Work and Simple
Work and Simple
Machines
What is work?
What is work?
In science, the word
In science, the word
work
work
has a
has a
different meaning than you may be
different meaning than you may be
familiar with.
familiar with.
The scientific definition of work is:
The scientific definition of work is:
using a force to move an object a
using a force to move an object a
distance (when both the force and
distance (when both the force and
the motion of the object are in the
the motion of the object are in the
Work or Not?
Work or Not?
According to the
According to the
scientific definition,
scientific definition,
what is work and
what is work and
what is not?
what is not?
a teacher lecturing a teacher lecturing to her class
to her class
a mouse pushing a a mouse pushing a piece of cheese
piece of cheese
with its nose across
with its nose across
the floor
Work or Not?
Work or Not?
According to the
According to the
scientific definition,
scientific definition,
what is work and
what is work and
what is not?
what is not?
a teacher lecturing a teacher lecturing
to her class
to her class
a mouse pushing a a mouse pushing a
piece of cheese
piece of cheese
with its nose across
with its nose across
the floor
What’s work?
What’s work?
A scientist delivers a speech to an
A scientist delivers a speech to an
audience of his peers.
audience of his peers.
A body builder lifts 350 pounds
A body builder lifts 350 pounds
above his head.
above his head.
A mother carries her baby from room
A mother carries her baby from room
to room.
to room.
A father pushes a baby in a carriage.
A father pushes a baby in a carriage.
A woman carries a 20 kg grocery bag
A woman carries a 20 kg grocery bag
to her car?
What’s work?
What’s work?
A scientist delivers a speech to an
A scientist delivers a speech to an
audience of his peers.
audience of his peers. No
No
A body builder lifts 350 pounds
A body builder lifts 350 pounds
above his head.
above his head. Yes
Yes
A mother carries her baby from room
A mother carries her baby from room
to room.
to room. No
No
A father pushes a baby in a carriage. Yes
A father pushes a baby in a carriage.
Yes
Formula for work
Formula for work
Work = Force x Distance
Work = Force x Distance
The unit of force is newtons
The unit of force is newtons
The unit of distance is meters
The unit of distance is meters
The unit of work is newton-meters
The unit of work is newton-meters
One newton-meter is equal to one joule
One newton-meter is equal to one joule
W=FD
W=FD
Work = Force x
Work = Force x
Distance
Distance
Calculate: If a
Calculate: If a
man pushes a
man pushes a
concrete block 10
concrete block 10
meters with a
meters with a
W=FD
W=FD
Work = Force x
Work = Force x
Distance
Distance
Calculate: If a man
Calculate: If a man
pushes a concrete
pushes a concrete
block 10 meters
block 10 meters
with a force of 20
with a force of 20
N, how much work
N, how much work
has he done?
has he done? 200
200
joules
joules
(W = 20N x 10m)
Power
Power
Power is the rate at which work is
Power is the rate at which work is
done.
done.
Power = Work
Power = Work
**/Time
/Time
**
(force x distance)
(force x distance)
Check for Understanding
Check for Understanding
1.
1.
Two physics students, Ben and Bonnie, are
Two physics students, Ben and Bonnie, are
in the weightlifting room. Bonnie lifts the 50
in the weightlifting room. Bonnie lifts the 50
kg barbell over her head (approximately .60
kg barbell over her head (approximately .60
m) 10 times in one minute; Ben lifts the 50
m) 10 times in one minute; Ben lifts the 50
kg barbell the same distance over his head
kg barbell the same distance over his head
10 times in 10 seconds.
10 times in 10 seconds.
Which student does the most work?
Which student does the most work?
Which student delivers the most
Which student delivers the most
power?
power?
Explain your answers.
Ben and Bonnie do the Ben and Bonnie do the same amount of work; same amount of work;
they apply the same they apply the same force to lift the same force to lift the same
barbell the same barbell the same
distance above their distance above their
heads. heads.
Yet, Ben is the most Yet, Ben is the most
powerful since he does powerful since he does
the same work in less the same work in less
time. time.
Power and time are Power and time are
2. How much power will it take to
2. How much power will it take to
move a 10 kg mass at an acceleration
move a 10 kg mass at an acceleration
of 2 m/s/s a distance of 10 meters in 5
of 2 m/s/s a distance of 10 meters in 5
seconds? This problem requires you to
seconds? This problem requires you to
use the formulas for force, work, and
use the formulas for force, work, and
power all in the correct order.
power all in the correct order.
Force=Mass x Acceleration
Force=Mass x Acceleration
Work=Force x Distance
Work=Force x Distance
Power = Work/Time
2. How much power will it take to move a 10 kg
2. How much power will it take to move a 10 kg
mass at an acceleration of 2 m/s/s a distance of
mass at an acceleration of 2 m/s/s a distance of
10 meters in 5 seconds? This problem requires
10 meters in 5 seconds? This problem requires
you to use the formulas for force, work, and
you to use the formulas for force, work, and
power all in the correct order.
power all in the correct order.
Force=Mass x Acceleration
Force=Mass x Acceleration
Force=10 x 2
Force=10 x 2
Force=20 N
Force=20 N
Work=Force x Distance
Work=Force x Distance
Work = 20 x 10
Work = 20 x 10
Work = 200 Joules
Work = 200 Joules
Power = Work/Time
Power = Work/Time
Power = 200/5
Power = 200/5
Power = 40 watts
History of Work
History of Work
Before engines and motors were invented,
Before engines and motors were invented,
people had to do things like lifting or pushing
people had to do things like lifting or pushing
heavy loads by hand. Using an animal could help,
heavy loads by hand. Using an animal could help,
but what they really needed were some clever
Simple Machines
Simple Machines
Ancient people invented simple machines
Ancient people invented simple machines
that would help them overcome resistive
that would help them overcome resistive
forces and allow them to do the desired
forces and allow them to do the desired
Simple Machines
Simple Machines
The six simple machines are:
The six simple machines are:
Lever
Lever
Wheel and Axle
Wheel and Axle
Pulley
Pulley
Inclined Plane
Inclined Plane
Wedge
Wedge
Simple Machines
Simple Machines
A machine is a device that helps
A machine is a device that helps
make work easier to perform by
make work easier to perform by
accomplishing one or more of the
accomplishing one or more of the
following functions:
following functions:
transferring a force from one place to
transferring a force from one place to
another,
another,
changing the direction of a force,
changing the direction of a force,
Mechanical Advantage
Mechanical Advantage
It is useful to think about a machine
It is useful to think about a machine
in terms of the
in terms of the
input force
input force
(the force
(the force
you apply) and the
you apply) and the
output
output
force
force
(force which is applied to the task).
(force which is applied to the task).
When a machine takes a small input
When a machine takes a small input
force and increases the magnitude of
force and increases the magnitude of
the output force, a
the output force, a
mechanical
mechanical
advantage
Mechanical Advantage
Mechanical Advantage
Mechanical advantage is the ratio of output force Mechanical advantage is the ratio of output force divided by input force. If the output force is
divided by input force. If the output force is
bigger than the input force, a machine has a
bigger than the input force, a machine has a
mechanical advantage greater than one.
mechanical advantage greater than one.
If a machine increases an input force of 10 If a machine increases an input force of 10 pounds to an output force of 100 pounds, the
pounds to an output force of 100 pounds, the
machine has a mechanical advantage (MA) of 10.
machine has a mechanical advantage (MA) of 10.
In machines that increase distance instead of In machines that increase distance instead of force, the MA is the ratio of the output distance
force, the MA is the ratio of the output distance
and input distance.
and input distance.
No machine can increase
No machine can increase
both
both
the magnitude and
the magnitude and
the distance of a force at
the distance of a force at
the same time.
The Lever
The Lever
A lever is a rigid bar A lever is a rigid bar that rotates around a
that rotates around a
fixed point called the
fixed point called the
fulcrum.
fulcrum.
The bar may be either The bar may be either straight or curved.
straight or curved.
In use, a lever has In use, a lever has both an effort (or
both an effort (or
applied) force and a
The 3 Classes of Levers
The 3 Classes of Levers
The class of a lever
The class of a lever
is determined by
is determined by
the location of the
the location of the
effort force and the
effort force and the
load relative to the
load relative to the
fulcrum.
First Class Lever
First Class Lever
In a first-class lever the fulcrum is
In a first-class lever the fulcrum is
located at some point between the
located at some point between the
effort and resistance forces.
effort and resistance forces.
Common examples of first-class levers
Common examples of first-class levers
include crowbars, scissors, pliers, tin
include crowbars, scissors, pliers, tin
snips and seesaws.
snips and seesaws.
A first-class lever always changes the
A first-class lever always changes the
direction of force (I.e. a downward effort
direction of force (I.e. a downward effort
force on the lever results in an upward
force on the lever results in an upward
movement of the resistance force).
Fulcrum is between EF (effort) and RF (load) Fulcrum is between EF (effort) and RF (load)
Effort moves farther than Resistance.
Effort moves farther than Resistance.
Second Class Lever
Second Class Lever
With a second-class lever, the load is
With a second-class lever, the load is
located between the fulcrum and the effort
located between the fulcrum and the effort
force.
force.
Common examples of second-class levers
Common examples of second-class levers
include nut crackers, wheel barrows,
include nut crackers, wheel barrows,
doors, and bottle openers.
doors, and bottle openers.
A second-class lever does not change the
A second-class lever does not change the
direction of force. When the fulcrum is
direction of force. When the fulcrum is
located closer to the load than to the effort
located closer to the load than to the effort
force, an increase in force (mechanical
RF (load) is between fulcrum and EF RF (load) is between fulcrum and EF
Effort moves farther than Resistance.
Effort moves farther than Resistance.
Third Class Lever
Third Class Lever
With a third-class lever, the effort
With a third-class lever, the effort
force is applied between the fulcrum
force is applied between the fulcrum
and the resistance force.
and the resistance force.
Examples of third-class levers include
Examples of third-class levers include
tweezers, hammers, and shovels.
tweezers, hammers, and shovels.
A third-class lever does not change the
A third-class lever does not change the
direction of force; third-class levers
direction of force; third-class levers
always produce a gain in speed and
always produce a gain in speed and
distance and a corresponding decrease
distance and a corresponding decrease
in force.
EF is between fulcrum and RF (load) EF is between fulcrum and RF (load)
Does not multiply force Does not multiply force
Resistance moves farther than Effort.
Resistance moves farther than Effort.
Wheel and Axle
Wheel and Axle
The wheel and axle is The wheel and axle is a simple machine
a simple machine
consisting of a large
consisting of a large
wheel rigidly secured
wheel rigidly secured
to a smaller wheel or
to a smaller wheel or
shaft, called an axle.
shaft, called an axle.
When either the wheel When either the wheel or axle turns, the
or axle turns, the
other part also turns.
other part also turns.
One full revolution of
One full revolution of
either part causes one
either part causes one
full revolution of the
Pulley
Pulley
A pulley consists of a grooved wheel A pulley consists of a grooved wheel
that turns freely in a frame called a that turns freely in a frame called a block.
block.
A pulley can be used to simply change A pulley can be used to simply change
the direction of a force or to gain a the direction of a force or to gain a mechanical advantage, depending on mechanical advantage, depending on how the pulley is arranged.
how the pulley is arranged.
A pulley is said to be a A pulley is said to be a fixed pulleyfixed pulley if it if it
does not rise or fall with the load being does not rise or fall with the load being moved. A fixed pulley changes the moved. A fixed pulley changes the direction of a force; however, it does direction of a force; however, it does not create a mechanical advantage. not create a mechanical advantage.
A A moveable pulleymoveable pulley rises and falls with rises and falls with
the load that is being moved. A single the load that is being moved. A single moveable pulley creates a mechanical moveable pulley creates a mechanical advantage; however, it does not
advantage; however, it does not change the direction of a force. change the direction of a force.
The mechanical advantage of a The mechanical advantage of a
moveable pulley is equal to the moveable pulley is equal to the number of ropes that support the number of ropes that support the moveable pulley.
Inclined Plane
Inclined Plane
An inclined plane is
An inclined plane is
an even sloping
an even sloping
surface. The
surface. The
inclined plane
inclined plane
makes it easier to
makes it easier to
move a weight
move a weight
from a lower to
from a lower to
higher elevation.
Inclined Plane
Inclined Plane
The mechanical The mechanical
advantage of an
advantage of an
inclined plane is equal
inclined plane is equal
to the length of the
to the length of the
slope divided by the
slope divided by the
height of the inclined
height of the inclined
plane.
plane.
While the inclined plane While the inclined plane
produces a mechanical
produces a mechanical
advantage, it does so
advantage, it does so
by increasing the
by increasing the
distance through which
distance through which
the force must move.
Although it takes less force for car A to get to the top of the ramp, Although it takes less force for car A to get to the top of the ramp,
Inclined Plane
Inclined Plane
A wagon trail on a A wagon trail on a
steep hill will often
steep hill will often
traverse back and forth
traverse back and forth
to reduce the slope
to reduce the slope
experienced by a team
experienced by a team
pulling a heavily loaded
pulling a heavily loaded
wagon.
wagon.
This same technique is This same technique is
used today in modern
used today in modern
freeways which travel
freeways which travel
winding paths through
winding paths through
steep mountain passes.
Wedge
Wedge
The wedge is a The wedge is a
modification of the inclined
modification of the inclined
plane. Wedges are used as
plane. Wedges are used as
either separating or
either separating or
holding devices.
holding devices.
A wedge can either be A wedge can either be
composed of one or two
composed of one or two
inclined planes. A double
inclined planes. A double
wedge can be thought of
wedge can be thought of
as two inclined planes
as two inclined planes
joined together with their
joined together with their
sloping surfaces outward.
Screw
Screw
The screw is also a
The screw is also a
modified version of
modified version of
the inclined plane.
the inclined plane.
While this may be
While this may be
somewhat difficult
somewhat difficult
to visualize, it may
to visualize, it may
help to think of the
help to think of the
threads of the
threads of the
screw as a type of
screw as a type of
circular ramp (or
circular ramp (or
inclined plane).
Efficiency
Efficiency
We said that the input force times the distance equals We said that the input force times the distance equals
the output force times distance, or:
the output force times distance, or:
Input Force x Distance = Output Force
Input Force x Distance = Output Force x Distancex Distance
However, some output force is lost due to friction.
However, some output force is lost due to friction.
The comparison of work input to work output is called The comparison of work input to work output is called
efficiency.
Practice Questions
Practice Questions
1. Explain who is doing more work and why: a bricklayer
1. Explain who is doing more work and why: a bricklayer
carrying bricks and placing them on the wall of a building
carrying bricks and placing them on the wall of a building
being constructed, or a project supervisor observing and
being constructed, or a project supervisor observing and
recording the progress of the workers from an observation
recording the progress of the workers from an observation
booth.
booth.
2. How much work is done in pushing an object 7.0 m across
2. How much work is done in pushing an object 7.0 m across
a floor with a force of 50 N and then pushing it back to its
a floor with a force of 50 N and then pushing it back to its
original position? How much power is used if this work is done
original position? How much power is used if this work is done
in 20 sec?
in 20 sec?
3. Using a single fixed pulley, how heavy a load could you lift
Practice Questions
Practice Questions
4.
4. Give an example of a machine in which friction is Give an example of a machine in which friction is both an advantage and a disadvantage.
both an advantage and a disadvantage.
5. Why is it not possible to have a machine with
5. Why is it not possible to have a machine with
100% efficiency?
100% efficiency?
6. What is effort force? What is work input? Explain
6. What is effort force? What is work input? Explain
the relationship between effort force, effort
the relationship between effort force, effort
distance, and work input.
Practice Questions
Practice Questions
1. Explain who is doing more work and why: a bricklayer carrying 1. Explain who is doing more work and why: a bricklayer carrying
bricks and placing them on the wall of a building being bricks and placing them on the wall of a building being
constructed, or a project supervisor observing and recording the constructed, or a project supervisor observing and recording the
progress of the workers from an observation booth.
progress of the workers from an observation booth. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work.
2. How much work is done in pushing an object 7.0 m across a 2. How much work is done in pushing an object 7.0 m across a
floor with a force of 50 N and then pushing it back to its original floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? position? How much power is used if this work is done in 20 sec?
Work = 7 m X 50 N X 2 = 700 N-m or J; Power = 700 N-m/20 sec = 35 W
3. Using a single fixed pulley, how heavy a load could you lift? 3. Using a single fixed pulley, how heavy a load could you lift?
Practice Questions
Practice Questions
4.
4. Give an example of a machine in which friction is both an Give an example of a machine in which friction is both an advantage and a disadvantage.
advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a
desired height without slipping. Disadvantage of friction: It reduces efficiency.
5. Why is it not possible to have a machine with 100% efficiency? 5. Why is it not possible to have a machine with 100% efficiency?
Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100%
efficient.
6. What is effort force? What is work input? Explain the relationship 6. What is effort force? What is work input? Explain the relationship
between effort force, effort distance, and work input.
