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Work and Energy

Work and Energy. Work is done when an external force is used to change the energy of the system. Energy is the ability to create change or do work. Energy and work are both measured in Joules (J =Nm). Energy and work are scalar quantities. They only have magnitude, no direction.

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Work and Energy

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  1. Work and Energy

  2. Work is done when an external force is used to change the energy of the system.

  3. Energy is the ability to create change or do work. • Energy and work are both measured in Joules (J =Nm). • Energy and work are scalar quantities. They only have magnitude, no direction

  4. There are many different forms of Energy:

  5. Kinetic Energy Is the object moving? The energy of motion.

  6. Gravitational Potential Energy Does the object have a height? The energy due to the height of an object.

  7. Elastic Potential Energy Is there a loaded spring? The energy stored in a stretched or compressed spring. k = The spring constant (N/m) x = distance stretched or compressed (m)

  8. Internal Energy Is there a force of friction acting? The energy transferred to the molecules of the objects in the due to friction. HEAT f = The force of friction. ∆x = The distance traveled.

  9. Chemical Potential Energy Is there a chemical reaction occurring? The energy released due to a chemical reaction. ASK A CHEMISTRY TEACHER FOR THE FORMULA

  10. Conservation of Energy For a closed system the sum of the original energy (Eo) and the work (W) done is equal to the final energy (Ef).

  11. Using Pie Charts to understand Energy transfers

  12. A v = 0m/s B Eint C A C B D = = = Eint Eint Ug K D Example 1: A ball is dropped from rest. (Include air friction) K Ug Ug K h = 0

  13. V=0m/s A B D h =0 A C B D E Ug Ug = = = = Example 2: A pendulum swings from A to E (Neglect air resistance) V=0m/s E C Ug K K K Ug

  14. v=0m/s v=0m/s v v A C B D = = = Eint Eint Example 3: A spring launches a block across a horizontal table. A D B C Eint K Us K

  15. v v v A C B D = = = 8m v Ug Ug K K K K Ug UC UC UC UC Example 4: A biker rides up a hill with at a constant speed. D C B h = 0 A

  16. Let’s do some quantitative problems:

  17. v Example 1: A ball is dropped from a height of 15 meters. What is its velocity just before it hits the ground? v = 0m/s 15m h = 0

  18. Example 2: A pendulum is released from rest at point A and has a velocity of 6 m/s at point C. Find the initial height (h) from which the pendulum was released. (Neglect air resistance) V=0m/s A h v = 6m/s C

  19. v=0m/s v=0m/s 5m Example 3: A spring is compressed 20cm and launches a 400 gram block across a horizontal table. The block comes to rest after traveling 5 meters. The coefficient of friction is 0.6. What is the spring constant (k)?

  20. Example 4: A 70kg biker has a velocity of 10m/s at the bottom of a 8 meter hill. The biker does 6000J of work in climbing the hill and 2000J is transferred to internal energy as he climbs the hill. What is the final velocity of the biker? v 8m 10m/s

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