward is greater
than the distance the car is
pushed upward.
 The jack increases the applied
force, but doesn't increase the
work done.
Force and Distance
 The work done in lifting an object
depends on the change in height of
the object.
 The same amount of work is done
whether the mover pushed the
furniture up the long ramp or lifts
it straight up.
 If work stays the same and the
distance is increased, then less
force will be needed to do the
work.
Changing Direction
 Some machines change the
direction of the force you
apply.
 The wedge-shaped blade of an
ax is one example.
The Work Done by Machines
 When you use an ax to split
wood, you exert a downward
force as you swing the ax
toward the wood.
 The blade changes the
downward force into a
horizontal force that splits the
wood apart.
The Work Done by Machines
 When you use a machine such
as a crowbar, you are trying to
move something that resists
being moved.
 If you use a crowbar to pry the
lid off a crate, you are working
against the friction between
the nails in the lid and the
crate.
The Work Done by Machines
 You also could use a crowbar to
move a large rock
 In this case, you would be
working against gravity—the
weight of the rock.
Input and Output Forces
 Two forces are involved when a
machine is used to do work.
 The force that is applied to
the machine is called the input
force.
 Fin stands for the effort force.
 The force applied by the
machine is called the output
force, symbolized by Fout.
Input and Output Forces
 Two kinds of work need to be
considered when you use a
machine—the work done by you on
the machine and the work done by
the machine.
 The work done by you on a machine
is called the input work and is
symbolized by Win.
 The work done by the machine is
called the output work and is
abbreviated Wout.
Conserving Energy
 When you do work on the machine,
you transfer energy to the
machine.
 When the machine does work on an
object, energy is transferred from
the machine to the object.
 The amount of energy the machine
transfers to the object cannot be
greater than the amount of energy
you transfer to the machine.
Work
 Conservation of Energy
 can never get more work out
than you put in
 trade-off between force and
distance
Win = Wout
Fe × de = Fr × dr
Ideal Machines
 Suppose a perfect machine
could be built in which there
was no friction.
 None of the input work or
output work would be
converted to heat.
 For such an ideal machine, the
input work equals the output
work.
Ideal Machines
 Suppose the ideal machine
increases the force applied to
it.
 This means that the output
force, Fout, is greater than the
input force, Fin.
 Recall that work is equal to
force times distance.
Ideal Machines
 If Fout is greater than Fin, then
Win and Wout can be equal only
if the input force is applied
over a greater distance than
the output force is exerted
over.
Mechanical Advantage
 The ratio of the output force
to the input force is the
mechanical advantage of a
machine.
 The mechanical advantage of a
machine can be calculated from
the following equation.
Force
 Effort Force (Fe)
 force applied to the machine
 “what you do”
 Resistance Force (Fr)
 force applied by the machine
 “what the machine does”
Mechanical Advantage
 Mechanical Advantage (MA)
 number of times a machine
increases the effort force
Fr
MA 
Fe
 MA
 MA
 MA
> 1 : force is increased
< 1 : distance is increased
= 1 : only direction is changed
Mechanical Advantage
 Window blinds are a machine
that changes the direction of
an input force.
 A downward pull on the cord is
changed to an upward force on
the blinds.
Mechanical Advantage
 The input and output forces
are equal, so the MA is 1.
Ideal Mechanical Advantage
 The mechanical advantage of a
machine without friction is
called the ideal mechanical
advantage, or IMA.
 The IMA can be calculated by
dividing the input distance by
the output distance.
Efficiency
 Efficiency
 measure of how completely
work input is converted to
work output
Wout
Efficiency 
 100%
Win
 always
less than 100% due to
friction
Calculating Efficiency
 In an ideal machine there is no
friction and the output work
equals the input work. So the
efficiency of an ideal machine
is 100 percent.
 The efficiency of a real
machine is always less than 100
percent.
Increasing Efficiency
 Machines can be made more
efficient by reducing friction.
This usually is done by adding a
lubricant, such as oil or grease,
to surfaces that rub together.
 A lubricant fills in the gaps
between the surfaces, enabling
the surfaces to slide past each
other more easily.
Mechanical Advantage
GIVEN:
 A worker applies an effort force of
20 N to open a window with a
resistance force of 500 N. What is
the crowbar’s MA?
Fe = 20 N
Fr = 500 N
MA = ?
Fr
MA Fe
WORK:
MA = Fr ÷ Fe
MA = (500 N) ÷ (20 N)
MA = 25
Mechanical Advantage
 Find the effort force needed to lift a
2000 N rock using a jack with a
mechanical advantage of 10.
GIVEN:
Fe = ?
Fr = 2000 N
MA = 10
Fr
MA Fe
WORK:
Fe = Fr ÷ MA
Fe = (2000 N) ÷ (10)
Fe = 200 N