1. technology and computing
2. mp3 and midi

Chapter 4 Machines,
Work, and Energy
4.2 Simple Machines
What is a Machine?
 A machine is a device with moving parts
that work together to accomplish a task.
 A bicycle is made of a combination of
machines that work together.
 All the parts of a bicycle work as a system
to transform forces from your muscles
into motion.
 A bicycle allows you to travel
at faster speeds and for
greater distances than
possible on foot.
The Concepts of Input and
Output
 Machines are designed to do something.
 The input includes everything you do to make
the machine work, like pushing on the bicycle
pedals.
 The output is what the machine does for you,
like going fast or climbing a steep hill.
 The input and output may be force, power, or
energy.
The Beginning of Technology
 A simple machine is an unpowered
mechanical device that accomplishes a
task with only one movement (such as a
lever).
 A lever allows you to move a rock that
weighs 10 times (or more) what you
weigh.
Input Force and Output Force
 Simple machines work with forces.
 The input force is the force you apply to the
machine.
 The output force is the force the machine
applies to what you are trying to move.
Ropes and Pulleys
 A rope and pulley
system is a simple
machine made by
connecting a rope to
one or more pulleys.
 You apply the input
force to the rope and
the output force is
exerted on the load
you are lifting.
Machines within Machines
 Most of the
machines we use
today are made up
of combinations of
different types of
simple machines.
Types of Machines
 Bicycle uses wheels
and axels, levers (the
pedals and kickstand),
and gears.
 A complex machine like a
VCR contains simple
machines of every type
including screws, ramps,
pulleys, wheels, gears,
and levers.
Ratio of Output to Input Force
 The mechanical advantage of a machine is the ratio of
the output force to the input force.
 If the mechanical advantage of a machine is larger
than one, the output force is larger than the input force.
 A mechanical advantage smaller than one means the
output force is smaller than the input force.
 Mechanical advantage is a ratio of forces, so it is a
pure number without any units.
Input and Output Work
 A simple machine does work because it
exerts forces over a distance.
 If you are using the machine you also do
work, because you apply forces to the
machine that move its parts.
Definition of a Simple
Machine
 A simple machine has no source of energy
except the immediate forces you apply.
 The only way to get output work FROM a
simple machine is to do input work ON the
machine.
 The output work done by a simple machine can
never exceed the input work done on the
machine.
Perfect Machines
 In a perfect machine, the output work
equals the input work.
 Friction always converts some of the input
work to heat and wear,
so the output work is
always less than the
input work.
The Cost of Multiplying Force
 The output work of a machine can never
be greater than the input work.
 This rule is true for all machines.
 The force and distance are related by the
amount of work done.
 In a perfect (theoretical) machine, the
output work is exactly equal to the input
work.
Force or Distance
 Many problems give
three or four quantities:
input force, input
distance, output force,
and output distance.
 If the input and output
work are equal then force
x distance at the input of
the machine equals force
x distance at the output.
Parts of the Lever
 All levers include a stiff structure that rotates
around a fixed point called the fulcrum.
 The side of the lever where the input force is
applied is called the input arm.
 The output arm is the end of the lever that applies
the output force.
Effort
Load
Fulcrum
Changing Direction
 When the fulcrum is in the middle of the lever,
the input and output forces are the same.
 The input and output forces are different if the
fulcrum is not in the center of the lever.
 The side of the lever with the longer arm has
the smaller force.
Input work
di x Fi
Output work
=
do x Fo
di = Fo
do Fi
Mechanical Advantage of a
Lever
 The output work is the output force
multiplied by the output distance.
 The input work is the input distance
multiplied by the input force.
Mechanical Advantage of a
Lever
 By setting the input
and output work
equal, you see that
the ratio of forces is
the inverse of the
ratio of distances.
 The larger (input)
distance has the
smaller force.
 The ratio of distances
is equal to the ratio of
the lengths of the two
arms of the lever.
Three Types of Levers
 Levers are used in
many common
machines: pliers,
wheelbarrow, and
the human biceps
and forearm.
 The mechanical
advantage is
always the ratio of
lengths of the input
arm to the output
arm.
How a Rope and Pulley
System Works
 The force in a rope is called tension and is a pulling
force that acts along the direction of the rope.
 The tension is the same at every point in a rope.
 If the rope is not moving, its tension is equal to the
force pulling on each end.
 Ropes or strings do not carry pushing forces.
Mechanical Advantage
 The mechanical advantage of a
pulley system depends on the
number of strands of rope directly
supporting the load.
 To make a rope and pulley system
with a greater mechanical
advantage, you can increase the
number of strands directly
supporting the load by taking more
turns around the pulleys.
Gears and Ramps
 Many machines require that rotating motion
be transmitted from one place to another.
 The transmission of rotating motion is often
done with gears.
 Some machines that use gears, such as small
drills, require small forces at high speeds.
 Other machines, such as the paddle wheel on
the back of a steamboat, require large forces
at low speed.
How Gears Work
 The rule for how two gears turn depends
on the number of teeth on each gear.
Ramps
 A ramp is another type of simple machine that allows
you to push a heavy object to a higher location with
less force than is needed to lift the object straight up.
 Ramps reduce the input force needed by increasing
the distance over which the input force acts.
 The output work is work done against gravity.
Mechanical Advantage of a Ramp
 The input work is the input force
multiplied by the length of the ramp.
1m
Ramp
Mechanical advantage = ramp length
height
Screws
 A screw is a simple
machine that turns
rotating motion into
linear motion.
 A screw works just like
a ramp that curves as it
get higher.
 The “ramp” on a screw
is called a thread.