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Chapter 8: Energy

Chapter 8: Energy. 8.1 Work. Work = force × distance W=Fd Measured joules (J); 1 joule = 1 newton-meter If you lift an object work is done on it; more work is done if… the object was heavier you raised it higher When work is done on an object, the object’s energy changes. Practice.

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Chapter 8: Energy

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  1. Chapter 8: Energy

  2. 8.1 Work • Work = force × distance W=Fd Measured joules (J); 1 joule = 1 newton-meter • If you lift an object work is done on it; more work is done if… • the object was heavier • you raised it higher • When work is done on an object, the object’s energy changes

  3. Practice • How many joules of work are done on an object when a force of 20 N pushes it a distance of 5 m? • Which requires more work, lifting a 5 kg barbell a vertical distance of 4 m or lifting a 10 kg barbell a vertical distance of 2 m?

  4. 8.2 Power • Power is the rate at which work is done • If an engine is twice as powerful than another engine, it can do… • twice the work in the same amount of time • same amount of work in half the time • Measured in watts (W)

  5. Practice • How much power is required to do 200 J of work on an object in a time of 2 seconds? • How much power is required if the same work is done in 0.5 seconds?

  6. 8.3 Mechanical Energy • Energy enables objects to do work • Energy is measured in joules (J) • Mechanical energy is energy due to position (potential energy) or movement (kinetic energy)

  7. 8.4 Potential Energy • Potential energy (PE) is stored energy due to an objects position • Work is required to elevate objects against the earth’s gravity • Gravitational potential energy is the potential energy due to elevated positions • Gravitational potential energy= weight × height PE=mgh

  8. Practice • How much work is done on a 200 N boulder when you lift it 3 m? • How much work is done on the boulder if it is only lifted half as high?

  9. 8.5 Kinetic Energy • Kinetic energy is the energy due to an object’s motion • Kinetic energy =½ mass × speed2 KE=½mv2

  10. The kinetic energy of a moving object is equal to the work required to bring it to that speed from rest, or the work the object can do while being brought to rest • Net force × distance = kinetic energy • Fd=½mv2

  11. Practice A 1,000 kg car is driving at a speed of 30 m/s. What is its kinetic energy? If the brakes provide a force of 900 N, causing the car to stop, what distance does the car travel while braking?

  12. 8.6 Conservation of Energy • Law of conservation of energy: • Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes

  13. Practice • What will the kinetic energy of an arrow having a potential energy of 100 J after it is shot from a bow? • If a 5 kg object is dropped from a height of 30 m, how fast will it be going when it hits the ground?

  14. 1/8 m 8.7 Machines • A machine is a device used to change the magnitude (size) and/or direction of a force • work input = work output • (force × distance) input = (force × distance) output • The pivot point of a lever is called the fulcrum • A lever can multiply forces; however, no machine can multiply work or energy

  15. Practice A lever is used to lift a heavy object. When a 100 N force pushes one end of the lever down 2 m, the object rises .5 m. Calculate the weight of the object.

  16. 1/8 m • Mechanical advantage is the ratio of output force to input force for a machine and/or the ratio of input distance to output distance What is the mechanical advantage of the above set-up

  17. Types of Levers Type 1 lever: fulcrum between the force and load Type 2 lever: load is between the fulcrum and input force Type 3 lever: fulcrum is at one end, load is at the other, and input force is between them

  18. Types of Pulleys Pulley is a kind of lever used to change the direction of a force and/or its magnitude

  19. 8.8 Efficiency Efficiency is the ratio of useful work output to total work input Efficiency is the ratio of actual mechanical advantage (with friction) to the theoretical (ideal) mechanical advantage (without friction)

  20. Efficiency is always less than 1 Multiply by 100 to convert to a percent

  21. Practice In raising a 2,500 N piano with a pulley system, the workers note that for every 1 m of rope pulled down, the piano rises 0.2 m. Ideally, how much force is required to lift the piano? If the workers actually pull with 1,000 N of force to lift the piano, what is the efficiency of the pulley system?

  22. 8.9 Energy of Life • Living cells need energy • Living organisms react hydrocarbon compounds with oxygen to produce energy and carbon dioxide • Plants use light energy, water, and carbon dioxide to produce hydrocarbon compounds like sugar

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