Work, Power, and Machines
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Transcript Work, Power, and Machines
Work, Power, and
Machines
9.1
Work
A
quantity that
measures the effects of
a force acting over a
distance
Work = force x distance
W = Fd
Work
Work
is measured
in:
Nm
Joules (J)
Work Example
A
crane uses an
average force of 5200
N to lift a girder 25 m.
How much work does
the crane do?
Work Example
Work
= Fd
Work = (5200 N)(25m)
Work = 130000 N m
= 130000 J
Power
A
quantity that
measures the rate at
which work is done
Power = work/time
P = W/t
Power
Watts
(W) is the SI
unit for power
1 W = 1 J/s
Power Example
While
rowing in a race,
John uses 19.8 N to
travel 200.0 meters in
60.0 s. What is his
power output in Watts?
Power Example
Work
Work
= Fd
= 19.8 N x 200.0 m= 3960 J
Power
= W/t
Power = 3960 J/60.0 s
Power = 66.0 W
Machines
Help
us do work by
redistributing the force
that we put into them
They do not change the
amount of work
Machines
Change
the direction
of an input force (ex
car jack)
Machines
Increase
an output
force by changing the
distance over which the
force is applied
(ex ramp)
Multiplying forces
Mechanical Advantage
A
quantity that
measures how much a
machine multiples force
or distance.
Mechanical Advantage
Input distance
Mech. Adv =
Output Distance
Mech. Adv. =
Output Force
Input Force
Mech. Adv. example
Calculate
the
mechanical advantage
of a ramp that is 6.0 m
long and 1.5 m high.
Mech. Adv. Example
Input
= 6.0 m
Output = 1.5 m
Mech. Adv.=6.0m/1.5m
Mech. Adv. = 4.0
Simple Machines
9.2
Simple Machines
Most
basic machines
Made up of two
families
Levers
Inclined planes
The Lever Family
All
levers have a rigid
arm that turns around a
point called the
fulcrum.
The Lever Family
Levers
are divided into
three classes
Classes depend on the
location of the fulcrum
and the input/output
forces.
First Class Levers
Have
fulcrum in middle
of arm.
The input/output forces
act on opposite ends
Ex. Hammer, Pliers
First Class Levers
Output Force
Input Force
Fulcrum
Second Class Levers
Fulcrum
is at one end.
Input force is applied to
the other end.
Ex. Wheel barrow,
hinged doors,
nutcracker
Second Class Levers
Output Force
Fulcrum
Input Force
Third Class Levers
Multiply
distance
rather than force.
Ex. Human forearm
Third Class Levers
The
muscle contracts
a short distance to
move the hand a
large distance
Third Class Levers
Output distance
Fulcrum
Input Force
Pulleys
Act
like a modified
member of the
first-class lever family
Used to lift objects
Pulleys
Output
Force
Input force
The Inclined Plane
Incline
planes multiply
and redirect force by
changing the distance
Ex loading ramp
The Inclined Plane
Turns
a small input
force into a large
output force by
spreading the work out
over a large distance
A Wedge
Functions
like two
inclined planes back
to back
A Wedge
Turns
a single
downward force into
two forces directed out
to the sides
Ex. An axe , nail
Or Wedge Antilles
from Star Wars
Not to be mistaken
with a wedgIEEEEE
A Screw
Inclined
plane
wrapped around a
cylinder
A Screw
Tightening
a screw
requires less input force
over a greater distance
Ex. Jar lids
Compound Machines
A
machine that
combines two or more
simple machines
Ex. Scissors, bike gears,
car jacks
Energy
9.3-9.4
Energy and Work
Energy
is the ability to
do work
whenever work is done,
energy is transformed
or transferred to
another system.
Energy
Energy
is measured in:
Joules (J)
Energy can only be
observed when work is
being done on an
object
Potential Energy PE
the
stored energy
resulting from the
relative positions of
objects in a system
Potential Energy PE
PE
of any stretched
elastic material is called
Elastic PE
ex. a rubber band,
bungee cord, clock
spring
Gravitational PE
energy
that could
potentially do work on
an object do to the
forces of gravity.
Gravitational PE
depends both on the
mass of the object
and the distance
between them
(height)
Gravitational PE
Equation
grav. PE= mass x gravity x height
PE = mgh
or
PE = wh
PE Example
A
65 kg rock climber
ascends a cliff. What is
the climber’s
gravitational PE at a
point 35 m above the
base of the cliff?
PE Example
PE
= mgh
PE=(65kg)(9.8m/s2)(35m)
4
PE = 2.2 x 10 J
PE = 22000 J
Kinetic Energy
the
energy of a moving
object due to its motion.
depends on an objects
mass and speed.
Kinetic Energy
What
influences energy
more: speed or mass?
ex. Car crashes
Speed does
Kinetic Energy
Equation
KE=1/2 x mass x speed squared
KE = ½
2
mv
KE Example
What
is the kinetic
energy of a 44 kg
cheetah running at
31 m/s?
KE Example
KE
=½
KE=
2
mv
2
½(44kg)(31m/s)
KE=2.1
4
10
x
J
KE = 21000 J
Mechanical Energy
the
sum of the KE and
the PE of large-scale
objects in a system
work being done
Nonmechanical
Energy
Energy that lies at
the level of atoms
and does not affect
motion on a large
scale.
Atoms
Atoms
have KE, because
they at constantly in
motion.
KE particles heat up
KE particles cool down
Chemical Reactions
during
reactions stored
energy (called chemical
energy)is released
So PE is converted to
KE
Other Forms
nuclear
fusion
nuclear fission
Electricity
Light
Energy
Transformations
9.4
Conservation of
Energy
Energy
is neither
created nor destroyed
Energy is transferred
Energy
Transformation
PE
becomes KE
car going down a
hill on a roller
coaster
Energy
Transformation
KE
can become PE
car going up a hill
KE starts converting
to PE
Physics of roller coasters
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