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Work, Power, and Machines

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Work, Power, and Machines What is Work? transfer of energy to a body by application of a force that causes body to move in direction of force. W = F d What is Work? – PowerPoint PPT presentation

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Title: Work, Power, and Machines


1
Work, Power, and Machines
2
What is Work?
  • transfer of energy to a body by application of a
    force that causes body to move in direction of
    force.
  • W F ? d

3
What is Work?
Chapter 12
  • SI units
  • joules (J).
  • 1 J 1 Nm 1 kgm2/s2

4
WORK
  • Imagine a father playing with his daughter by
    lifting her repeatedly in the air. How much work
    does he do with each lift, assuming he lifts her
    2.0 m and exerts an average force of 190 N?

GIVEN W ? F 190 N d 2.0 m
WORK W Fd W (190 N) (2.0 m) W 380 J
5
WORK
  • A crane uses an average force of 5200 N to lift
    a girder 25 m. How much work does the crane do
    on the girder?

GIVEN W ? F 5200 N d 25 m
WORK W Fd W (5200 N) (25 m) W 130,000 J or
1. 3 x 105 J
6
WORK
  • An apple weighing 1 N falls through a distance
    of 1 m. How much work is done on the apple by
    the force of gravity?

GIVEN W ? F 1 N d 1m
WORK W Fd W (1 N) (1 m) W 1 J
7
Power
Chapter 12
  • rate at which work is done or energy is
    transformed.
  • SI Unit
  • watts.
  • watt (W) 1 J/s
  • Power work
  • time

p W/t
8
POWER
  • It takes 100 kJ of work to lift an elevator 18
    m. If this is done in 20 s, what is the average
    power of the elevator during the process?

GIVEN p ? W 1 x 105 J t 20 s
WORK p W/t p 1 x 105 J / 20 s p 5 x 103 W
or 5 kW
9
POWER
  • While rowing across the lake during a race, John
    does 3960 J of work on the oars in 60.0 s. What
    is his power output in watts?

GIVEN p ? W 3960 J t 60 s
WORK p W/t p 3960 J / 60 s p 66.0 W
10
POWER
  • Using a jack, a mechanic does 5350 J of work to
    lift a car 0.500 m in 50.0 s. What is the
    mechanics power output?

GIVEN p ? W 5350 J t 50 s
WORK p W/t p 5350 J / 50 s p 107 W
11
Machines
  • Machines
  • multiply and redirect forces.
  • help people by redistributing work put into them.
  • change either size or direction of input force.
  • allows same amount of work to be done by
  • either decreasing distance while increasing force
    or
  • by decreasing force while increasing distance.

12
Force and Work
Chapter 12
13
Mechanical Advantage
  • tells how much a machine multiplies force or
    increases distance.
  • mechanical advantage output force input
    distance
  • input force output distance

14
MECHANICAL ADVANTAGE
  • Calculate the mechanical advantage of a ramp
    that is 5.0 m long and 1.5 m high.

GIVEN ma ? id 5.0 m od 1.5 m
WORK ma id/od ma 5.0 m / 1.5 m ma 3.3
15
MECHANICAL ADVANTAGE
  • Calculate the mechanical advantage of a ramp
    that is 6.0 m long and 1.5 m high.

GIVEN ma ? id 6.0 m od 1.5 m
WORK ma id/od ma 6.0 m / 1.5 m ma 4
16
MECHANICAL ADVANTAGE
  • Determine the mechanical advantage of an
    automobile jack that lifts a 9900 N car with an
    input force of 150 N.

GIVEN ma ? of 9900 N if 150 N
WORK ma of/if ma 9900 N / 150 N ma 66
17
SIMPLE MACHINES
18
The Lever Family
  • simple machines
  • One of six basic types of machines which are
    basis for all other forms of machines.
  • have a rigid arm and a fulcrum.
  • six types divided into two families.

19
First Class Levers
  • fulcrum located between points of application of
    input and output forces.

20
Second Class Levers
  • fulcrum is at one end of arm and input force is
    applied to other end.

21
Third Class Levers
  • multiply distance rather than force.
  • have a mechanical advantage of less than 1.

22
Pulleys
  • are modified levers.
  • point in middle of a pulley is like fulcrum of a
    lever.
  • single, fixed pulley has a m. a. of 1.
  • block and tackle Multiple pulleys working
    together

23
Wheel Axle
  • a lever or pulley connected to a shaft.
  • steering wheel of a car, screwdrivers, and cranks

24
The Inclined Plane Family
  • multiply and redirect force.
  • turns small input force into large output force
    by spreading work out over a large distance.

25
Simple Inclined Plane
  • Changes both magnitude direction of force

26
Wedge
  • Functions as two inclined planes back to back.
  • Turns single downward force into two forces
    directed out to sides.

27
Screw
  • an inclined plane wrapped around a cylinder.

28
Compound Machines
Chapter 12
  • machine made of more than one simple machine
  • Examples
  • scissors
  • two first class levers joined at a common fulcrum
  • car jack
  • combination of lever with a large screw

29
What is Energy?
30
Energy
Chapter 12
  • Energy
  • ability to do work.
  • When you do work on an object, you transfer
    energy to that object.
  • Whenever work is done, energy is transformed or
    transferred to another system.
  • SI Units joules (J)

31
Potential Energy
  • energy that an object has because of position,
    shape, or condition
  • stored energy.
  • Elastic potential energy
  • energy stored in any type of stretched or
    compressed elastic material, (spring or a rubber
    band).
  • Gravitational potential energy
  • energy stored in gravitational field which exists
    between any two or more objects.

32
Gravitational Potential Energy
  • depends on both mass and height.
  • PE mgh
  • The height can be relative.
  • height used in above equation is usually measured
    from ground.
  • However, it can be a relative height between two
    points, such as between two branches in a tree.

33
GRAVITATIONAL POTENTIAL ENERGY
  • A 65 kg rock climber ascends a cliff. What is
    the climbers gravitational potential energy at a
    point 35 m above the base of the cliff?

GIVEN m 65 kg h 35 m g 9.8 m/s2 PE ?
WORK PE mgh PE (65 kg) (35 m) (9.8 m/s2) PE
2.2 x 104 kgm2/s2 2.2 x 104 J
34
Kinetic Energy
  • energy of a moving object due to objects motion
  • depends on mass and speed.
  • depends on speed more than mass.

35
KINETIC ENERGY
  • What is the kinetic energy of a 44 kg cheetah
    running at 31 m/s?

GIVEN KE ? m 44 kg v 31 m/s
WORK KE ½ mv2 KE ½ (44 kg) (31 m/s)2 KE
2.1 x 104 kg x m2/s2 or 2.1 x104 J
36
KINETIC ENERGY
  • Calculate the kinetic energy in joules of a 1500
    kg car moving at 29 m/s.

GIVEN KE ? m 1500 kg v 29 m/s
WORK KE ½ mv2 KE ½ (1500 kg) (29 m/s)2 KE
6.3 x105 J
37
KINETIC ENERGY
  • Calculate the kinetic energy in joules of a 1500
    kg car moving at 18 m/s.

GIVEN KE ? m 1500 kg v 18 m/s
WORK KE ½ mv2 KE ½ (1500 kg) (18 m/s)2 KE
2.4 x105 J
38
Other Forms of Energy
  • mechanical energy
  • amount of work an object can do because of
    objects kinetic potential energies
  • you can SEE it
  • Large scale basis
  • nonmechanical energy.
  • you CANNOT SEE it
  • X rays, microwaves
  • Small scale basis (atomic)

39
Other Forms of Energy
Chapter 12
  • Atoms and molecules
  • kinetic energy of particles related to heat and
    temperature.
  • Chemical reactions
  • Breaking bonds exothermic/endothermic
  • Photosynthesis
  • turn energy in sunlight into chemical energy.

40
Other Forms of Energy
  • nuclear fusion reactions
  • Combining of atomic nuclei
  • Electricity.
  • derived from flow of charged particles
  • bolt of lightning or in a wire.
  • electromagnetic waves.
  • Light energy from sun

41
CONSERVATION OF ENERGY
42
Energy Transformations
  • readily changes from one form to another.
  • Potential energy changes into kinetic energy.
  • car goes down a hill on a roller coaster,
    potential energy changes to kinetic energy.
  • Kinetic energy changes into potential energy.
  • kinetic energy a car has at bottom of a hill can
    do work to carry car up another hill.

43
Energy Transformations
  • Mechanical energy can change to
  • nonmechanical energy as a result of
  • friction,
  • air resistance,
  • or other means.

44
The Law of Conservation of Energy
  • energy cannot be created or destroyed.
  • doesnt disappear, it changes to another form.
  • if total energy in a system increases, it must be
    due to energy that enters the system from an
    external source.

45
SYSTEMS
  • closed system
  • when flow of energy into and out of a system is
    small enough that it can be ignored
  • open systems (most)
  • exchange energy with the space that surrounds
    them.

46
Efficiency of Machines
  • Not all of the work done by a machine is useful
    work.
  • cannot do more work than work required to operate
    machine.
  • Because of friction, work output of a machine is
    always somewhat less than work input.
  • Efficiency
  • ratio of useful work out to work in.
  • measure of how much useful work it can do.
  • expressed as a percentage.

47
Efficiency of Machines
  • Efficiency Equation
  • Machines need energy input.
  • Because energy always leaks out of a system,
    every machine needs at least a small amount of
    energy input to keep going.

48
EFFICIENCY
  • A sailor uses a rope and an old, squeaky pulley
    to raise a sail that weighs 140 N. He finds that
    he must do 180 J of work on the rope in order to
    raise the sail by 1 m (doing 140 J of work on the
    sail). What is the efficiency of the pulley?
    Express your answer as a percentage.

GIVEN eff ? uwo 140 J wi 180 J
WORK eff uwo/wi eff 140 J / 180 J eff 0.78
or 78
49
EFFICIENCY
  • Alice Jim calculate that they must do 1800 J
    of work to push a piano up a ramp. However,
    because they must also overcome friction, they
    must actually do 2400 J of work. What is the
    efficiency of the ramp?

GIVEN eff ? uwo 1800 J wi 2400 J
WORK eff uwo/wi eff 1800 J / 2400 J eff
0.75 or 75
50
EFFICIENCY
  • It takes 1200 J of work to lift the car high
    enough to change a tire. How much work must be
    done by the person operating the jack if the jack
    is 25 efficient

GIVEN eff 25 uwo 1200 J wi ?
WORK wi uwo/eff wi 1200 J / .25 wi 4800 J
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