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  1. Physics: Motion, Forces and Motion, Forces in Fluids and Work and Machines

  2. Motion 11-1 • An object is in motion if it changes position relative to a reference point • Stationary objects make good reference points

  3. To describe motion accurately and completely, a frame of reference is necessary. • The answer to “How fast is the butterfly is moving?” depends on which frame of reference you use to measure its motion. • The answer to “ How fast are the train passengers moving?” depends on the frame of reference you use measure their motion. • Choosing a meaningful frame of reference allows you to describe motion in a clear and relevant manner.

  4. Relative Motion • Whether or not an object is in motion depends on the reference point you choose.

  5. Distance and Displacement • Distance is the total length of the actual path between two points. Displacement is the length and direction of a straight line between starting and ending points. What is the total distance this person traveled (in blocks)? 7 Blocks What is the total displacement of this person? 5 Blocks Northeast

  6. More on Displacement: Vectors • Vector quantities that have both a magnitude and a direction

  7. Calculating Speed 11-2 • If you know the distance an object travels in a certain amount of time, you can calculate the speed of the object.

  8. Average Speed • The speed of most moving objects is not constant

  9. Instantaneous Speed • Rate at which object is moving at a given instant in time

  10. A few more practice problems…

  11. Velocity • Speed in a given direction • Velocity is a vector because it has both magnitude and direction • Changes in velocity may be due to changes in speed, changes in direction, or both • A cheetah’s speed may be as fast as 90 km/h but to describe its velocity you must know the direction in which it is moving. • As the sailboat’s direction changes, its velocity also changes, even if its speed stays the same.

  12. Graphing Motion • You can use distance-versus-time graphs to interpret motion.

  13. Let’s Review!

  14. Review Questions 1. Is a moving bus a good reference point from which to measure your position? a. No, because it is often late. b. No, because it is not a stationary object. c. Yes, because it is very large. d. Yes, because it can travel very far.

  15. Review Questions 1. Is a moving bus a good reference point from which to measure your position? a. No, because it is often late. b. No, because it is not a stationary object. c. Yes, because it is very large. d. Yes, because it can travel very far.

  16. Review Questions 2. To describe a friend’s position with respect to you, you need to know a. Your friend’s distance from you. b. The direction your friend is facing. c. Your friend’s distance and direction from you. d. Your friend’s distance from a nearby object.

  17. Review Questions 2. To describe a friend’s position with respect to you, you need to know a. Your friend’s distance from you. b. The direction your friend is facing. c. Your friend’s distance and direction from you. d. Your friend’s distance from a nearby object.

  18. Review Questions 3. Two cars traveling in the same direction pass you at exactly the same time. The car that is going faster a. moves farther in the same amount of time. b. has more mass. c. has the louder engine. d. has less momentum.

  19. Review Questions 3. Two cars traveling in the same direction pass you at exactly the same time. The car that is going faster a. moves farther in the same amount of time. b. has more mass. c. has the louder engine. d. has less momentum.

  20. Review Questions 4. To describe an object’s motion, you need to know its a. position. b. change in position. c. distance. d. change in position over time.

  21. Review Questions 4. To describe an object’s motion, you need to know its a. position. b. change in position. c. distance. d. change in position over time.

  22. Acceleration 11-3 • The rate at which velocity (speed and direction) changes • Is a vector quantity • In science, acceleration refers to changes in speed, changes in direction or both • Decreasing speed = deceleration • Describe the acceleration occurring at this instant on the roller coaster.

  23. Calculating Acceleration • To determine the acceleration of an object, you must calculate its change in velocity per unit of time.

  24. Let’s Try a Problem • Calculate the plane’s acceleration in the first 5 seconds of motion. A= Vf – Vi time A = 40 m/s – 0 m/s 5 s A = 8 m/s2

  25. Calculating Acceleration • As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration? • Read and Understand • What information have you been given? • Initial velocity = 4 m/s • Final velocity = 22 m/s • Time = 3 s

  26. Calculating Acceleration • As a roller-coaster car starts down a slope, its velocity is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration? • Plan and Solve • What quantity are you trying to calculate? • The acceleration of the roller-coaster car = __ • What formula contains the given quantities and the unknown quantity? • Acceleration = (Final velocity - Initial velocity)/Time • Perform the calculation. • Acceleration = (22 m/s - 4 m/s)/3 s = 18 m/s/3s • Acceleration = 6 m/s2 • The acceleration is 6 m/s2 down the slope .

  27. Calculating Acceleration • Practice Problem • A falling raindrop accelerates from 10 m/s to 30 m/s in 2 seconds. What is the raindrop’s acceleration? • (30 m/s - 10 m/s) ÷ 2 seconds = 10 m/s2

  28. Calculating Acceleration • Practice Problem • A certain car can accelerate from rest to 27 m/s in 9 seconds. Find the car’s acceleration. • (27 m/s - 0 m/s) ÷ 9 s = 27 m/s ÷ 9 s = 3 m/s2

  29. Graphing Acceleration • You can use both a speed-versus-time graph and a distance-versus-time graph to analyze the motion of an accelerating object.

  30. Mr. Edmonds!! • http://youtu.be/4CWlNoNpXCc

  31. 12-1: Forces

  32. What is a force? • A push or pull that acts on an object • Is a vector quantity • Described by its magnitude and by the direction in which it acts • Arrow represents direction • Magnitude unit = Newton (N) • 1N = 1 kg*m/s²

  33. Net Force • Often there is more than one force acting on an object at the same time • Pushing a car that’s run out of gas • The result is net force, a combination of all the forces • Net force determines whether an object moves and in which direction • Sometimes the net force is zero

  34. Combining Force Vectors • The strength and direction of the individual forces determine the net force.

  35. Unbalanced Forces • Unbalanced forces acting on an object result in a net force and cause a change in the object’s velocity.

  36. Balanced Forces • Balanced forces acting on an object do not change the object’s velocity.

  37. Friction • A force 2 surfaces exert on each other when they rub against each other • Acts as an unbalanced force to slow motion down • The strength of the force of friction depends on the types of surfaces involved and how hard the surfaces push together • 4 types of friction

  38. 1) Static Friction • Acts on objects that are not moving • You must exert a force greater than the force of static friction to make the object move

  39. 2) Sliding Friction • Occurs when two solid surfaces slide over each other

  40. 3) Rolling Friction • Occurs when an object rolls across a surface • Rolling friction is less than sliding friction for similar surfaces

  41. 4) Fluid Friction • Occurs when a solid object moves through a fluid, such as air, water, oil, etc. • Fluid friction is usually less than sliding friction

  42. Gravity • Force that pulls towards the center of the earth • Newton realized that gravity acts everywhere, not just on earth • Called the Law of Universal Gravitation • Any 2 objects in the universe attract each other

  43. Gravity Between Objects • The force of gravity between objects increases with greater mass and decreases with greater distance.

  44. Mass and Weight • Mass is how much matter is in an object • The gravitational force exerted on a person or object at the surface of a planet is known as weight. • Weight = Mass x Acceleration due to gravity Acceleration due to gravity at Earth’s surface = 9.8 m/s2

  45. Free Fall • If the only force acting on the object is gravity, it is said to be in free fall • An object in free fall is accelerating because of the force of gravity at a rate of 9.8 m/s/s • This means that every second an object is free falling, it increases its velocity 9.8 m/s • Is this affected by mass? • If dropped from the same height at the same time, will a heavier object fall faster?

  46. Free Fall • No! If there are no other forces to consider, then the objects will fall at the same rate, regardless of mass.

  47. Air resistance • A type of fluid friction that acts on objects falling through the air • An upward force acting on a falling object