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:
Nm
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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