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WARM UP

WARM UP. FRI SEPT 6 TH EXAM – 1 Measurement-Math-Lab Skills READ TEXT (pgs. 32-37) re: Motion. UNIT 2A. Linear Motion. Unit 2A : Linear Motion. distance | speed | direction acceleration. You can describe the motion of an object by its:. DAILY WARM UP. Is motion relative?

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WARM UP

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  1. WARM UP FRI SEPT 6TH EXAM – 1 Measurement-Math-Lab Skills READ TEXT (pgs. 32-37) re: Motion

  2. UNIT 2A Linear Motion

  3. Unit 2A: Linear Motion distance | speed | direction acceleration You can describe the motion of an object by its:

  4. DAILY WARM UP • Is motion relative? • Does it depend on your perspective?

  5. 2.1Motion Is Relative • How do you know if an object is moving? • Is your book moving? • The book is at rest, • relative to the table, BUT • It’s moving at about • 30 km/s relative to the sun. An object is moving if its positionrelative to a fixedpoint is changing.

  6. 2.1Motion Is Relative • An object’s motion must be described relative to something else. • shuttle 8 km/s relative to Earth below • race car 300 km/h relative to the track • The speedsof things on Earth are usuallymeasuredrelativeto the Earth’ssurface.

  7. Reference point Motion Problem: You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward. WHAT?? You have set yourself as the reference point as the car rolls slightly backward.

  8. MIKE’S PHYSICS Video

  9. Methods of Motion Honors Physics

  10. The bottom line….Motion is RELATIVE It depends completely on how you want to look at the moving object. You establish a frame of reference! Example: You are sitting in an airplane which is moving at a speed of 100 km/h and there is a fly sitting on your head. • What is your speed relative to the ground? • What is your speed relative to the seat you're sitting it? • What is the speed of the fly relative to you? 100 km/hr 0 km/hr 0 km/hr

  11. Displacement Distance - How far you travel regardless of direction. Displacement (x or y)   "Change in position" It is not necessarily the total distance traveled. In fact, displacement and distance are entirely different concepts. Displacement is relative to an axis. • "x" displacement means you are moving horizontally either right or left. • "y" displacement means you are moving vertically either up or down. • The word change is expressed using the Greek letter DELTA ( Δ ). • To find the change you ALWAYS subtract your FINAL - INITIAL position • It is therefore expressed as either   Δx = xf - xi or Δy = yf - yi

  12. Example The answer is positive so the person must have been traveling horizontally to the right. Suppose a person moves in a straight line from the lockers( at a position  x = 1.0 m) toward the physics lab(at a position x = 9.0 m) , as shown below

  13. Example Suppose the person turns around! The answer is negative so the person must have been traveling horizontally to the left What is the DISPLACEMENT for the entire trip? What is the total DISTANCE for the entire trip?

  14. Average Velocity Average SPEED is simply the “RATE at which DISTANCE changes”. Velocity is defined as: “The RATE at which DISPLACEMENTchanges”. Rate = ANY quantity divided by TIME.

  15. Example Let's look at the defender's average velocity: “m/s” is the derived unit for both speed and velocity. Let's look at the defender's speed: A quarterback throws a pass to a defender on the other team who intercepts the football. Assume the defender had to run 50 m away from the quarterback to catch the ball, then 15 m towards the quarterback before he is tackled. The entire play took 8 seconds.

  16. Slope – A basic graph model A basic model for understanding graphs in physics is SLOPE. Using the model - Look at the formula for velocity. Who gets to play the role of the slope? Who gets to play the role of the y-axis or the rise? Who get to play the role of the x-axis or the run?  What does all the mean? It means that if your are given a Displacement vs. Time graph, to find the velocity of an object during specific time intervals simply find the slope. Velocity Displacement Time

  17. Displacement vs. Time graph What is the velocity of the object from 0 seconds to 3 seconds? The velocity is the slope!

  18. Displacement vs. Time graph What is the velocity of the object from 7 seconds to 8 seconds?  Once again...find the slope! A velocity of 0 m/s. What does this mean? It is simple....the object has simply stopped moving for 1 second.

  19. Displacement vs. Time graph What is the velocity from 8-10 seconds? You must remember! To find the change it is final - initial. The answer is negative! It is no surprise, because the slope is considered to be negative. This value could mean several things: The object could be traveling WEST or SOUTH.The object is going backwards - this being the more likely choice! You should also understand that the slope does NOT change from 0-3s , 5 to 7s and 8- 10s. This means that the object has a CONSTANT VELOCITY or IT IS NOT ACCELERATING.

  20. Example It is very important that you are able to look at a graph and explain it's motion in great detail. These graphs can be very conceptual. Look at the time interval t = 0 to t = 9 seconds. What does the slope do? It increases, the velocity is increasing Look at the time interval t = 9 to t = 11 seconds. What does the slope do? No slope. The velocity is ZERO. Look at the time interval t = 11 to t = 15 seconds. What does the slope do? The slope is constant and positive. The object is moving forwards at a constant velocity. Look at the time interval t = 15 to t = 17 seconds. What does the slope do? The slope is constant and negative. The object is moving backwards at a constant velocity.

  21. 2.2Speed 400 yrs ago, people described motion as simply “slow” or “fast.” Galileo was the first to measure speed by the distance covered and the time it takes. 5 mi 0.20 h distance time avg. speed = speed = 25 mi/h avg. speed =

  22. 2.2Speed

  23. 2.2Speed • Cars do not always move at a constant speed. • You can tell the speed of the car at any instant by looking at the car’s speedometer. • instantaneous speed: • the speed at any instant Instantaneous Speed • average speed: • total distance • time

  24. 2.2Speed If we know average speed and travel time, the distance traveled is easy to find. Example: If your average speed is 80 km/h on a 4-hour trip, then how far did you travel? distance = 80 km = x km 1 h 4 hr distance time speed = distance = speed x time 320 km

  25. 2.2Speed If a cheetah can maintain a constant speed of 25 m/s, it will cover 25 meters every second. At this rate, how far will it travel in 10 seconds? distance = (25 m) = (x m) = (1 s) 10 s In 1 minute? distance = (25 m) x (x m) = (1 s) (60 s) 250 m distance = speed x time 1500 m

  26. 2.2Speed The speedometer in every car also has an odometer that records the distance traveled. If the odometer reads zero at the beginning of a trip and 35 km a half hour later, what is the average speed? distance time 35 km 0.5 h speed = = = 70 km/h

  27. Quick Quiz! • Jake walks east through a passenger car on a train that moves 10 m/s in the same direction. Jake’s speed relative to the car is 2 m/s. Jake’s speed relative to an observer at rest outside the train is ___. • 2 m/s • 5 m/s • 8 m/s • 12 m/s 2.1

  28. Quick Quiz. • A gazelle travels 2 km in a half hour. The gazelle’s average speed is ___. • 1/2 km/h • 1 km/h • 2 km/h • 4 km/h 2.2

  29. WARM UP

  30. 2.3Velocity • In physics, • Velocity:is speed in a direction. • speed: 60 km/h • velocity: 60 km/h north, or right, or down… ∆: change in… (final – initial) (df – di) t ∆d t m s (m/s) v =

  31. 2.3Velocity • If either the speedorthe direction (or both) • changes, then the velocitychanges. • constant speed and constant velocity are NOT the same. The car speedometer always reads 30 km/h. Is speed constant? Is velocity constant? Y N

  32. 2.4Acceleration We can change an object’s motion by changing its speed, its direction, or both. Accelerationis the rate at which velocitychanges. ∆v t (vf – vi) t a = acceleration can increaseordecrease speed,

  33. 2.4Acceleration We can change an object’s motion by changing its speed, its direction, or both. Acceleration:is the rate at which velocitychanges. ∆v t (vf – vi) t a = acceleration can increaseordecrease speed, deceleration is really negativeacceleration (–a)

  34. 2.4Acceleration Acceleration concerns change in velocity so any a change in directionis acceleration. The car speedometer always reads 30 km/h. Is velocity constant? Is there an acceleration? N Y

  35. 2.4Acceleration a in the same direction as v:speed up

  36. 2.4Acceleration a in the same direction as v:speed up a in the opp. direction as v:slow down

  37. 2.4Acceleration a in the same direction as v:speed up a in the opp. direction as v:slow down a at an angle to v:change direction

  38. 2.4Acceleration • vunits are in distance per time: (m/s) • a is the change in v per change in time. • aunits are v per time: (m/s per s) or (m/s2) • changing v from 0 m/s to 10 m/s in 1 s, • a is… ∆v t m/s s m s2 or a = 10 m/s – 0 m/s 1 s 10 m/s 1 s a = 10 m/s2 = =

  39. 2.4Acceleration In 5 seconds a car increases its speed from 8 m/s to 18 m/s, while a truck goes from rest to 10 m/s in a straight line. Which undergoes greater acceleration? 18 m/s – 8 m/s 5 s 10 m/s 5 s acar = 2 m/s2 = = 10 m/s – 0 m/s 5 s 10 m/s 5 s atruck = 2 m/s2 = =

  40. Quick Quiz! • Constant speed in a constant direction is… • constant velocity. • constant acceleration. • instantaneous speed. • average velocity. 2.3

  41. Quick Quiz. • A vehicle undergoes acceleration when it __. • gains speed. • decreases speed. • changes direction. • ALL of the above Check off the learning targets you can do after today. 2.4

  42. Acceleration Honors Physics

  43. The RATE of CHANGE of VELOCITY D = Change = FINAL - INITIAL Dv = Final velocity – Initial velocity • Acceleration – The Definition

  44. Example: A Cessna Aircraft goes from 0 m/s to 60 m/s in 13 seconds. Calculate the aircraft’s acceleration. • Example • 4.62 m/s/s

  45. Example: The Cessna now decides to land and goes from 60 m/s to 0 m/s in 11 s. Calculate the Cessna’s ? • Example • deceleration • - 5.45 m/s/s

  46. If an object is in FREE FALL in the VERTICAL DIRECTION, the acceleration is due to GRAVITY. • Free-Fall Acceleration • It NEVER ceases to exist • It ALWAYS works DOWN • It is NEVER zero • This is ONLY true in a vacuum (no air)

  47. Example: A person throws a ball straight upward into the air. Q1: What is the Acceleration at the TOP of its path? • Acceleration due to Gravity -9.8 m/s/s • Q2: What is the VELOCITY at the TOP of its path? ZERO

  48. Q3: What is the magnitude(#value) and direction of the acceleration, HALF way up? • Acceleration due to Gravity • -9.8 m/s/s - ALWAYS DOWNWARD • Q4: What is the magnitude(#value) and direction of the acceleration, HALF way down? • -9.8 m/s/s - ALWAYS DOWNWARD • THE BOTTOM LINE:EVERYTHING will accelerate at -9.8 m/s/s in a VACUUM, • that is any situation involving NO AIR.

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