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## Work, Energy and Power

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### Work, Energy and Power Chapter 5 Section 4 Wagon Example Push a wagon on a sidewalk and it starts to roll down the sidewalk. The wagon eventually comes to a stop ... – PowerPoint PPT presentation

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Title: Work, Energy and Power

1
Work, Energy and Power
• Chapter 5 Section 4

2
Wagon Example
• Push a wagon on a sidewalk and it starts to roll
down the sidewalk.
• The wagon eventually comes to a stop shortly
after the push.
• Friction slows the wagon down.
• Mechanical Energy is not conserved in the wagon
since there is a change in kinetic energy.

3
Work-Kinetic Energy Theorem
• Work-Kinetic Energy Theorem The net work done
on an object is equal to the change in the
kinetic energy of the object.

4
Work-Kinetic Energy Theorem Equation
• Wnet ?KE
• Wnet Net Work
• ?KE Change in kinetic Energy
• Force is not required and applies to all objects
universally.

5
Friction
• When dealing with the work done by friction, the
Work-Kinetic Energy Theorem can be put into an
alternative form.
• Wfriction ?ME

6
Frictionless
• When a problem deals with frictionless objects or
where friction is neglected.
• Wfriction 0
• ?ME 0
• MEi MEf
• This is the Conservation of Mechanical Energy

7
Work-Kinetic Energy Theorem Work
• It doesnt matter if friction is present or its
frictionless, the Theorem demonstrates that work
is a method of transferring energy.
• Perpendicular forces to the displacement cause no
work, cause the energy is not transferred.

8
Distinction Between W and Wnet
• Its important to make the distinction between the
two expressions
• W Fd(cos?)
• This expression applies to the work done on an
object due to another object
• Definition of work
• Wnet ?KE
• Shows only the NET FORCE on an object
• Relates to the net work done on an object to
change the kinetic energy of an object

9
Example Problem
• A 10.0 kg shopping cart is pushed from rest by a
250.0 N force against a 50.0 N friction force
over a 10.0 meter distance.
• How much work is done by each force on the cart?
• How much kinetic energy has the cart gained?
• What is the carts final speed?

10
1. 2500 J
2. 2000 J
3. 20 m/s

11
Everyday Power
• What is power?
• A few everyday uses of power.
• Electricity
• Engines
• Etc
• Basically any time work is done, power is
generated.

12
Power
• Power The rate at which energy is transferred.
• In other words, power is the rate at which energy
is transferred.

13
What is Power?
• Power is the amount of work done over a certain
time interval.
• P W/?t
• Power Work / time

14
Alternative Power Form
• Power can also be described through forces and
the speed of the object.
• P Fv
• Power Force Speed

15
SI Units of Power
• The SI units for Power is the watt
• Variable for a watt is a capital letter W
• A watt is equal to one joule per second
• Horsepower is often used with power when dealing
with mechanical devices such as engines.
• 1 horsepower 746 watts

16
• Why are many mountain roads built so that they
zigzag up the mountain rather than straight up?

17
• The same energy is needed to reach the top of the
mountain regardless of the path.
• Therefore the work is the same.
• The zigzag path has a longer distance and takes
more time to reach the top
• Therefore less power is needed on the zigzag path
vs. straight up.

18
Machine Power
• Machines with different power ratings do the same
work, but do so over different time intervals.
• The only main difference between different power
motors is that more powerful motors can do the
work in a shorter time interval.

19
Example Problem
• Two horses pull a cart. Each horse exerts a
250.0 N force at a 2 m/s speed for 10.0 minutes.
• Calculate the power delivered by the horses.
• How much work is done by the two horses?

20
1. 1000 watts
2. 600,000 Joules

21
Light Bulbs
• A common everyday thing that you take for granite
is artificial light.
• A light bulb usually has marked on it the wattage
it uses.
• Example 60 watt light bulb (most common)
• A 60 watt light bulb will use 60 joules of energy
over the course of 1 second.
• Where does the energy come from?

22
From Sunlight to Artificial Light
• Sunlight ? Plants ? Fossil Fuel (coal) ? Steam ?
Turbine ? Electricity ? Light
• Whenever energy is transferred, heat is produced.
• 2nd Law of Thermodynamics
• So it takes light to produce light and its very
inefficient.