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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 shortly after the push. Friction slows the wagon down.

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Work, Energy and Power

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  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 = ΔKE • Wnet = Net Work • ΔKE = 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 = ΔME

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

  7. Work-Kinetic Energy Theorem & Work • It doesn’t 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(cosθ) • This expression applies to the work done on an object due to another object • Definition of work • Wnet = ΔKE • 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 cart’s final speed?

  10. Example Problem Answers • 2500 J • 2000 J • 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 P = Power (watt) W = Work (J) t = Time (s)

  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” • A watt is equal to one joule or energy per second • Horsepower is often used with power when dealing with mechanical devices such as engines. • 1 horsepower = 746 watts

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

  17. 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.

  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. Example Problem Answers • 1000W or 1kW • 600,000 Joules or 0.60MJ

  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.

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