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Physics 101: Lecture 9 Work and Kinetic Energy

Physics 101: Lecture 9 Work and Kinetic Energy. Today’s lecture will be on Textbook Sections 6.1 - 6.4. Energy. Forms Kinetic Energy (Motion) Today, Wed Potential Energy (Stored) Today, Wed Heat Later

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Physics 101: Lecture 9 Work and Kinetic Energy

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  1. Physics 101: Lecture 9Work and Kinetic Energy • Today’s lecture will be on Textbook Sections 6.1 - 6.4

  2. Energy • Forms • Kinetic Energy (Motion) Today, Wed • Potential Energy (Stored) Today, Wed • Heat Later • Mass (E=mc2) Phys 102 • Units Joules = kg m2 / s2

  3. Energy is Conserved • Energy is “Conserved” meaning it cannot be created nor destroyed • Can change form • Can be transferred • Total Energy does not change with time. “no such thing as a free lunch” • This is a BIG deal.

  4. Tb Tb Ta mg mg Work: Energy Transfer due to Force • Force to lift trunk at constant speed • Case a Ta – mg = 0 Ta = mg • Case b 2Tb - mg =0 or Tb = ½ mg • But in case b, trunk only moves ½ distance you pull rope. • F * distance is same in both! (a) (b) “Work” W = F dcos(q)

  5. F q F Dr F Dr F q Dr Dr F Work by Constant Force A) W>0 B) W=0 C) W<0 • Only component of force parallel to direction of motion does work! • W = F Dr cos q Note Change in r! 1) 2) 3) WF = 0. q =90  cos(q) =0 WF < 0. 90 < q < 270  cos(q) < 0 WF > 0. 0< q < 90  cos(q) > 0

  6. ACTS: Ball toss You throw a ball up in the air. The work done by gravity by the time the ball reaches the top is … A) Positive B) Negative C) Zero The ball then falls to the ground. The work done by gravity during this part of the ball’s motion is … A) Positive B) Negative C) Zero

  7. Work by Constant Force • Example:You pull a 30 N chest 5 meters across the floor at a constant speed by applying a force of 50 N at an angle of 30 degrees. How much work is done by the 50 N force? 50 N 30

  8. Where did the energy go? • Example:You pull a 30 N chest 5 meters across the floor at a constant speed, by applying a force of 50 N at an angle of 30 degrees. • How much work did gravity do? • How much work did friction do?

  9. Preflight 1 You are towing a car up a hill with constant velocity. The work done on the car by the normal force is: 1. positive2. negative3. zero V

  10. Preflight 2 You are towing a car up a hill with constant velocity. The work done on the car by the gravitational force is: 1. positive2. negative3. zero V

  11. Preflight 3 You are towing a car up a hill with constant velocity. The work done on the car by the tension force is: 1. positive2. negative3. zero V

  12. Kinetic Energy: Motion • Apply constant force along x-direction to a point particle m. W = FxDx = m axDx = ½ m (vf2 – v02) • Work changes the quantity ½ m v2 • Define Kinetic Energy K = ½ m v2 W = D K(For Point Particles)

  13. Preflight 4 You are towing a car up a hill with constant velocity. The total work done on the car by all forces (including gravity) is: 1. positive2. negative3. zero

  14. y x Example: Block w/ friction • A block is sliding on a surface with an initial speed of 5 m/s. If the coefficent of kinetic friction between the block and table is 0.4, how far does the block travel before stopping? 5 m/s

  15. Falling Ball Example • Ball falls a distance 5 meters, What is final speed? mg

  16. Force F=kx Distance Work by Variable Force Force • W = FxDx • Work is area under F vs x plot • Spring F = k x • Area = ½ k x2 =Wspring Work Distance Work

  17. Summary • Energy is Conserved • Work = transfer of energy using force • Can be positive, negative or zero • W = F d cos(q) • Kinetic Energy (Motion) • K = ½ m v2 • Work = Change in Kinetic Energy • S W = DK

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