Conservation of Energy


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Notes from Ms. Parker's class the week of Nov 7 - 11, 2011

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Conservation of Energy

  1. 1. Good Morning! Today we will:  launch pennies into the air  takes some notes Please do before the tardy bell:  get your lab notebook  get out your notes from this week  get out something to write with
  2. 2. Warm-Up Using the formula for the coefficient of friction, solve this problem. Put your work in your notes from this week. Calculate the μ for wood sliding across carpet if the weight of the wood is 36N and the pulling force is 24N. You have 2 minutes
  3. 3. Warm-Up II Calculate the force of sliding friction for a 500 N person using a shoe with a μ of 0.4  Hint: use μ = Ff/FN Calculate the acceleration of the 500 N person (mass = 50 kg) due to the force of sliding friction.  (Hint: use F = ma) You have 6 minutes to complete
  4. 4. On back of quiz/warm-upYou are a scientist working for ACME Company andyour boss asks you to produce data to answer thefollowing question:What is the effect of velocity on the coefficient offriction between wood and carpet? Write a hypothesis. Create a procedure and a data table for the lab you would conduct to test your hypothesis.
  5. 5. What Do You See?
  6. 6. Launching Pennies Hold down one end of the track or wooden ruler on the table and press down on the other end. Try to get the penny to travel close to the height of the ceiling without hitting the ceiling. What factors (variables) about the track and how it is positioned determine the height the stone achieves? You have 8 minutes
  7. 7. Launching Pennies What is the effect on a penny when additional force (increased deflection) is applied to the ruler?  your hypothesis  the data you will record  tools you will need to make your measurements You have 8 minutes
  8. 8. Launching Pennies Perform your experiment You have 10 minutes
  9. 9. Law of Conservation of Energy
  10. 10. Law of Conservation of Energy When a net force acts on an object, what happens?  either the speed or position of the object (or both) change – in other words, the object accelerates Think about throwing a ball vertically into the air. Draw a sketch of what the ball’s path would look like.
  11. 11. Law of Conservation of Energy The moment the ball leaves your hand, it has all of the vertical speed it will have – as the ball rises into the air, what happens to its vertical speed?  the vertical speed of the ball decreases
  12. 12. Law of Conservation of Energy At some point, the ball will reach its maximum height. At this point, the ball’s vertical velocity is zero. You know what happens next, but do you know what speed the ball will be when it reaches your hand again?  when the ball reaches it’s launch height, it will be traveling at exactly the same speed as it was when it left your hand.
  13. 13. Law of Conservation of Energy If the speed when you launch the ball is exactly the same as when the ball returns back to the same point, then something is conserved. What do you think is conserved?  seriously – you better be able to figure out the answer 
  14. 14. Law of Conservation of Energy The Law of Conservation of Energy is pretty simple:Energy can be neither created nordestroyed; it can only be transformed fromone form to another. The total amount ofenergy remains constant.
  15. 15. Forms of Energy Energy comes in various forms. Today, we will be talking about three of them:  kinetic energy  gravitational potential energy  elastic potential energy
  16. 16. Vocabulary Alert!! kinetic energy is the energy of motion gravitational potential energy is the energy of position elastic potential energy is the energy of a spring due to compression or stretch
  17. 17. End Day 1 Notes
  18. 18. Day 2 Notes
  19. 19. Good Afternoon! Today we will:  finish taking notes on the conservation of energy  diagram, label, and describe energy transformations  use formulas to solve word problems Please do BEFORE THE TARDY BELL  get out your spiral/notes and look over the definitions for KE, GPE, and EPE  pick up the “sample problems” worksheet by the door  Pick up a whiteboard and a marker
  20. 20. Kinetic + Potential Energy = Total Energy In any system – whether it’s the ball you throw vertically in the air or the penny you launched with the ruler, the total kinetic energy + the total potential energy = the total energy in a system  KE + PE = total energy  Our friend Wil E Coyote
  21. 21. Kinetic + Potential Energy = Total Energy Fun with bowling balls Do you trust physics? And now for a song you’ll never get out of your head: Ole!
  22. 22. Concept Check Think about the lab we did on Monday when we launched pennies into the air.  elastic potential energy (EPE)  gravitational potential energy (GPE)  kinetic energy (KE) were all involved in the energy transformations.
  23. 23. Concept Check On the back of the sample problems handout, sketch the lab we did Monday:  Label the maximum elastic potential energy, the maximum gravitational potential energy, and the maximum kinetic energy  Below the sketch, describe the entire path of the penny in terms of EPE, KE, and GPE and their energy transformations You have 12 minutes
  24. 24. Concept Check II True or False: If you know the maximum kinetic energy in a system, you know the maximum potential energy as well. TRUE
  25. 25. That was a lot of work! Or was it? Johnnie pushes against a wall until his muscles tremble. Carol Anne picks up her pencil. Who worked harder?
  26. 26. Work To a scientist, the word work has a very specific meaning. Work is defined as a force applied to an object over a distance. Work = force x distance
  27. 27. Work So, back to Johnnie and Carol Ann. Who did more work – Johnnie pushing against a wall with all of his might or Carol Ann picking up her pencil? Let’s “work” a couple of problems 
  28. 28. UNIT WARNING! Before we go much further, we need to emphasize WHAT a Newton is A Newton is a unit of force that is equal to:  1 kg•m/s2 So BEFORE you start ANY word problem dealing with work or energy, convert your units to kilograms, meters, and seconds!
  29. 29. Formula for Gravitational PotentialEnergy The formula for gravitational potential energy is GPE = mghm = mass (kg), g = gravity (m/s2),h = height (m)
  30. 30. Gravitational Potential Energy GPE = mgh GPE = Work Done Work = fd How are these two quantities related?  mass x gravity = weight (a type of force)  height = a type of distance What ever GPE an object has, it has it because your did that much WORK on it.
  31. 31. Units If we solve a gravitational potential energy problem AND carry our units all the way through LIKE YOU SHOULD ALWAYS DO, you end up with a Newton-meter. GPE = (mass)(gravity)(distance) (kg)(m/s2)(m) Nm Solve sample problem #3 in your handout
  32. 32. Formula for Kinetic Energy The formula to calculate kinetic energy is KE = 0.5mv2where m = mass (kg) and v = velocity (m/s)
  33. 33. Kinetic Energy KE = 0.5mv2 looking at the formula, which quantity has the largest influence on the amount of kinetic energy – mass or velocity?  Velocity
  34. 34. Units If we solve a kinetic energy problem AND carry our units all the way through LIKE YOU SHOULD ALWAYS DO, you end up with a Newton-meter. (kg)(m/s)(m/s) = kgm/s2 x m Nm Solve sample problem #4 in your handout
  35. 35. Elastic Potential Energy The formula for elastic potential energy is EPE = 0.5kx2k = the spring constant (N/m)x = amount of bending in metersthe spring constant has to be given – it’sdifferent for different objects
  36. 36. Units If we solve an elastic potential energy problem AND carry our units all the way through LIKE YOU SHOULD ALWAYS DO, you end up with something called a Newton-meter. EPE = 0.5 (k)(x2) N/m  m2 Nm You guessed it - time to work some sample problems!
  37. 37. Newton-Meters Energy is measured in Newton-meters Usually, you’ll see it reported as something else, though: A Newton-meter is called a Joule (J)
  38. 38. Show How Much You Know The spring constant for the track we used in this lab is 280 N/m A penny minted after 1982 has a mass of 2.5 grams (o.0025 kg) If you deflect the track 0.03 m (3 cm)  what will be the coin’s maximum velocity  how high in the air will it travel  how long will it stay in the air no air resistance, penny travels at a 90° angle to the track, you catch the penny when it returns to the track