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Chapter 7 Energy

Chapter 7 Energy . The “money” of physics. Get paid. Primitive Economics. Do your job. Buy stuff. Do your job. Get paid. Modern Economics. Using money simplifies economics and accounting. Why Energy Helps. Motion, in general, is hard to calculate.

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Chapter 7 Energy

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  1. Chapter 7Energy The “money” of physics Physics 1 (Garcia) SJSU

  2. Get paid Primitive Economics Do your job Physics 1 (Garcia) SJSU

  3. Buy stuff Do your job Get paid Modern Economics Using money simplifies economics and accounting. Physics 1 (Garcia) SJSU

  4. Why Energy Helps Motion, in general, is hard to calculate. Using forces, momentum, acceleration, etc. gets complicated because they are all vectors (have magnitude & direction). Energy is not a vector; it’s just a number. Can predict motion by figuring out how much energy that motion will “cost.” Physics 1 (Garcia) SJSU

  5. Potential Energy (PE) Gravitational potential energy of an object is, (Potential Energy) = (Weight) x (Height) Think of potential energy as stored energy or energy “in the bank.” Metric unit of energy is Joules (same as for work). Physics 1 (Garcia) SJSU

  6. Sample Problem 6 kg What is the potential energy of a 6kg bowling ball at a height of 20 meters? What is potential energy at zero height? 20 m Physics 1 (Garcia) SJSU

  7. Kinetic Energy (KE) Kinetic energy of an object is, (Kinetic Energy) = ½ x (Mass) x (Speed)2 Kinetic energy is the energy of motion. A stationary object has zero kinetic energy. Kinetic energy is related to momentum but concepts are not the same! Physics 1 (Garcia) SJSU

  8. Sample Problem 6 kg What is the kinetic energy of a 6kg bowling ball, falling from a height of 20 meters, just as it reaches the ground? 20 m 20 m/s Physics 1 (Garcia) SJSU

  9. 10 m PE = 600 J, KE = 600 J The Big Idea PE = 1200 J, KE = 0 J 6 kg Energy is the “currency” of motion 20 m PE = 0 J, KE = 1200 J Physics 1 (Garcia) SJSU

  10. Conservation of Energy Conservation of mechanical energy is (Potential Energy) + (Kinetic Energy) stays constant during motion. Energy “bookkeeping” makes motion simple. Physics 1 (Garcia) SJSU

  11. Maximum Height Maximum Speed Maximum Height, again Pendulum Energy exchange from PE to KE and back. Can predict speed from height since PE+KE constant Note: Use this concept in lab experiment entitled “Projectiles” Physics 1 (Garcia) SJSU

  12. Demo: Don’t Flinch When pendulum returns, never hits you. Physics 1 (Garcia) SJSU

  13. Movie: Bowling Ball Pendulum Physics 1 (Garcia) SJSU

  14. Demo: Interrupted Pendulum By conservation of energy we know that the pendulum ball will never rise above its initial height. Physics 1 (Garcia) SJSU

  15. Demo: Bunny Bash www.bunnybash.org Physics 1 (Garcia) SJSU

  16. Demo: Bunny Bash Would the ball strike at a higher speed by falling straight down instead of swinging as a pendulum? No, by energy conservation the kinetic energy at impact only depends on the initial potential energy (on initial height). Physics 1 (Garcia) SJSU

  17. Demo: Bunny Bash The following year, we simplified the process. Physics 1 (Garcia) SJSU

  18. Demo: Ball Races Can predict ball speeds along the tracks. Ball on track B goes the same speed as ball on track A whenever the two balls are at the same height Physics 1 (Garcia) SJSU

  19. Demo: Blaster Balls How high does the ping-pong ball go? After collision the speed of ping-pong ball is 3x larger Ping pong ball Golf ball Speed after collision is 3 times larger so kinetic energy is 9 times greater. Ping-pong ball rises to 9 times its original height (9x original potential energy). Physics 1 (Garcia) SJSU

  20. Work Define work done on an object by a force as (Work) = (Force) X (Distance traveled) Force acting in direction of motion: Positive work. Force acting in opposite direction: Negative work. Force perpendicular to motion: Zero work Physics 1 (Garcia) SJSU

  21. Check Yourself Slaves pull a heavy load. Work done by slaves is positive, negative, or zero? Work done by friction force? Work done by the ground? Support Pull Friction LOAD Physics 1 (Garcia) SJSU

  22. Work & Energy When forces do work on an object, the work done equals the change in energy. (small force) X (LONG DISTANCE) (BIG FORCE) X (short distance) Two persons do the same work in different ways. Physics 1 (Garcia) SJSU

  23. Jumping How high you jump depends on the force and on the distance over which you apply that force This is because the work done, (Force)x(distance), equals the energy gained. Can only push while in contact with the ground so squatting helps by increasing distance. Physics 1 (Garcia) SJSU

  24. Check Yourself Compared with going 30 mph, a car going 60 mph has ____ times the kinetic energy. Four times the KE means ____ times the work required to stop the car. Four times the work means ____ times the distance (same friction force on brakes). Physics 1 (Garcia) SJSU

  25. Stopping & Braking Distance 55’ 35’ 30 mph 90’ Reaction Distance Braking Distance Stopping Distance 45 mph 70’ 220’ 290’ 60 mph At twice the speed, braking distance is four times longer Physics 1 (Garcia) SJSU

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