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Chapter 10

Chapter 10. Energy, Work, and Simple Machines. 10.1 Energy and Work. Energy is the ability to produce change in itself or the environment. Energy of Motion: v f 2 = v i 2 + 2 ad Rearranging v f 2 - v i 2 = 2 ad Substituting Newton’s second law v f 2 - v i 2 = 2 Fd/m.

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Chapter 10

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  1. Chapter 10 Energy, Work, and Simple Machines

  2. 10.1 Energy and Work • Energy is the ability to produce change in itself or the environment. • Energy of Motion: vf2 = vi2 + 2ad Rearranging vf2 - vi2 = 2ad Substituting Newton’s second law vf2 - vi2 = 2Fd/m

  3. Multiply both sides by ½ m ½ mvf2 – ½ mvi2 = Fd • Kinetic energy KE = ½ mv2 • Work-Energy Theorem ΔKE = W • When work is done on an object, a change in kinetic energy results.

  4. Work: • Through the process of doing work, energy can transfer from the environment to the object and back again! • The equation for work is • W = Fd For constant forces in the direction of the motion. Unit of work is the joule (J)

  5. Constant force at an angle: • Work (angle between force and displacement). • W = Fd cos 

  6. Power: • The rate at which work is done • P = W/t • Power is measured in watts (joule/second)

  7. PSS • Sketch the problem. • Establish a coordinate system • Draw a vector diagram. • List known and unknowns. • Use the basic equation for work when a constant force is exerted in the same direction as the displacement. Or the equation W = Fd cos which will work in all situations.

  8. Use the work-energy theorem to determine the change in energy of the system. • Use work and time to find power. • Check your answer.

  9. 10.2 Machines • Simple and Compound Machines • The output work can never be greater than the input work • The machine simply aids in the transfer of energy. • Mechanical Advantage • The force you exert on a machine is called the effort force Fe • The force exerted by the machine is called the resistance forceFr

  10. The ratio of resistance force to effort force, is called the mechanical advantage. • Mechanical Advantage • MA = Fr / Fe • This equation can be rewritten using the definition of work • Wo = Wi or • Frdr = Fede • Rearranging this gives • Fr/Fe = de/dr

  11. We know that the mechanical advantage is given by MA = Fr/Fe • For an ideal machine, MA = de/dr • Because this equation is characteristic of an ideal machine, the mechanical advantage is called the Ideal Mechanical Advantage • IMA = de/dr

  12. Efficiency: • The efficiency of a machine is defined as the ratio of output work to input work. • Efficiency (%) = Wo/Wi X 100 • An ideal machine has equal output and input work and the efficiency is 100%. • In terms of mechanical advantage and ideal machine advantage • Efficiency (%) = (Fr/Fe)/(de/dr) X 100 • Efficiency (%) = MA / IMA X 100

  13. Simple machines: • Most simple machines are combinations of one or more of the six simple machines. • They are the lever, pulley, wheel and axel, inclined plane, wedge, and screw. • The IMA of all machines is the ratio of distances moved.

  14. Compound machines: • A compound machine consists of two or more simple machines linked so that the resistance force of one machine becomes the effort force of the second. • The mechanical advantage of a compound machine is the product of the mechanical advantage of the simple machines it is made up of. • MA = MA machine 1 X MA machine 2

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