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Chapter 2

Newton’s Laws. Chapter 2. Opening Discussion. Imagine a WWII battle, a massive convoy is moving slowly though enemy waters with gun crews on alert. Suddenly a lone aircraft swoops from the sky diving straight for a battleship. Every gun crew in the convoy sees the aircraft and opens fire.

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Chapter 2

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  1. Newton’s Laws Chapter 2

  2. Opening Discussion Imagine a WWII battle, a massive convoy is moving slowly though enemy waters with gun crews on alert. Suddenly a lone aircraft swoops from the sky diving straight for a battleship. Every gun crew in the convoy sees the aircraft and opens fire. If the plane is destroyed, will the battleship be harmed?

  3. Opening Discussion • If the plane is destroyed, will the battleship be harmed? • Well it depends on a few things: • Gravity • Air Resistance • Distance

  4. Opening Discussion • What would happen if we took this same battle scenario into space? • NOTE: Assume the Star Destroyer and its shields have been disabled. • Low gravity and no air resistance means the debris will just keep moving in a straight line.

  5. Opening Discussion

  6. Newton’s First Law • Both had inertia, which kept the debris moving forward • Inertia is an objects resistance to change in motion • Change in direction • Change in speed • What did both of those fighters have in common?

  7. Newton’s First Law • There were other forces present on Earth! • A force is a push or pull on an object • These forces were present… • Air resistance • Gravity • Why did the two battles have different results?

  8. Newton’s First Law • Objects at rest will stay at rest • Objects in motion will continue in a straight line unless acted on by another force • Also called the “law of inertia” • How does this all tie into Newton’s First Law?

  9. Newton’s First Law • Friction! • Two types: • Kinetic – two surfaces move past one another • Static – two surfaces that are not moving • Friction must be overcome to get an object moving AND to keep it moving • Air resistance is simply friction created by air • What force keeps us from sliding all over the place?

  10. Newton’s Laws and Movie Physics • Video 1 - In Young Guns 2, how did Billy the Kid violate Newton’s 1st Law?

  11. Newton’s Laws and Movie Physics • Video 2 - In Road to Perdition, how did the gunman with the shotgun obey Newton’s 1st Law?

  12. Newton’s Laws and Movie Physics • Video 3 - In the Matrix Revolutions, identify times when Newton’s 1st Law was violated?

  13. Newton’s Laws and Movie Phsyics • In Young Guns 2, how did Billy the Kid violate Newton’s 1st Law? • The window (and everything else) behind the candles is unharmed and the spinning candelabra • In Road to Perdition, how did the gunman with the shotgun obey Newton’s 1st Law? • The buckshot goes through the wall and kills the accountant on the other side • In Matrix Revolutions, identify times when Newton’s 1st Law was violated? • Quickly moving machines just drop when they are killed 2 feet in front of an APU

  14. Opening Discussion (Part 2) • Is it easier to pull the tablecloth off a table with full dishes or empty ones? Explain.

  15. Opening Discussion (Part 2) • Would it be easier to move a boulder in space? Explain.

  16. Opening Discussion (Part 2) • If you are in an airplane and flip a coin into the air, where would it land and why?

  17. A Measure of Inertia • Mass is the amount of material in an object • The more massive an object is, the more force it takes to change its state of motion • What determines the inertia of a substance?

  18. Mass and Volume • NO! • Volume is how much space an object takes up • Two objects can have the same mass, but have different volumes and vice versa • Are mass and volume the same thing?

  19. Mass and Weight • NO! • Weight is determined by gravity • Heavy objects on Earth would not be heavy in space, but they still have the same mass • Are mass and weight the same thing?

  20. Mass and Weight • NO! • Inertia and mass are directly related! • Gravity does not affect mass! • If the mass of an object doesn’t change, does its inertia change?

  21. The Moving Earth • Earth moves at 30 km/s • This means you, the school and ground are also moving at 30 km/s… Inertia keeps us moving • Vertical and horizontal motion are not connected • If you jumped off the school roof, why doesn’t the Earth move underneath you?

  22. Full Circle • Is it easier to pull the tablecloth off a table with full dishes or empty ones? Explain. • Would it be easier to move a boulder in space? • If you are in an airplane and flip a coin into the air, where would it land and why? • Full – because they have more mass and more inertia (don’t want to move) • No, it still has the same mass and inertia • Coin moves with you because it has inertia and wants to move forward

  23. Tablecloth Pull - BMW

  24. Tablecloth Pull - MythBusters

  25. Cruise Ship Crash • A runaway cruise ship, its engines unstoppable, is headed straight for a massive wooden dock. Its passengers scream as the boat slowly rips through the dock and two passengers get hurled through some of the frontward facing windows. • Why weren’t the passengers in danger? • Should the two passengers have got hurled through the window? Explain.

  26. Car Crash • A family of four is driving from a city to a seaside resort when the father loses control of the car and crashes it into an oncoming minibus. • What happens to the passengers and the car?

  27. Car Crash

  28. Crashes Combined • Both vehicles were slowing down • Both ran into other objects • Moving at different initial speeds • Slowed down at different rates • Had different masses • Stopping distance • How are the two crashes similar? • How are the two crashes different?

  29. Newton’s Second Law • Force and acceleration • Force – push or pull on an object • Units: Newton (N) • Acceleration – change in velocity over time • What two quantities does Newton’s 2nd Law connect?

  30. Force and Acceleration • They are directly proportional • As force increases, so will acceleration and vice versa • The net force and the acceleration are always pointing in the same direction • How are force and acceleration connected?

  31. Net Force • What is net force? • The sum of all of the forces acting on an object 5 N 8 N 5 N 2 N

  32. Mass and Acceleration • They are indirectly (inversely) proportional • As mass increases acceleration decreases and vice versa • Full cart vs. empty cart • How are mass and acceleration connected?

  33. Newton’s Second Law • What does Newton’s second law state? • Acceleration is proportional to force and mass • F = ma “Big force equals your ma” F m a

  34. Newton’s Second Law • How much force is required to accelerate a 30000 kg jet 1.5 m/s2? • If a 500 kg car has 2000 N of force, what is its acceleration? • What if its mass was 1000 kg? • F = ma • F = 30000(1.5) • F = 45000 N • F = ma • 2000 = 500a • a = 4 m/s2 • a = 2 m/s2 (double the mass, half the acceleration)

  35. Newton’s Second Law • What is the mass of a ball that needs 45 N of force to accelerate it 36 m/s2? • What is the acceleration of a 2.5 kg box if it is pushed with 7.7 N of force? • Calculate the force of a 3000 kg car going from 4 m/s to 24 m/s in 5 seconds. • 1.25 kg • 3.08 m/s2 • 12000 N

  36. Cruise Ship Crash Revisited • Deceleration was gradual because the ship was so large • People decelerated with the ship • No, same reason as above • Plus, ship windows are built tough • Explain why the passengers were not in any danger. • Should the two passengers have got hurled through the window?

  37. Car Crash Revisited • Car stops almost instantly and gets partially crushed because of inertia • Passengers continue to move forward because of inertia until another force stops them, friction • What happens to the passengers and the car?

  38. Car Crash Revisited • The car is traveling at 20 m/s and stops within 0.1 seconds. • What is the acceleration? • Slows down at 200 m/s2 • How many g’s is this? (Simply divide by the acceleration due to gravity, 10 m/s2.) • 20 g’s • The man has a mass of 100 kg. • What is the force of the man? • 20000 N

  39. Forces • Forces always act in pairs, so therefore there will always be an even number of them • What is the general rule about forces?

  40. Forces • A visual representation of all of the forces acting on an object • Forces have direction so we will use arrows to represent them • What is a free body diagram?

  41. Forces • Two • Force of gravity (FG) • Downward direction • Normal force (FN) • Support force created by a surface or by tension • Upward direction • How many forces act on a box at rest on a flat surface?

  42. Forces • Four • Force of gravity (FG) • Normal force (FN) • Applied force (Fa) • A push or pull created by a machine or person • Whatever direction is indicated • Friction force (Ff) • Kinetic friction • Works against applied force in the opposite direction • How many forces act on a box sliding across a surface?

  43. Forces • Two • Force of gravity (FG) • Air resistance (Fair) • Special type of kinetic friction caused by particles in our atmosphere • Works in the opposite direction that the object is moving • How many forces act on a falling box?

  44. Equilibrium • What is equilibrium? • The net force is zero • An object will then either stay at rest or move at a constant velocity • Ex. Plane or Hanging Object

  45. Darius Stone • In the second xXx movie, Darius Stone jumps off a 160 mph train that is on a bridge over a body of water. The bridge was approximately 300 ft. in height. The jump is done incorrectly in order for Darius to remain unharmed. • What did Darius do correctly and incorrectly?

  46. Superman • Lois Lane falls from the 15th floor and Superman whisks her away seconds before she hits the pavement. At the time that she was rescued, Lois was traveling at approximately 120 mph. • If Superman were to exist, what is the problem with this scenario?

  47. Experiencing g’s • What does it mean to experience g’s? • g is the acceleration due to gravity • g = 10 m/s2 • Weight = mg • 5 g’s is 5 times gravity, so 5 times as heavy • Can feel changes in g’s by flying, riding rollercoasters, spinning, etc.

  48. Its Not the Fall • Sidewalk moves a little bit/cracks • Stopping distance provided by the person • 25% body crumple because inertia keeps the person moving • Person experiences approximately 327 g’s • What happens when a person falls feet first at the speed of a skydive (120 mph) onto a sidewalk?

  49. Its Not the Fall • Yes • A Russian airman once fell 22000 feet without a parachute (other pilots have also survived) • Is any fall from a great height survivable?

  50. Its Not the Fall • Extend the stopping distance provided by crumpling other materials, not the person • Experience a longer time slowing down and less g’s • Snow, bushes, trees, a hillside and water are all good examples • How is a great fall survivable?

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