Transcript Work, Power, and Energy [CH 14

Work, Power & Energy
Work
• Work is the product of force and distance
– 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.
– Any part of a force that does not act in the
direction of motion does no work on an
object.
Work
• Work depends on direction
– If all of the force acts in the same direction
as the motion, all of the force does work.
– If part of the applied force acts in the
direction of motion, that part of the force
does work.
– If none of the force is applied in the
direction of the motion, the force does no
work.
Calculating Work
When using SI units in the work formula, the
force is in newtons, and distance is in meters.
The joule (J) is the SI unit of work. A joule is
equal to 1 newton-meter.
Problem
A weight lifter raises a 1600-newton barbell to
a height of 2.0 meters.
Power
• Power is the rate at which work is done
• 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.
Power
• The SI unit for Power is the watt (W) which
equals 1 joule per second
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?
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?
Problem
You lift a book from the floor to a bookshelf
1.0 m above the ground. How much power is
used if the upward force is 15.0 N and you do
the work in 2.0 s?
Problem
You apply a horizontal force of 10.0 N to pull a
wheeled suitcase at a constant speed of 0.5
m/s across flat ground. How much power is
used? (Hint: The suitcase moves 0.5 m/s.
Consider how much work the force does each
second and how work is related to power.)
Horsepower
• 1 horsepower = 746 watts
• Defined by James Watt
– Way to compare power output of steam engines
in 1700s
Work and Machines
• A machine is something that changes a force
– Machines make work easier to do
• Change size of force needed
• Change the direction of the force
• Change the distance over which the force acts
• Because of friction work done by a machine
(output) is always less than the work done on
the machine (input)
Work Input and Work Output
• Input force
– Force exerted on a machine (this is done by you)
• Input distance
– Distance the input force acts through
• Remember W=Fd
• Work input=input force x input distance
Work Input and Work Output
• Output force
– Force exerted by a machine (what the machine
does)
• Output distance
– Distance the output force is exerted through
• Work output
– Output force multiplied by the output distance
Problem
What is the output distance of a machine that
requires 2 newtons of force exerted over
6 meters and whose output force is
4 newtons?
Mechanical Advantage
• Mechanical advantage of a machine is the
number of times that the machine increases
an input force
• Due to friction the actual mechanical
advantage is always less than the ideal
mechanical advantage
Actual Mechanical Advantage (AMA)
• The actual mechanical advantage (AMA)
equals the ratio of the output force to the
input force.
Ideal Mechanical Advantage (IMA)
• The ideal mechanical advantage (IMA) of a
machine is the mechanical advantage in the
absence of friction.
Problem
A woman drives her car up onto wheel ramps
to perform some repairs. If she drives a
distance of 1.8 meters along the ramp to raise
the car 0.3 meter, what is the ideal mechanical
advantage (IMA) of the wheel ramps?
Problem
A student working in a grocery store after
school pushes several grocery carts together
along a ramp. The ramp is 3 meters long and
rises 0.5 meter. What is the ideal mechanical
advantage of the ramp?
Problem
A construction worker moves a crowbar
through a distance of 0.50 m to lift a load 0.05
m off of the ground. What is the IMA of the
crowbar?
Efficiency
• Efficiency is the percentage of the work input that
becomes work output.
• Due to friction…guess what…efficiency is always
less than 100%
• Reducing friction increases efficiency
Problem
• If the work input of a machine is 250 J and the
work output is 193 J, than what is the total
efficiency of the machine?
Energy
• Energy is the ability to do work, therefore
work is the transfer of energy
– The SI unit for energy is a joule (J)
• The energy of motion is called kinetic energy
(KE)
Problem
A 0.10-kilogram bird is flying at a constant
speed of 8.0 m/s. What is the bird’s kinetic
energy?
Problem
A 50.0-kilogram cheetah has a kinetic energy
of 18,000 J. How fast is the cheetah running?
Energy
• Potential Energy (gravitational potential
energy) is energy stored as a result of position
or shape
Remember g = 9.8 m/s2
Problem
What is the potential energy relative to the
water surface of a diver at the top of a 10.0meter-high diving platform. Suppose she has a
mass of 50.0 kilograms.
Elastic Potential Energy
• The potential energy of an object that is
stretched or compressed
Forms of energy
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Mechanical energy
Thermal energy
Chemical energy
Electrical energy
Electromagnetic energy
Nuclear energy
Mechanical Energy
• The energy associated with the motion and
position of everyday objects is mechanical
energy.
• Mechanical energy is the sum of an object’s
potential energy and kinetic energy.
Thermal Energy
• The total potential and kinetic energy of all
the microscopic particles in an object make up
its thermal energy.
• When an object’s atoms move faster, its
thermal energy increases, and the object
becomes warmer.
Chemical, Electrical, EM, and Nuclear
Energy
• Chemical energy is the energy stored in chemical bonds.
• Electrical energy is the energy associated with electric
charges.
• Electromagnetic energy is a form of energy that travels
through space in the form of waves.
• The energy stored in atomic nuclei is known as nuclear
energy
– Nuclear Fission – releases energy by splitting an atom
– Nuclear Fusion releases energy by combining atoms
Energy Conversion
• Energy can be converted from one form to
another
Law of Conservation of Energy
• Energy cannot be created nor destroyed, but
only transformed
• In a closed system, energy input = energy
output
– Ex. Falling objects: PE=KE; mgh=1/2mv2
Mechanical Energy
• Mechanical Energy (ME) = KE + PE
Problem
At a construction site, a 1.50-kg brick is
dropped from rest and hits the ground at a
speed of 26.0 m/s. Assuming air resistance can
be ignored, calculate the gravitational
potential energy of the brick before it was
dropped.
Problem
A 10-kg rock is dropped and hits the ground
below at a speed of 60 m/s. Calculate the
gravitational potential energy of the rock
before it was dropped. You can ignore the
effects of friction.
Problem
A diver with a mass of 70.0 kg stands
motionless at the top of a 3.0-m-high diving
platform. Calculate his potential energy
relative to the water surface while standing on
the platform, and his speed when he enters
the pool. (Hint: Assume the diver’s initial
vertical speed after diving is zero.)
Problem
A pendulum with a 1.0-kg weight is set in
motion from a position 0.04 m above the
lowest point on the path of the weight. What
is the kinetic energy of the pendulum at the
lowest point? (Hint: Assume there is no
friction.)
Energy and Mass
• Einstein’s Theory of Special Relativity
– E=mc2
– Energy and mass are equivalent and can be
converted into each other
– E is energy, m is mass, and c is the speed of light
– c = 3x108 m/s or 176,000 mi/s