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NOTES 17 - Topic 2 - Mechanics

NOTES 17 - Topic 2 - Mechanics - ------------------------------------------------------------------------------------------------ 2 .3.6 The Law of Conservation of Energy; Th e total energy of a system remains constant in spite of being transformed from one form to another.

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NOTES 17 - Topic 2 - Mechanics

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  1. NOTES 17 - Topic 2 - Mechanics ------------------------------------------------------------------------------------------------- 2.3.6 The Law of Conservation of Energy; The total energy of a system remains constant in spite of being transformed from one form to another. Etot = KE + PE + Q (heat) + W = constant

  2. 2.3.7 List different forms of energy and describe examples of the transformation of energy from one form to another; Different forms of Energy: Kinetic Energy (KE) - energy of motion; Potential Energy (PE) - energy of position or condition; Heat or Thermal Energy (Q); Nuclear Energy - energy holding the nuclei of atoms together; Work (W) - sometimes called “mechanical energy”; Sound Energy;

  3. Energy Transformations Energy can be transformed from W to KE to PE to KE to W, etc., back and forth. Frictional forces almost always causes heat (Q) to be generated. As Q is generated, energy is "lost" from KE, PE, and W. Example (assuming no frictional forces and no transfer to Q): 1. A 10. kg object is raised 10. m above the ground by 980 J of work; 2. The PEg has been increased by 980. J by being raised 10. m; 3. When allowed to fall to the ground, the object achieves a vf of 14 ms-1 and a KE of 980. J; 4. Upon striking a stake in the ground, the falling object can do 980 J of work on the stake, driving it into the ground a certain distance, depending on the frictional force between the stake and the ground; NOTICE: the energy involved is always 980. J. PEg and KE as a Function of Time

  4. 2.3.8 Elastic versus Inelastic Collisions Elastic Collision - objects are deformed by a collision but resume 100% of their original shape; most collisionsare not 100% elastic, but some are close...pool balls and steel ball bearings; the closest to 100% elastic collisions are those betweeen atoms and molecules; momentum is conserved; KE is conserved; Inelastic Collision - objects are deformed by a collision and do NOT resume their original shape; most collisions are inelastic; momentum is conserved; KE is NOT conserved;

  5. 2.3.9 Mechanical Power; POWER - the amount of work done in a specific amount of time; Power (P) = work / time = W / t = Js-1 = N•ms-1 = kgm2s-3 =Watts (W); Horse Power - the amount of work done per second by an average 19th century English work horse; 1.00 HP = 746 J s-1 = 746 W; The Most Powerful Diesel Engine in the World http://www.bath.ac.uk/%7Eccsshb/12cyl/ A Ship That Would Use Such an Engine http://www.fas.org/man/dod-101/sys/ship/president_polk.htm

  6. 2.5.10 Define and apply the concept of EFFICIENCY; Efficiency - the fraction of energy and work that is produced and is not transformed into heat; Sample Problem: Efficiency (show solution in NB) A particular pulley system requires 1200. J of work to raise a 10. kg object 10. m into a building. What is the efficiency of the pulley system? Given: Unknown: Equations:

  7. 2.3.11 Solve work, energy, and power problems. Sample Problem 8: A 20.0 kg box is pulled up a frictionless ramp a distance of 10.0 m to the top in 30.0 seconds. If the plane is inclined at 30o above the horizontal, (A) how much work is done? (show solutions in NB) (A) Given: Unknown: Equation:

  8. (B) What is the power of the motor that accomplishes the work? Given: Unknown: Equation:

  9. (C) What is the horsepower rating of the motor? Given: Unknown: Equation:

  10. Sample Problem 9: A railway car, m = 20,000. kg, is sitting motionless on a track. An identical car, moving at 5.0 ms-1, collides with the stationary car, they couple, and move off together. (A) Calculate the final speed of the coupled cars. (show solutions in NB) (A) Given: Unknown: Equation:

  11. (B) How much KE was “lost” as a result of this collision? (C) What has happened to this “lost” KE? Given: Unknown: Equation: (C)

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