Chapter 12 – Work and Machines
► Work
is when a force is used to make
something move.
Work = Force (Distance)
► Work
units
 English units will be foot.pounds
 Metric units will be newton.meters (which
is called a joule)
Example problem:
Use the four step method to determine
how much work is done if a 50 newton force
is used to drag a rock 30 meters.
The force used in this equation is the
force in the direction of motion.
30 newtons
Force
40
newtons
Force
30 newtons
Force used to
move box
►
►
►
Machines
A machine is a device with which you
can do work in a way that is easier or
more effective
A machine can be simple or complex
A machine makes work easier by:
a.
b.
c.
Changing the amount of force you exert or
Changing the distance over which you exert
the force
The direction in which you exert your force
► Input force – force you exert on the machine
► Output force – force exerted by the machine
Mechanical advantage =
Output Force
Input Force
► Multiplying
Force (mechanical advantage
greater than 1)
If the input force is less than the output force,
the input force must be exerted over a greater
distance.
 Examples: a car jack, pulling nails with a hammer
► Multiplying
distance (mechanical
advantage less than 1)
If the input force is greater than the
output force, the output end of the
machine will move a greater distance.
 Examples: a fan, a hockey stick, high
gear on a bicycle or car
► Changing
Direction of Force
 Example: a pulley, a lever
► Efficiency
Efficiency is the total work or energy
input into a machine compared to the
useful work or energy output of the
machine
Output Work
Efficiency = ------------------ x 100
Input Work
►
►
If there was no energy loss through
friction or other causes, the efficiency
of a machine would be 100%
No machine is 100% efficient,
although some simple machines are
very close to 100% efficient
For simple machines
Input work ≈ Output work
Push down
a distance
of 5 ft.
Lift a distance
of 2 ft.
20 lbs
50 lbs
Input work = F (D)
Output work = F (D)
Input work = 20 lb (5 ft.)
Output work = 50 lb (2 ft.)
Input work = 100 ft-lbs
Output work = 100 ft-lbs
100 ft-lbs ≈ 100 ft-lbs
Use the efficiency equation to determine
the approximate efficiency of the lever
system.
output work
Efficiency =
x 100 =
input work
100 ft-lbs
x 100
100 ft-lbs
efficiency = 100%
Example efficiency problems: Use the four
step method to solve these problems
Determine the efficiency of a machine
that requires a work input of 224 foot
pounds for a work output of 200 foot
pounds.
A hydraulic jack requires thirty ½ foot
down ward strokes of 20 pounds each
to lift an 1150 pound object a distance
of .25 feet. Calculate the efficiency of
the jack.
► Ideal
Mechanical Advantage – the
mechanical advantage of a machine
if there was no friction. This can be
calculated.
► Actual
Mechanical Advantage – the
real mechanical advantage of a
machine. This can be different for
each machine and must be measured.
Part 2 – Simple Machines
The 6 types of simple machines are:
1. The inclined plane
2. The wedge
3. The screw
4. The lever
5. The wheel and axle
6. The pulley
The Inclined Plane
Mechanical advantage (MA)= output force/input
force
Ideal MA = length of the incline/height of incline
If there was no friction the actual MA would
equal ideal MA. In real life it is always less.
Incline
height
Incline
length
Wedge
►A
wedge is like a moving inclined
plane to split or cut things apart.
Screw
A screw is also related to the inclined plane.
The threads of a screw are like a spiral inclined plane.
The closer the threads, the greater the MA of the screw.
Levers (3 types)
► First
Class Lever (a pry
or teeter-totter)
► Second
class Lever
(wheel barrel)
► Third
class Lever
Output
force
Output force
Input
force
Input
force
Input
force
Output
force
Distance from fulcrum to input force
Ideal MA =
Distance from fulcrum to output force
Wheel and Axle
► The
wheel and axle are fastened
together so they rotate together.
(door
knob, screw driver,
steering
wheel of a car)
radius of wheel
mechanical advantage =
radius of axle
Pulley
►A
single fixed pulley has a mechanical advantage
of one.
► A single movable pulley has a mechanical
advantage of two
► We can add more movable pulleys to add
more mechanical advantage.
Compound Machine
• A machine that uses two or more simple
machines
• Identify the simple machines in each of
the following.