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Work, Energy and Power


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

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 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.

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.

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

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

  • 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

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.

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

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?

Example Problem Answers
  1. 2500 J
  2. 2000 J
  3. 20 m/s

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

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

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

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

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

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

The Physics Behind Road Design
  • 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.

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.

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?

Example Problem Answers
  1. 1000 watts
  2. 600,000 Joules

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?

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