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Physics Unit 3

Physics Unit 3. Year Long Plan. First Ten Minutes (Everyday) – Revision, questions Then Learning/ Pracs Homework… 40+ Club. What you need. Textbook Student Book Scientific Calculator. Topics. Unit 3: Motion Electronics and Photonics Unit 4: Electric Power

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Physics Unit 3

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  1. Physics Unit 3

  2. Year Long Plan • First Ten Minutes (Everyday) – Revision, questions • Then Learning/Pracs • Homework… • 40+ Club

  3. What you need • Textbook • Student Book • Scientific Calculator

  4. Topics • Unit 3: Motion • Electronics and Photonics • Unit 4: Electric Power • Interaction of light and matter • Detailed Study (Homework) • …Plus an extra project…?

  5. Project…? • Print off News article. • Youtube video

  6. Area of Study One Motion in One and Two Dimensions

  7. The Plan • Review of Motion • Projectile Motion • Momentum • Energy • Circular Motion • Gravity and Satellites

  8. Review of Motion • Place the following into a linked “Concept Map”. Label all the arrows that link the concepts together. • Forces, Newton’s First Law, Newtons Second Law, Newton’s Third Law, Eqns of motion, mometum, velocity, acceleration, Impulse, Work, kinetic energy, potential energy, Types of forces, Gravity, Normal Force, Springs, Inclined Planes

  9. Review • CUPS 2

  10. Scalars and Vectors • Scalar: Physical quantity represented by only a number • Eg Mass, temperature • Vector: Physical quantity requiring a direction AND number (magnitude) • Eg Force, velocity

  11. Distance and Displacement • Distance Length of an object has travelled e.g. total distance of travel Scalar • Displacement Change in position of an object. e.g. final position – initial position Vector

  12. Speed or Velocity? Magnitude only • Speed: Scalar quantity. • Velocity: Vector quantity. Magnitude AND Direction

  13. Velocity • To find velocity, when travelling at a constant velocity (no acceleration) OR to find the average velocity:

  14. Questions • Tim travels 80m North up North Rd. He then turns and travels 60m East along east road. This travel takes 24s. • Draw a diagram of the situation • Calculate the total distance • Calculate the total displacement • Calculate the average speed • Calculate the average velocity

  15. Centre of mass motion • Usain Bolt runs the 100m with a speed of 10ms-1 • Do all parts of his body move at 10ms-1? • His arms? His legs? His Head? • No. His arms often move faster (and slower) than 10ms-1

  16. Centre of Mass Motion • As physicists, we simplify the problem, and approximate Usain Bolt as a point, at his centre of mass. • So all calculations of his speed are based on his centre of mass

  17. Joke • A Statistician, Engineer and Physicist go to the horse track. Each have their system for betting on the winner and they're sure of it. After the race is over, the Statistician wanders into the nearby bar, defeated. He notices the Engineer, sits down next to him, and begins lamenting: "I don't understand it. I tabulated the recent performance of all these horses, cross-referenced them with trends for others of their breed, considered seasonal variability, everything. I couldn't have lost.“ "Yeah," says the Engineer, "well, forget that. I ran simulations based on their weight, mechanical ratios, performance models, everything, and I'm no better off.“ Suddenly, they notice a commotion in the corner. The Physicist is sitting there, buying rounds and counting his winnings. The Engineer and Statistician decide they've got to know, so they shuffle over and ask him, "what's your secret, how'd you do it?“ The Physicist leans back, takes a deep breath, and begins, "Well, first I assumed all the horses were spherical and identical..."

  18. Acceleration • Acceleration is a measure of how much velocity changes over time • Change in velocity: • Acceleration:

  19. Graphing motion • We can graph either the displacement, velocity, acceleration as time changes • The Gradient of a graph is the slope. • The area under the graph is the solid area between the line and the axis • Eg …

  20. Zero Slope/ Gradient Positive Slope/ Gradient Negative Slope/ Gradient Area under graph

  21. Graphing Motion • Prac: Graphing Motion

  22. Displacement-Time Graph A puppy runs after a stick. It runs 10m to the stick (it takes 10s). It then waits by the stick for 10s, and finally brings the stick back to its owner over 10s. Draw a displacement-time graph for this scenario

  23. Displacement-Time Graph

  24. Displacement-Time Graphs Gradient of the displacement-time graph is the velocity Questions: • Calculate the velocity over the first 10s • Calculate the velocity over the next 10s • Calculate the velocity over the last 10s • Calculate the average velocity over the full 30s

  25. Velocity-Time Graph • Gradient of a velocity-time graph is acceleration • Area under the velocity graph is the displacement • Eg…

  26. Velocity-Time Graph

  27. Velocity-Time Graph • What is the initial velocity? • What is the change in velocity over the 30s of motion • What is the acceleration? • Is this a constant acceleration? • What is the total displacement between 0 and 30s?

  28. Acceleration-Time Graph • The area under an acceleration-time graph is the change in velocity.

  29. Acceleration-Time Graph

  30. Acceleration-Time Graph • What is the acceleration at 0s? • What is the acceleration at 10s? • What could this graph be describing? • Find the change in velocity between 0s and 10s • If the initial velocity is 2ms-1, what is the velocity after 10s?

  31. Questions • Draw a displacement-time, velocity-time, and acceleration-time graph for the following: • Sky diver – no air resistance • Sky diver – air resistance • Person walking at a constant speed along a path • Person 20m away from their house, standing still

  32. Equations of Motion • Any object moving with a constant acceleration, use the equations of motion

  33. Example • A car accelerates from rest for 10s at an acceleration of 1.5ms-2 • What is the final speed? • What distance does the car travel over this time

  34. Questions • A car, travelling at 30ms-1, accelerates to 40ms-1 in order to pass a slower car. This acceleration takes 20s. What distance does he travel during this acceleration? • Q4 [Pg12] A car travelling at a constant speed of 80km/h passes a stationary motorcycle policeman. The policeman sets of in pursuit, accelerating uniformly to 80km/h in 10s and reaching a constant speed of 100km/h after a further 5s. At what time will the policeman catch up with the car. • Extension question: Mr McGovern is driving down a country road at 100km/h when a beautiful duck steps out 75m in front of his car. His reaction time is 0.6s, then he applies the brakes, decelerating at 15ms-2. Will the duck live?

  35. Vertical Motion • Vertical motion is accelerated motion where the acceleration equals gravity (10ms-2)

  36. Vertical Motion How high does the tennis ball go? • Time how long it takes to get to the top of its flight • What was the initial velocity? • How high did the ball go? • After the ball left the hand, draw the forces acting on it • When the ball was at the top of its flight, draw the forces acting on it

  37. Question • A footy ball is kicked vertically upwards with an initial speed of 22ms-1 • How high does it reach? • After what time does it hit the ground again?

  38. Forces • When we add all the forces acting on a body, we add the forces head to tail • eg

  39. Forces • The total force is found by drawing a new arrow from the tail of the first to the head of the last

  40. Find the total forces

  41. Q3 2012 • A metal ring is to be held stationary by three forces. Which configuration would make the ring stationary, and why?

  42. Newton’s Laws • First Law: Unless acted on by a net force, an object will continue its motion (whether that’s stationary or constant velocity • New Name: • Second Law: If acted on by a net force, an object will accelerate • New Name:

  43. Newton’s Laws • Third Law: For every action force on object A, there is an equal and opposite reaction force on object B • New Name:

  44. Newton’s Laws • Why is there misunderstanding in Newton’s Laws? • Rules for next 6 examples. • Quietly think of which answer you like. • Text in answer • Then we will work together to decide how most people think… • And what Newton’s Laws predict the answer should be

  45. Newton’s Laws: Example 1 • Victoria Azerenka throws a tennis ball upwards for her serve. Consider the forces on the tennis ball after it has left the hand, but before she hits it on the way down. Is there…?

  46. Newton’s Laws: Example 1 • A downwards force of gravity, along with a steady decreasing upwards force • A steadily decreasing upward force from the moment it leaves her hand until it reaches its highest point, on the way down a steadily increasing downwards force of gravity • An almost constant downwards force of gravity

  47. Newton’s Laws: Example 1 • What do you think the answer is? • What do you think most of the general population will think? • What answer does Newton’s Laws predict?

  48. Newton’s Laws: Example 2 An elevator is being lifted by a steel cable at a constant speed. The forces on the elevator are…

  49. Newton’s Laws: Example 2 • The upwards force of the cable is greater than the downward force of gravity • The upwards force of the cable is equal to the downwards force of gravity • The upwards force of the cable is smaller than the downwards force of gravity • None of the above: The elevator goes up simply because the cable is being shortened

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