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Conservation of Energy and Work

Conservation of Energy and Work. Principles of Physics - Foederer. Conservation of Energy. All energy in existence is already here. It cannot be created or destroyed, only transferred among systems. So, E total before transfer = E total after transfer. Example.

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Conservation of Energy and Work

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  1. Conservation of Energy and Work Principles of Physics - Foederer

  2. Conservation of Energy All energy in existence is already here. It cannot be created or destroyed, only transferred among systems. So, Etotalbefore transfer = Etotalafter transfer

  3. Example A 0.100kg egg is dropped from the top of the bleachers which are 5.5 m high. How fast was it travelling when it hit the ground if there was no air resistance? PE→KE Since energy is conserved, PEtop= KEbottom mgh =1/2 mv2 (0.100 kg)(9.8 m/s2)(5.5 m) = 1/2 (0.100 kg)v2 v = 10.4 m/s

  4. Work How does energy change form or enter/leave a system? Work is done on an object or by an object • Work = change in energy • a force is applied to the object to change its motion or condition W = Fd W = work (J) F = force (N) d = distance (m) **This force could be applied, friction, gravitational, etc. for the distance that it is causing the change

  5. Energy is conserved Open system – energy can be transferred out of the system to another system. Example • A box slides down an incline. Its PE changes to KE • There is less KE at the bottom of the incline than there was PE at the top • Energy was lost as heat from friction as the box slid

  6. Example (Open system) A car with mass 200 kg has a motor that supplies a force of 950 N. The car travels a distance of 6.3 m when the motor is turned on. During the 6.3 m distance 5, 557 J of work is done on the car by friction. What is the velocity of the car after travelling 6.3 m? W→KE Since energy is lost because of friction, Wmotor- Wfriction = KE Fd – Wfriction=1/2 mv2 (950N)(6.3 m) – 5557 J= 1/2 (200 kg)v2 v = 2.1 m/s

  7. Energy is conserved Closed system – all energy stays within the system Example • The box slides down an incline with no friction. Its entire PE changes to KE.

  8. Pendulum (Closed System) Excellent example of conservation of energy Work is done on system to raise bob to position A (W = Etotal) • Work is relative to its height at equilibrium (C) As pendulum is being raised, work energy is changing to PE When at position A all work is now PE (Etotal = PE) As pendulum swings from its initial position A and back again, PE changes to KE and back Etotal = PEA = PEB + KEB= KEC= PED + KED= PEE

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