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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
• tells how much a machine multiplies force or
increases distance.
• mechanical advantage output force input
distance
• input force output distance

14
• 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
• 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
• 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?

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