07_InstSolManual_PDF_Part5

07_InstSolManual_PDF_Part5 - Work and Energy 7-5(b Since s...

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(b) Since s is proportional to doubling increases s by a factor of 4; s therefore becomes 154 m. (c) The original kinetic energy was converted into thermal energy by the negative work of friction. Reflect: To stop the car friction must do negative work equal in magnitude to the initial kinetic energy of the car. 7.20. Set Up: For part (a), estimate that the average person weighs 150 lb, walks at approximately and jogs at roughly For part (b), estimate that the car weighs 4000 lb and has a speed of 60.0 mph. Finally, for part (c), the work-energy principle tells us that the amount of work done by gravity must equal the gain in kinetic energy: Estimating the person’s height to be her shoulder height would be roughly or Solve: (a) (b) (c) The change in kinetic energy is where Note the positive sign is a result of the force, acting in the direction of motion and therefore doing positive work. Substituting, Reflect: While we estimated that the person’s speed doubled by jogging, his kinetic energy increased by a factor of 4.
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