Work and Simple
Machines
What is work?
 In
science, the word work has a
different meaning than you may be
familiar with.
 The scientific definition of work is:
using a force to move an object a
distance (when both the force and
the motion of the object are in the
same direction.)
Work or Not?

According to the
scientific
definition, what is
work and what is
not?


a teacher lecturing
to her class
a mouse pushing a
piece of cheese
with its nose
across the floor
Work or Not?

According to the
scientific
definition, what is
work and what is
not?


a teacher lecturing
to her class
a mouse pushing a
piece of cheese
with its nose
across the floor
5
What’s work?
A
scientist delivers a speech to an
audience of his peers.
 A body builder lifts 350 pounds above
his head.
 A toddler pushes a stationary wall.

A father pushes a baby in a carriage.
A
woman places a 20 kg grocery bag
into her car
What’s work?
A
scientist delivers a speech to an
audience of his peers. No
 A body builder lifts 350 pounds
above his head. Yes
 A toddler pushes a stationary wall.
No

A father pushes a baby in a carriage. Yes
A
woman places a 20 kg grocery bag
into her car Yes
Formula for work
Work = Force x Distance
 The
unit of force is newtons
 The unit of distance is meters
 The unit of work is newton-meters
 One newton-meter is equal to one joule
 So, the unit of work is a joule
W=FD
Work = Force x
Distance
Calculate: If a man
pushes a concrete
block 10 meters
with a force of 20
N, how much work
has he done?
W=FD
Work = Force x
Distance
Calculate: If a man
pushes a concrete
block 10 meters
with a force of 20
N, how much work
has he done? 200
joules
(W = 20N x 10m)
Power
 Power
is the rate at which work is
done.
 Power
 The
= Work*/Time
*(force x distance)
unit of power is the watt.
Check for Understanding
How much power will it take to move
a 10 kg mass at an acceleration of 2
m/s/s a distance of 10 meters in 5
seconds? This problem requires you
to use the formulas for force, work,
and power all in the correct order.
Force=Mass x Acceleration
Work=Force x Distance
Power = Work/Time
How much power will it take to move a 10 kg
mass at an acceleration of 2 m/s/s a distance of
10 meters in 5 seconds? This problem requires
you to use the formulas for force, work, and
power all in the correct order.
Force=Mass x Acceleration
Force=10 x 2
Force=20 N
Work=Force x Distance
Work = 20 x 10
Work = 200 Joules
Power = Work/Time
Power = 200/5
Power = 40 watts
Definitions:
Force: A Push or a Pull
Energy: Ability to do work
Work= Force x Distance
Power= Work / Time
History of Work
Before engines and motors were invented, people
had to do things like lifting or pushing heavy loads by
hand. Using an animal could help, but what they really
needed were some clever ways to either make work
easier or faster.
Simple Machines
Ancient people invented simple
machines that would help them overcome
resistive forces and allow them to do the
desired work against those forces.
Simple Machines
A
machine is a device that helps
make work easier to perform by
accomplishing one or more of the
following functions:
 transferring
a force from one place to
another,
 changing the direction of a force,
 increasing the magnitude of a force, or
 increasing the distance or speed of a
force.
Mechanical Advantage
 It
is useful to think about a machine
in terms of the input force (the force
you apply) and the output force
(force which is applied to the task).
 When a machine takes a small input
force and increases the magnitude of
the output force, a mechanical
advantage has been produced.
Mechanical Advantage




Mechanical advantage is the ratio of output force
divided by input force. If the output force is
bigger than the input force, a machine has a
mechanical advantage greater than one.
If a machine increases an input force of 10
pounds to an output force of 100 pounds, the
machine has a mechanical advantage (MA) of 10.
In machines that increase distance instead of
force, the MA is the ratio of the output distance
and input distance.
MA = output/input
No machine can increase
both the magnitude and
the distance of a force at
the same time.
Efficiency

We said that the input force times the distance
equals the output force times distance, or:
Input Force x Distance = Output Force x Distance
However, some output force is lost due to friction.

The comparison of work input to work output is
called efficiency.

No machine has 100 percent efficiency due to
friction.
The 6 Simple Machines
Inclined Plane
Screw
Pulley
Lever
Wedge
Wheel and Axle
Inclined Plane
Inclined Plane

The Egyptians used simple machines to build the
pyramids. One method was to build a very long
incline out of dirt that rose upward to the top of the
pyramid very gently. The blocks of stone were
placed on large logs (another type of simple machine
- the wheel and axle) and pushed slowly up the long,
gentle inclined plane to the top of the pyramid.
Inclined Planes

An inclined plane is
a flat surface that is
higher on one end
 Inclined planes
make the work of
moving things easier
Work input and output
 Work
input is the amount of work
done on a machine.
 Input force x input distance
 Work
output is the amount of work
done by a machine.
 Output force x output distance
Wout = Win
Dout
Fout x Dout = Fin x Din
3m
10N x 3m = 2N x 15m
10 N
Din 15 m
Fin
Inclined Plane Mechanical Advantage


The mechanical
advantage of an
inclined plane is equal
to the length of the
slope divided by the
height of the inclined
plane.
While the inclined
plane produces a
mechanical advantage,
it does so by
increasing the distance
through which the
force must move.
Screw
The mechanical advantage of an screw can be
calculated by dividing the circumference by the pitch of
the screw.
Pitch equals 1/ number of turns per inch.
Wedges
Two inclined
planes joined
back to back.
 Wedges are used
to split things.

Wedge – Mechanical Advantage

The mechanical advantage of a wedge can be found
by dividing the length of either slope (S) by the
thickness (T) of the big end.
S
T

As an example, assume that the length of the slope
is 10 inches and the thickness is 4 inches. The
mechanical advantage is equal to 10/4 or 2 1/2. As
with the inclined plane, the mechanical advantage
gained by using a wedge requires a corresponding
increase in distance.
First Class Lever
Fulcrum is between EF (effort) and RF (load)
Effort moves farther than Resistance.
Multiplies EF and changes its direction
The mechanical advantage of a lever is the ratio of the length
of the lever on the applied force side of the fulcrum to the
length of the lever on the resistance force side of the fulcrum.
First Class Lever
.

Common examples
of first-class
levers include
crowbars, scissors,
pliers, tin snips and
seesaws.
Second Class Lever
RF (load) is between fulcrum and EF
Effort moves farther than Resistance.
Multiplies EF, but does not change its direction
The mechanical advantage of a lever is the ratio of the
distance from the applied force to the fulcrum to the
distance from the resistance force to the fulcrum.
Second Class Lever
 Examples
of secondclass levers include nut
crackers, wheel barrows,
doors, and bottle openers.
Third Class Lever
EF is between fulcrum and RF (load)
Does not multiply force
Resistance moves farther than Effort.
Multiplies the distance the effort force travels
The mechanical advantage of a lever is the ratio of the
distance from the applied force to the fulcrum to the
distance of the resistance force to the fulcrum
Third Class Lever
 Examples
of
third-class
levers include
tweezers, arm
hammers, and
shovels.
Pulleys

Pulley are wheels
and axles with a
groove around the
outside
 A pulley needs a
rope, chain or belt
around the groove
to make it do work
Diagrams of Pulleys
Fixed pulley:
Movable Pulley:
A fixed pulley changes the
direction of a force;
however, it does not create
a mechanical advantage.
The mechanical advantage
of a moveable pulley is
equal to the number of
ropes that support the
moveable pulley.
COMBINED PULLEY

The effort needed to
lift the load is less
than half the weight
of the load.
 The main
disadvantage is it
travels a very long
distance.
WHEEL AND AXEL

The axle is stuck
rigidly to a large
wheel. Fan blades
are attached to the
wheel. When the
axel turns, the fan
blades spin.
Wheel and Axel

The mechanical advantage of a wheel and axle is the
ratio of the radius of the wheel to the radius of the axle.
1

5
In the wheel and axle illustrated above, the radius of the
wheel is five times larger than the radius of the axle.
Therefore, the mechanical advantage is 5:1 or 5.
 The wheel and axle can also increase speed by
applying the input force to the axle rather than a wheel.
This increase is computed like mechanical advantage.
This combination would increase the speed 5 times.
GEARS-Wheel and Axel

Each gear in a
series reverses the
direction of
rotation of the
previous gear. The
smaller gear will
always turn faster
than the larger
gear.
Rube Goldberg Machines




Rube Goldberg machines are
examples of complex machines.
All complex machines are made
up of combinations of simple
machines.
Rube Goldberg machines are
usually a complicated combination
of simple machines.
By studying the components of
Rube Goldberg machines, we
learn more about simple machines
Safety Device for Walking on Icy Pavements
When you slip on ice, your foot kicks paddle (A),
lowering finger (B), snapping turtle (C) extends neck
to bite finger, opening ice tongs (D) and dropping pillow (E),
thus allowing you to fall on something soft.
Mousetrap
 http://www.cnn.com/2012/10/10/us/idea
s-lifesizemousetrap/index.html?iref=allsearch