Transcript Ch 14 Work, Power and Simple Machines

Chapter 14
Work, Power and Simple Machines
Questions to think about before…
• What does work mean to you???
• List some examples of work:
Is this work???
Work & Science
• Now...think about work in terms of science...it
probably means something very different than
what you listed above.
14.1: Work and Power
• What is work?
• Recall...From Chapter 12
• Question: How does an unmoving object begin
moving?
Answer…
• Answer: When an unbalanced force acts on it.
• Work: the product of force and distance
• Work is done when a force acts on an object in the
direction the object moves.
I s wo r k b e i n g d o n e ?
Work Requires Motion
Question: Does a weight lifter do work on the
barbell to lift it over his head?
Answer: yes, force is up and barbell moves up
Stationary Objects
• Question: Is the weight lifter doing work while he
holds the barbell stationary over his head?
ANSWER
• Answer: NO, the barbell is stationary
• For a force to do work on an object, some of the
force must act in the same direction as the object
moves. If there is NO movement, NO work is
done!!!
Work Depends on Direction
• The amount of work done on an object, if any,
depends on the direction of the force and the
direction of the movement.
• A force does not have to act entirely in the
direction of movement to do work.
Is work being done?
Is work being done????
• The force acts upward and to the right.
• The suitcase only moves to the right.
• Any part of a force that does not act in the
direction of motion does NO work on an object
Calculating Work
• Work = Force x Distance
• Units of Work
– SI unit for force is newtons
– SI unit for distance is meters
JOULE
• The SI unit for work is newton-meter or the
JOULE (J)
• When a force of 1 newton moves an object 1
meter in the direction of the force, 1 joule of work
is done.
Practice Problem
• Imagine the weight lifter. The weight lifter lifts a
1600 newton barbell over his head. Assume the
barbell is lifted to a height of 2.0 meters. What is
the work done?
• Work = Force x Distance
Practice Problem Answered
Work = 1600 N x 2.0 m
Work = 3200 N m = 3200 J
What is Power?
• Power: the RATE of doing work
• Doing work at a faster rate requires more
power. To increase power, you can increase the
amount of work done in a given time, or you can
do a given amount of work in less time
Q: Does a person shoveling snow do work?
• Answer: YES, because the shovel is moving in
the same direction as the force being applied
Q: Does a snow blower do work?
• Answer: YES, but because the snow blower does
the work in less time it has more POWER!!!
Calculating Power
• Power = Work / Time
– Work is in joules (J)
– Time is in seconds (s)
• The SI unit for POWER is the watt (W) =
one joule per second
– Thus, a 40-watt light bulb requires 40 joules
each second that it is lit.
Practice Problem
• You exert a vertical force of 72 newtons to lift a
box to a height of 1.0 meter in a time of 2.0
seconds. How much power is used to lift the
box?
Practice Problem Answered
Power = work / time
OR can be written as:
Power = (Force x Distance) / Time
(72 N x 1.0 m)/ 2.0 s = 36 J/s = 36 Watts
James Watt and Horsepower
Horsepower
• Horsepower (hp): common unit for power. One
horsepower is equal to about 746 watts.
• FYI...Interesting side note: Horsepower is literally
based on the power output of a very strong
horse!!!
14.2 Work and Machines
• Machine = a device that changes a force
• Machines make work easier to do. They can:
– Change the size of the force needed
– The direction of a force
– The distance over which the force acts
– However…
They can’t do work for us!
Increasing a force
• Ex: a car jack
– Each rotation of the jack applies a small force
over a large distance and the car is lifted a small
distance
• Tradeoff = total distance traveled is much greater
Increasing Distance
• Ex: oars of a boat
– You move oars a small distance and the end in
the water moves a large distance
• Tradeoff = increased travel of the oar requires you
to exert a greater force
Changing Direction
• Ex: pulley
– You pull down on the rope and the load moves
up
14.3 Mechanical Advantage
• Mechanical Advantage = the number of times that
the machine increase an input force
• MA = load force/effort force
• Q: Using a lever, a person is able to lift a 100N
object using only 20N of force. Calculate the MA
of this machine
• A: AMA = 100/20 = 5
• In other words, this machine has multiplied the
effort force 5 times.
• Ideal Mechanical Advantage = MA without
friction
• IMA = Input Distance/Output Distance
• Q: A woman drives her car onto a ramp. She
drives 1.8 meters along the ramp to raise it 0.3m
off the ground. Calculate IMA
• A: IMA = 1.8m/0.3m = 6
14.4 Simple Machines
• The six types of simple machines are:
– Lever
– Wheel and axle
– Inclined plane
– Wedge
– Screw
– Pulley
Lever
3 classes of levers
Wheel and axle
Inclined Plane
Wedge
Screw
Pulley
Chapter 15 Energy
15.1 Energy and Its Forms
What is Energy?
• Energy- the ability to do work
• Energy is transferred by a force moving an
object through a distance
Work & Energy
• Energy is closely related to work
– Work is a transfer of energy
– When work is done on an object, energy
is transferred to that object
– Both are typically measured in joules (J)
Types of Energy
• Energy can be classified as two general
types:
– kinetic energy
– potential energy.
Kinetic Energy
Kinetic Energy
• Kinetic energy - (KE) the energy of motion
• The kinetic energy of any moving object
depends on two things:
– Mass of the object
– Speed of the object
• To calculate the KE of an object, use the
following formula:
KE = ½ mv2
KE = ½mv2
• Notice that doubling the mass doubles the
KE
• But, if you double the speed you quadruple
the KE!
Practice Problem
• A 70kg man is walking at a speed of 2m/s.
Calculate his KE.
• Show your work!
Practice Problem Solved
• KE = ½ 70kg x (2m/s)2
• KE = 35kg x 4m/s = 140J
Potential energy
Potential Energy
• Potential energy: energy that is stored as a
result of position or shape
• Energy that is stored has the ability to do
work!
• There are two types of potential energy:
– Gravitational potential energy and
– Elastic potential energy
GPE
• Gravitational potential energy depends on
an object’s mass, height, and acceleration
due to gravity.
• GPE = m x g
–
–
–
–
x
h or GPE = w
m = mass (kg)
g= acceleration due to gravity
h= height
Remember m x g = w (N)
x
h
GPE
Calculate the GPE in the picture below
Show your work here:
• 75kg x 9.8 m/s/s x 4m = 2940 J
Practice Problem
• A diver at the top of a 10 m high platform
has a mass of 50kg. Calculate GPE
Practice Problem Solved
• GPE = 50kg x 9.8m/s2 x 10m = 4900J
Elastic Potential Energy
• Elastic potential energy – the PE of an
object that is stretched or compressed.
– Something is said to be elastic if it
springs back to its original shape after
being stretched or compressed
– Example: rubber band, basketball
EPE
Mechanical Energy
• Mechanical energy- the energy associated
with the motion and position of everyday
objects
– The sum of an object’s PE and KE
Further Classification of Energy
• Energy can be potential or kinetic, but it
can be further classified into different
types of energy:
– Thermal energy
– Electrical energy
– Nuclear energy
– Chemical Energy
– Electromagnetic Energy
Thermal Energy
Thermal Energy
• Thermal energy- the total potential and
kinetic energy of all the microscopic
particles in an object
– When atoms move faster thermal energy
increases causing the object to become
warmer
Chemical Energy
Chemical Energy
• Chemical energy- energy stored in chemical
bonds.
– When the bonds are broken and new
bonds form, the released energy can do
work
– Examples:
• fuel like gasoline
• Food
• Any chemical fuel stores energy
Electrical Energy
Electrical Energy
• Electrical energy- energy associated with
moving electric charges
– Electric charges exert forces that do
work
– Examples:
• electricity
• lightning
Electromagnetic Energy
Electromagnetic Energy
• Electromagnetic energy- energy that
travels through space in the form of waves
– Can travel long distances through air and
space
– Often used for communication
– Examples:
• visible light
• x-rays
• radio waves
Nuclear Energy
Nuclear Energy
• Nuclear energy- energy stored in atomic
nuclei
– Fission- release of energy by splitting
nuclei
– Fusion- release of energy when less
massive nuclei combine to form a more
massive nuclei
– Example: heat and light from the sun
15.2 Conversion and Conservation of Energy
Conversion
• Energy can be converted from one form to
another
• Energy conversion = the process of
changing energy from one form into
another
Example: a wind-up toy converts PE into KE
when it unwinds
Energy Conservation
• As one form of energy converts into
another form the total energy remains the
same!!!
• The law of conservation of energy states
that energy can NOT be created or
destroyed.
Energy Conservation
• Question: Why do you slow down after you
stop pedaling your bike?
• Where did the bike’s KE go?
Energy Conservation
• Answer: Friction!
– Since we do not live in a frictionless
world, we have to take it into
consideration…
– The work done by this frictional force
changes KE into thermal energy.
– When the energy lost to frictional
forces is accounted for all energy is
conserved!
GPE to KE
The gravitational PE of an object is converted to the KE
of motion as the object falls.
Pendulum Conversions
Bouncing ball
Energy Conversion Calculations
• When friction is small enough to be
ignored, an object’s mechanical energy does
not change.
• Remember: mechanical energy is the
TOTAL KE and TOTAL PE of an object
• Mechanical Energy = KE + PE
Energy is Conserved
• The total mechanical energy at the
beginning of the conversion must equal the
total mechanical energy at the end!
(KE + PE)beginning = (KE + PE)end
Practice Problem
• At a construction site, a 1.5kg brick is
dropped from rest and hits the ground at a
speed of 26 m/s. Assuming air resistance
can be ignored, calculate the GPE of the
brick before it was dropped.
Practice Problem Answered
• (KE + PE)beg = (KE + PE)end
• (½
x
1.5kg
x
• 507 J = PE
(26m/s)2 + 0)end = (0 + PE)beg
Tying it all in to Nuclear Chemistry
• Nuclear Chemistry Connection/Review:
– Remember Einstein’s equation? E = mc2
– This equation says that energy and mass are
equivalent and can be converted into each
other.
Nuclear Chemistry
• In other words, energy is released as
matter is destroyed and matter can be
created from energy.
• Remember the law of conservation of mass
was modified to account for this, and says
that mass and energy together are always
conserved.