Chapter 5 Chapter 5 Work, Energy, Power Work, Energy, Power
Chapter 5Chapter 5
Work, Energy, PowerWork, Energy, Power
WorkWorkThe work done by force is defined
as the product of that force times the parallel distance over which it acts.
cosFsW The unit of work is the newton-
meter, called a joule (J)
EnergyEnergyThe amount of energy transferred to the object is equal
to the work done.
Types of Energy
• Kinetic Energy = “Motion Energy”
• Potential Energy = “Stored Energy”
Kinetic EnergyKinetic EnergyKinetic Energy is the
energy possessed by an object because it is in motion.
221 mvKE
Work-Kinetic Energy Theorem
When work is done by a net force on an object and the only change in the object is its speed, the work done is equal to the change in the object’s kinetic energy
– Speed will increase if work is positive– Speed will decrease if work is negative
net fiW KE KE KE
Ex: Work and Kinetic EnergyThe hammer head
has a mass of 0.4 kg and speed of 40 m/s when it drives the nail. If the nail is driven 3.0 cm into the wood and all of the kinetic energy is transferred to the work one on the nail, What is the average force exerted on the nail.
Example: Block w/ frictionA block is sliding on a surface with an initial
speed of 5 m/s. If the coefficient of kinetic friction between the block and table is 0.4, how far does the block travel before stopping?
5 m/sx
y
Gravitational Gravitational Potential EnergyPotential Energy
Gravitational Potential Energy is the energy possessed by an object because of a gravitational interaction.
mghPEg
Work and Gravitational Potential EnergyPE = mgy Units of Potential
Energy are the same as those of Work and Kinetic Energy
figravity PEPEW
Potential Energy in a Spring
Elastic Potential Energy– related to the work required to
compress a spring from its equilibrium position to some final, arbitrary, position x
– 2
s kx2
1PE
Force
Distance
F=kx
PowerPower
Power is the time rate of doing work or how fast you get work done.
SpeedForceFvt
Fd
erAveragePow
t
W P
work thisdo taken totime
force aby donework
PowerPowerThe unit of power is a joule per
second, called a Watt (W).
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Power - Examples
Watt is the work output if you perform 100 J of work in 1 s?
Run upstairs! If you raise your body (70 kg or 700 N) 3 m in 3 seconds, how powerful are you?
Shuttle puts out a few GW (gigawatts, or 109 W) of power!
Conservation of EnergyConservation of Energy
Energy can neither be created nor destroyed, but only transformed from one kind to another.
finalinital )PEKE(W)PEKE(
Energy is ConservedEnergy is “Conserved” meaning it
can not be created nor destroyed – Can change form – Can be transferred
– PE into KE, KE into PE, KE into HEAT
Total Mechanical Energy does not change with time.– ΔPE + ΔKE = 0
– PE + KE = constant
Energy Conservation ExampleDrop 1 kg ball dropped from 10 m.
– starts out with mgh = (1 kg)(9.8 m/s2)(10 m) = 98 J of gravitational potential energy
– halfway down (5 m from floor), has given up half its potential energy (49 J) to kinetic energy
• ½mv2 = 49 J v2 = 98 m2/s2 v 10 m/s
– at floor (0 m), all potential energy is given up to kinetic energy
• ½mv2 = 98 J v2 = 196 m2/s2 v = 14 m/s
10 m
8 m
6 m
4 m
2 m
0 m
P.E. = 98 JK.E. = 0 J
P.E. = 73.5 JK.E. = 24.5 J
P.E. = 49 JK.E. = 49 J
P.E. = 24.5 JK.E. = 73.5 J
P.E. = 0 JK.E. = 98 J
Roller Coasters
Since
PE + KE = Etotal ,
The shape of a potential energy curve is exactly the same as the shape of the track!
Roller Coasters - Example
If the height of the coaster at A is 60 m from the ground, how fast will you be moving at B, C, D?
Assume no friction and the height of B is 10 m and C is 20 m
On to problems...
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