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Chapter 6

Chapter 6 . Work & Energy. Work. Two thing are involved with work: The application of force The movement caused by force W = Fd Units (Nm) or J (joules). F. d. Work. Push a box across the floor. When a force is exerted over a distance, work is done.

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Chapter 6

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  1. Chapter 6 Work & Energy

  2. Work • Two thing are involved with work: • The application of force • The movement caused by force W = Fd Units (Nm) or J (joules)

  3. F d Work • Push a box across the floor. • When a force is exerted over a distance, work is done. • Work is the product of force and distance when the two are in the same direction. Same Direction

  4. F d Work and Direction • Consider a sled being dragged across the snow by a diagonal rope. • The angle between the quantities is denoted as q. What is the direction of motion of the sled? What is the direction of the force acting on the sled?

  5. F F d d Work and Direction (cont.) • Only the horizontal component of the force is acting in the direction of motion Fy Fx

  6. d F Sample Work Problem 2 • The box (m = 15kg) is now lifted a height of 1.5m. • How much work is done? The force required to lift an object is equal to its weight. Lift

  7. d F Practice Problem 2 (Work) • A Lawnmower is being pushed across the grass. The angle of the handle is 50° with the horizontal. The job requires the a force of 80N across the 33m yard. • How much work is done?

  8. F d When No Work Is Done • Carry a box horizontally across the room. • What is the direction of motion? • What is the direction of the force? • What is the angle q? • What is So when force and displacement vectors are perpendicular, no work is done.

  9. d F Work And Friction • Friction requires work to be done. • Consider a box being dragged across a sidewalk. • What is the direction of the displacement? • What is the direction of the frictional force? • What is the angle between the vectors below? • What is cos 180°? • This means that work can be negative. • This may include situations other than friction.

  10. Work Energy Theorem • WET=ΔKE How much work is required to accelerate a 1100kg car from 0m/s to 50m/s?

  11. Work (J) Power (W) Time (s) Power • Work is independent of time. • Lift a 10N box 2m in 2s W=20J • Lift a 10N box 2m in 4s W=20J • The equation for power is shown below.

  12. Machines Terms • Input Work (Wi): • The work you do • Output Work (Wo): • The work the machine does.

  13. dO FO FI dI Mechanical Advantage • Mechanical advantage is the ratio of output force to input force. • This shows how many times a force is multiplied by a machine. • If MA = 1, then no forces are multiplied. • If MA > 1, then forces are multiplied. • If MA < 1, then forces are divided. • The trade off is distance • If you cut the force in half, the distance doubles

  14. Pulleys • The approximate MA for a pulley system is the number of supporting strands. MA=1

  15. Pulleys • The approximate MA for a pulley system is the number of supporting strands. MA=2

  16. Pulleys • The approximate MA for a pulley system is the number of supporting strands. MA=2

  17. Pulleys • The approximate MA for a pulley system is the number of supporting strands. MA=3

  18. Efficiency

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