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Conservation of Energy

Conservation of Energy. Energy Work Kinetic and Potential Energy Conservative and non-conservative forces Other forms of energy. Introduction. Forms of energy Mechanical energy Focus for now Forms of energy Energy can be transformed from one form to another

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Conservation of Energy

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  1. Conservation of Energy • Energy • Work • Kinetic and Potential Energy • Conservative and non-conservative forces • Other forms of energy

  2. Introduction • Forms of energy • Mechanical energy • Focus for now • Forms of energy • Energy can be transformed from one form to another • Essential to the study of physics, chemistry, biology, geology, astronomy • Can be used in place of Newton’s law to solved certain problems more easily.

  3. Work • Provides a link between force and energy • Work is the product of the component of the force along the direction of the displacement and the magnitude of the displacement • W=F(cosq)d • F(cosq) is the component of the force in the direction of the displacement • d is the displacement

  4. Work • This gives no information about • The time it took for the displacement to occur • The velocity of acceleration of the object • Note: work is zero when • There is no displacement (holding a bucket) • Force and displacement are perpendicular to each other (if we are carrying the bucket horizontally, gravity does not work) http://lectureonline.cl.msu.edu/~mmp/kap5/work/work.htm

  5. More about Work • Work is a scalar quantity • Units of work are Nm or Joules (J) • Work can be positive or negative • Positive if the force and the displacement are in the same direction • Negative if the force and the displacement are in the opposite direction • Example lifting a cement block • Work done by the person • Is positive when lifting the box • Is negative when lowering the box

  6. Examples of Work Calculations W=F(cosq)d Since there is no angle W=Fd =(100N)5m = 500J • W=F(cosq)d • =(100N)(cos30)5m • = 433J W=F(cosq)d Since the force required to lift up is equal and opposite to gravity then F=+mg so W=+mgd W=(15kg)(9.81m/s2)5m W= 735J

  7. Example 4 • A 10-N forces is applied to push a block across a friction free surface for a displacement of 5.0 m to the right. • Since Fapp is the only horizontal force, it is the only force that does work • W = Fd • = (10N)(5.0m) • = 50J

  8. Example 5 • A 10-N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right Since the object moves horizontally, only horizontally forces will do work Wapp = Fappd W = 10N 5.0 m = 50 J Wfrct = Ff d = -10N(5.0 m) = -50J

  9. Graphing Work • A graph of force exerted over a displacement can be used to determine work. Since Work = Force x displacement and Area = length x width. If the axes on a graph are force and distance then the area under the line will be equivalent to work done. Find the work done over the 10 m displacement. Area = work, there are 3 distinct areas under the line the sum will equal total work done. Area = ½ bh + lw + ½ bh = ½ 3m(20N) + 5m(20N) + ½ 2m (20N) = 30 J + 100 J + 20 J = 150J Work done is 150 J

  10. Assignment 1 • Do questions 1 – 7 in workbook

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