36
Work, Power and Simple Machines Chapter 4 Physical Science
Work, Power and Simple
Machines
Chapter 4
Physical Science
Work, Power and Simple Machines
Machines make jobs
easier by increasing the
applied force on an
object.
The trade-off is that this
also requires an
increase in the distance
over which the force
must be applied.
Types of Simple Machines:
Types of Simple machines:
1. inclined plane
2. wedge
3. screw
4. lever
5. pulley
6. wheel and axel
Modern machines use a combination of these simple machines.
Work:
Work is the amount of force applied to an
object times the distance the object
moves in the direction of that force.
i.e. work = force • distance
or
W = Fd
1 joule (J) = 1 newton (N) • 1 meter (m) or
1 joule (J) = Newton•meter (N•m)
Common misconceptions about work:
misconception…Truth
Work involves time…It does NOT involve time.
Even if an object does not move, work is still
done…No, work is NOT being done.
Work is always positive…it CAN be negative.
Calculating Work:
Q. What amount of work is done when a force
of 300 N moves an object a distance of 2 m?
A. W = Fd
W = (300 N)(2 m)
= 600 N•m = 600 J
Q. A force of 550 N was used to move a stone
23 m. How much work was done?
A. W = Fd
= (550 N)(23 m)
= 12650 N•m = 12650 J
Calculating Work:
Q. If 34,560 J of work was done with a force
of 960 N, how far was the object moved?
A. W = Fd
d = W
F
= 34,560 J
960 N
= 36 m
Power:
Power is the rate at which work is done.
This requires a unit of time.
Power = work = W time t
Units of power are joules/second (J/s) or watt (W).
The watt rating on a lamp bulb or electric motor is a measure of its power.
https://www.khanacademy.org/video/power
Horsepower:
James Watt, Scottish
inventor, 1782, as a
marketing tool for
the sale of his improved
steam engine, developed
a comparative measure
of power.
Horsepower:
Horsepower (hp) is equal to the amount of
sustained power of 1 draft horse.
One draft horse can move a 330-pound
object 100 feet in one minute or 33,400 foot-
pounds per minute (550 ft-lbs/s).
The metric equivalent is a 750-newton object pulled 1
meter in 1 second.
i.e. 1hp = 750 watts (more precisely 745.56 W)
Horsepower:
Relating horsepower
Car engine = ~ 170 hp
Diesel train = ~ 10,000 hp
Nuclear power plant = ~ 300,000 hp
Although the European Union as of 1
January 2010 has banned the use of
horsepower rating, it will likely continue in
use for years to come in the U.S.
Calculating Power:
Q. What is the power of a 900 N
force applied over a distance of
40 m for 45 seconds? (2 steps)
1) W = Fd
900 N x 40 m = 36,000 N•m
= 36,000 J
2) P = W/t
36,000 J / 45 s = 800 J/s
= 800 W
Calculating Power:
Q. What is the power of a 300
N force applied over 20 m for
20 seconds? (2 steps)
W = Fd
300 N x 20 M = 6000N•m
= 6000 J
P = W/t
6000 J / 20 s = 300 J/s
= 300 W
Machines:
People by nature seek to make tasks easier (more
efficient use of the time.)
Machines make work easier by changing the size or
direction of the applied force.
Forces of Machine Use:
Effort Force (FE) is the force applied to
the machine.
Resistance Force (FR) is the force
applied by the machine. Resistance force always opposes effort force and
is usually equal to the weight of the object being
moved.
Effort:
When effort force is applied, work done is
measured as work input (WI).
Effort distance (dE) is the distance that a
machine moves due to effort force (FE)
WI = FE x dE
i.e. Work input using a lever is equal to the
force applied to the handle times the
distance the handle moves.
Resistance:
Resistance force (FR) is the force applied by the machine.
Resistance distance (dR) is the distance the machine moves the resistance.
Work output (WO) is the work done by the machine.
Resistance:
WO = FR x dR
Work output is
NEVER greater than
work input.
Machines multiply
force, not work.
Example of Work Output and Input:
A force of 50 N is required to move a 200-N block of granite a distance of 1 m using a lever. Assuming that WI and WO are equal, what is the effort distance of the lever?
FR =200 N FE = 50 N
de = ?
dR = 1 m
WO = FR x dR
Since WO = WI then WI =
dE = WI =
FE
200 J =
50 N
4 m = 200 N x 1 m
= 200 J
200 J
WI = FE x dE
Mechanical Advantage:
Mechanical advantage (MA) is number
of times a machine multiplies the effort
force.
MA = FR / FE
Back to the example:
Mechanical advantage of using the lever to
lift the granite block.
MA = 200 N / 50 N = 4
Mechanical Advantage:
A machine used to change the direction of an
object may have a mechanical advantage of
one.
e.g. If I need to lift a 1000-N object 1m
upward, it would be easier to use a lever or
fixed pulley and apply my 1000-N body to
raise the object rather than using my back.
A machine that has a mechanical advantage less
than one is used to move objects greater distances
or faster.
Efficiency:
Efficiency (Eff) is the ratio of WO to WI expressed as percent.
Eff = WO / WI x 100
WO can never be greater than WI because of friction.
i.e. Efficiency can never be greater than 100%. No machine is 100% efficient.
Modern machines use lubricants and bearings to reduce friction and increase efficiency.
Still, the average car is only about 20% efficient.
Simple Machines:
1. Inclined plane
An inclined plane is a slanted surface or ramp used to raise an object.
Inclined planes reduce the force needed to lift an object, but require a greater distance.
MA of inclined plane = length of plane
height of plane
Simple Machines:
• MA will always be greater than one, since
the length of the plane must be greater than
it’s height.
i.e. When an inclined plane is used, effort is
always less than resistance, but some work
is always lost by friction.
Resistance
Effort
2. Wedges:
A wedge is a
moveable inclined
plane.
The thinner (sharper) the
wedge, the less effort is
needed to overcome a
resistance force;
therefore, the greater the
MA.
3. Screws:
A screw is similar to
an inclined plane by
multiplying effort
force through a
longer effort
distance.
The closer the threads
are, the greater the
MA.
4. Levers:
A lever is any bar
that pivots on a
fulcrum (fixed
point).
4. Levers Cont’d:
Classes of Levers:
1st class - multiplies and changes the effort force.
e.g. pliers, crowbar, ice tongs
1st class levers can have a MA of one, more than one, or less than one, depending on the placement of the fulcrum.
4. Levers Cont’d
2nd class - multiplies the
effort with no change in
direction.
e.g. wheelbarrow, door
Resistance force is between
the fulcrum and the effort
force.
2nd class levers always
multiply the effort force (see
MA of levers below).
4. Levers Cont’d:
3rd class - multiplies
the distance, not the
effort force since the
effort arm is always
less than the resistance
arm.
e.g. hammer,
baseball bat
Effort is between the
resistance and the
fulcrum
Mechanical Advantage of Levers:
MA = effort arm length
resistance arm length
Where: effort arm length is the distance from the
fulcrum to the effort force and resistance arm
length is distance from the fulcrum to the
resistance force
If a lever has a mechanical advantage of one, then
the effort equals resistance
Resistance arm Effort arm
6. Wheel and Axle
A wheel and axle is a
rotating lever made up
of two different size
wheels.
The effort force applied
to a wheel is multiplied
at the axle (inner
wheel).
MA = wheel radius (rw)
axle radius (ra)
6. Wheel and Axle
The fulcrum is the
center of the wheel
and axel.
The wheel radius is
the effort arm.
The axle radius is
the resistance arm. i.e. The bigger the
wheel, the greater
the MA.
5. Pulley:
A pulley is a chain, belt, or rope wrapped around a wheel. Pulleys can either
change the direction or the amount of effort force.
Fixed pulleys (attached to a ceiling or frame) can only change the direction of an effort force, NOT increase an effort force.
i.e. MA = 1 using a fixed pulley
https://youtu.be/BJ9MELhhW6U
5. Pulley, Cont’d:
A moveable pulley can multiply an effort force, but the effort distance increases.
Additional pulleys added increase the MA. MA = number of
supporting ropes.
Compound Machines
Note: Compound
Machines use a
combination of 2 or
more simple
machines.
e.g. cars, washing
machines, tape
players, watches,
fishing reels, etc.
