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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.

Mar 27, 2015

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Page 1: 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.

Work, Power, and Machines

Page 2: 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.

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

Page 3: 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.

What is Work?

SI units: joules (J).• 1 J = 1 N•m = 1 kg•m2/s2

Chapter 12

Page 4: 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.

WORKImagine 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

FW

d

Page 5: 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.

WORKA 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 JFW

d

Page 6: 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.

WORKAn 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

FW

d

Page 7: 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.

Power

rate at which work is done or energy is transformed.

SI Unit: watts.• watt (W) = 1 J/s

Power = work

time

Chapter 12

p= W/t

Page 8: 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.

POWERIt 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

pW

t

Page 9: 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.

POWERWhile 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

pW

t

Page 10: 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.

POWERUsing a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output?

GIVEN:

p = ?

W = 5350 J

t = 50 s

WORK:

p = W/t

p = 5350 J / 50 s

p = 107 W

pW

t

Page 11: 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.

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.

Page 12: 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.

Force and Work

Chapter 12

Page 13: 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.

Mechanical Advantage

tells how much a machine multiplies force or increases distance.

mechanical advantage = output force = input distance input force output distance

ma

id

od ma

of

if

Page 14: 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.

MECHANICAL ADVANTAGECalculate 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

ma

id

od

Page 15: 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.

MECHANICAL ADVANTAGECalculate 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

ma

id

od

Page 16: 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.

MECHANICAL ADVANTAGEDetermine 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

ma

of

if

Page 17: 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.

SIMPLE MACHINES

Page 18: 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.

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.lever family: inclined plane family:

simple lever

(3 types)

simple inclined plane

pulley wedge

wheel and axle screw

Page 19: 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.

First Class Levers

fulcrum located between points of application of input and output forces.

Page 20: 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.

Second Class Levers

• fulcrum is at one end of arm and input force is applied to other end.

Page 21: 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.

Third Class Levers

• multiply distance rather than force. • have a mechanical advantage of less than 1.

Page 22: 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.

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

Page 23: 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.

Wheel & Axle

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

Page 24: 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.

The Inclined Plane Family

multiply and redirect force.• turns small input force into large output

force by spreading work out over a large distance.

Page 25: 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.

Simple Inclined Plane

Changes both magnitude & direction of force

Page 26: 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.

Wedge

Functions as two inclined planes back to back.

Turns single downward force into two forces directed out to sides.

Page 27: 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.

Screw

an inclined plane wrapped around a cylinder.

Page 28: 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.

Compound Machines

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

Chapter 12

Page 29: 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.

What is Energy?

Page 30: 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.

Energy

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)

Chapter 12

Page 31: 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.

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.

Page 32: 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.

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.

Page 33: 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.

GRAVITATIONAL POTENTIAL ENERGY

A 65 kg rock climber ascends a cliff. What is the climber’s 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 kg•m2/s2

2.2 x 104 J

Page 34: 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.

Kinetic Energy

energy of a moving object due to object’s motion depends on mass and speed. depends on speed more than mass.

kinetic energy 1

2mass speed squared

KE 1

2mv 2

Page 35: 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.

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

Page 36: 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.

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

Page 37: 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.

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

Page 38: 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.

Other Forms of Energy

mechanical energy: amount of work an object can do because

of object’s kinetic & potential energies you can SEE it Large scale basis

nonmechanical energy. you CANNOT SEE it X rays, microwaves Small scale basis (atomic)

Page 39: 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.

Other Forms of Energy

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.

Chapter 12

Page 40: 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.

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

Page 41: 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.

CONSERVATION OF ENERGY

Page 42: 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.

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.

Page 43: 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.

Energy Transformations

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

Page 44: 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.

The Law of Conservation of Energy

energy cannot be created or destroyed. doesn’t 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.

Page 45: 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.

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.

Page 46: 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.

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.

Page 47: 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.

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.

efficiency

useful work output

work input

Page 48: 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.

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 %

eff

uwo

wi

Page 49: 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.

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 %

eff

uwo

wi

Page 50: 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.

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

eff

uwo

wi