CHAPTER 14: MACHINES
14.1: Machines help people
do work
 Machine:
 Any device that helps people do work.
 Work is the use of force to move an object
 Does not decrease the amount of work that is
done
 Only changes the way the work is done
 Example:
 Ramp
 pulley
How do machines help?
 Machines make the work easier by changing:
 The size of the force needed to do the work
 the distance over which the force is applied
 The direction in which the force is exerted
 Can be powered by different types of energy
depending on the type of machine:
 Electronic machines use electrical energy
 Mechanical machines use mechanical energy
 Mechanical energy is usually supplied by the person
using the machine
Changing size and direction
 Some machines help by changing the size of
the force needed
 If a machine allows you to exert less force,
you must apply that force over a greater
distance:
 Total amount of work remains the same whether a
machine is used or not
 Work = force x distance
 Because a machine does not decrease the
amount of work to be done, less force must
mean a greater distance
 Some machines allow you to apply a greater
input force over a shorter distance (rake)
 You will move your hand a shorter distance to move
the end of the rake a longer distance
 Input force:
 The force exerted on a machine
 When using a rake, the input force is the force from
the boy on the rake
 Output force:
 The force that the machine exerts on an object
 This is the force that the rake exerts on the leaves
 Machines can help you do work by changing
the direction of a force:
 Flagpole:
 When raising the flag up the flagpole, you pull
down on the rope to raise the flag up the pole
 The rope system does not change the size of the
force, only the direction.
 The force pulling the flag up is equal to the force you
apply in your downward pull
 Shovel:
 Once you have the shovel in the ground, you push
the handle down to lift the dirt up.
 A shovel also changes the size of the force you
apply-you need less force to lift the dirt
Mechanical advantage of a
machine
 Mechanical advantage
 MA
 The number of times a machine multiplies the
input force
 Formulas to know:
 MA = output force ÷ input force
 Fout = MA x Fin
 Fin = Fout ÷ MA
 If your machine allows you to apply less force
over a greater distance (doorknob) the output
force is greater than the input force; MA is
greater than 1
 For machines that allow you to apply greater
force over a shorter distance (rake) the output
force is less than the input force; MA is less
than 1
 For machines that change only the direction
of a force (rope system of a flagpole) the
input and output forces are the same; MA is
equal to 1
 The output force of a machine is 600N and
the input force is 200N. What is the MA of
the machine?
 A machine has an input force of 150N and a
MA of 0.5. What is the output force?
 The output force of a machine is 135N and the
MA is 2.5. What is the input force?
Work transfers energy
 Machines transfer energy to objects on which
they do work.
 If the machine lifts an object it gives off
potential energy
 The higher you lift an object, the more work you
do and the more energy you give the object
 A machine that causes an object to move
gives the object kinetic energy
Output work is always less
than input work
 Efficiency:
 The ratio of a machine’s output work to the input
work
 An ideal machine would be 100% efficient so all of
the input work would be converted to output work
(not possible due to friction)
 Calculate efficiency:
 Efficiency (%) = (output work ÷ input work) x 100
 If someone does 500J of work on a pair of
pliers and the pliers do 300J of work on a wire,
what is the efficiency of the pliers?
E = Output ÷ Input x 100
Output =
Input =
 The more efficient the machine, the less
mechanical energy is lost
 Some energy is lost to heat (friction)
 The more moving parts the machine has, the
more energy is lost to friction
 Car engine:
 Efficiency is only about 25% due to the heat
generated
 Typical electric motors are about 80% efficient
 Increase efficiency:
 Decrease friction
 Decrease air resistance
14.2:Six simple machines
 There are six machines on which all other
mechanical machines are based:
 Inclined plane
 Lever
 Wheel and axle
 Pulley
 Wedge
 screw
Lever:
 A solid bar that rotates, or turns, around a




fixed point (fulcrum)
Bar can be straight or curved
Can multiply the input force
Can also change the direction of the input
force
3 classes of levers all with different
arrangements of the fulcrum, input force
(effort), and output force (resistance)
 First-class lever:
 Fulcrum is located between the input force and
the output force
 Used to change the direction and size of the force
 Second-class lever:
 Output force is located between the input force
and the fulcrum
 Used when a greater output force is needed
 Third-class lever:
 Input force is between the output force and the
fulcrum
 Used to reduce the distance over which you apply
the input force OR increase the speed of the end
of the lever
1st class
lever
system
2nd class
lever
system
3rd class
lever
system
Wheel and Axle
 Made of a wheel attached to a shaft or axle
 Act as a rotating collection of levers
 Axle at the wheel’s center is like a fulcrum
 Screwdrivers, steering wheels, doorknobs,
electric fans
Pulley
 A wheel with a grooved rim and a rope or
cable that rides in the groove
 As you pull the rope, the wheel moves
 Fixed pulley:
 Pulley that is attached to something that holds it
steady
 Makes work easier by changing the direction of
the force
 You must apply enough force to overcome the
weight of the load and any friction
 Distance you pull the rope is the same distance
that the object is lifted
 Movable pulley:
 One end of the rope is fixed but the wheel can also
move.
 Load is attached to the wheel
 Person pulling the rope provides the output force
that lifts the load
 Single movable pulley does not change the
direction of the force-it multiplies the force
 You would need only half the force which means you
need twice the distance
 Block and tackle system
 Contains both fixed and movable pulleys
 Used to haul and lift very heavy objects
Inclined plane
 A simple machine made of a sloped surface
(ramps)
 Makes work easier by supporting the weight of
the object over the distance it travels
 The less steep the incline, the less force you
need
 Which means the distance will increase
Wedge
 Simple machine with one thick edge and




one thin edge
Used to cut, split, or pierce objects; also to
hold objects together
Can be as simple as a doorstop, a chisel, or
an ice scraper, blade of an ax
Angle of cutting edge determines the
input force needed (thick wedge with large
angle needs more force to cut)
Thin edges provide a smaller surface area
for the force to act upon
Screw
 Inclined plane wrapped around a cylinder or
cone to form a spiral
 Used to raise and lower weights and to fasten
objects
 Distance between the threads of the screw
determine the amount of force needed:
 Threads close together = less force over greater
distance
Calculating MA of specific
machines
 Inclined plane:
 Ideal MA = length of incline ÷ height of incline
 Wheel and Axle:
 Ideal MA = Radius of input ÷ Radius of output
 Lever:
 Ideal MA = distance from input force to fulcrum
distance from output force to fulcrum