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POSITION, DISPLACEMENT

POSITION, DISPLACEMENT Kinematics is a branch of physics that is concerned with describing motion. The first concept of motion is position. Think of your position at this instant. Another way of asking this question is “Where are you?”.

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POSITION, DISPLACEMENT

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  1. POSITION, DISPLACEMENT Kinematics is a branch of physics that is concerned with describing motion. The first concept of motion is position. Think of your position at this instant. Another way of asking this question is “Where are you?”

  2. Position is defined as the distance and direction from a reference point. A reference point may be in fact moving (e.g. Earth). Reference points are arbitrary choices made by us. Position is a vector quantity because it has size or magnitude and direction. Scalar quantities have only size or magnitude. The symbol for position is

  3. Direction can be given above or below the horizon, within a 2-D system (NEWS), or within a 3-D system (on Star Trek). After the examples on the next slide complete the position worksheet.

  4. 15.41 cm (measure) 11.6o (measure) 6.52 cm (measure) 30.2o (measure) For this class use the 2-D map system. Record direction relative to north or south. r.p. 6.52 cm [S11.6oW] 15.41 cm [N59.8oE]

  5. DISPLACEMENT Displacement is defined as change in position. Displacement is a vector with magnitude and direction. It is found in the difference between two position vectors. Take note that it is always final position minus initial position.

  6. Displacement does not need a reference point. Displacement remains the same even if the reference point was changed. 25.31 cm (measure) Try worksheet initial point Arrow points to final position! r.p. final point 25.31 cm [S65.5oE] 24.5o (measure)

  7. Distance is defined as the length of path traveled. It is a scalar quantity and its symbol is d Solving problems involving displacement and distance demands we use proper form.

  8. Kinematics Problems -only one-dimensional problems will be studied -always establish a direction system for each question Adam is 59 m [E] of the flagpole when he chases Nick after Nick stole his lunch money. Later Adam is standing 24 m [W] of the flag pole. What was Adam’s displacement during the chase?

  9. East is positive Flagpole is r.p. The answer works out just as well with West is positive. Note thatthe path Adam took from start to finish had no effect on displacement.

  10. Meisha walks 350 m [N] and ends up standing 75 m [S] of the school. What was her starting position? South is positive School is r.p.

  11. Questions • Q stands 38.5 m [W] of Bell. He walks 82.7 m [E]. What is Q’s final position? • 2. Stacey finishes a canoe trip 15.6 km [S] of her starting position. Her final position is 18.9 km [S] of Palmer Rapids. What was Stacey’s initial position? • 3. Shawna jumps from a plane that is 9562 m above sea level and she pulls her rip cord at 1271 m above sea level. What was her displacement during her free fall?

  12. Answers 1. 44.2 m [E] 2. 3.3 km [S] 3. 8291 m [down]

  13. Displacement At this point displacement is defined as or change in position. Another way to calculate displacement is to sum up individual displacements. resultant or net displacement

  14. Distance is defined as the sum of individual path lengths regardless of direction. Kiah takes her dog for a walk from her house, she walks 300 m [E] followed by 200 m [W]. a) What is her resultant displacement? b) What is her total distance?

  15. East is positive with no vector above, this tells us it is path length a) b)

  16. Jason begins a walk 50 m [E] of Future Shop. Later he is 75 m [W] of Future Shop. Finally Jason is in Future Shop.(Assume Jason traveled in a line) • What was Jason’s displacement for each stage of his walk? (125 m [W], 75 m [E]) • What was Jason’s displacement for the walk? (50 m [W]) • What distance did Jason travel on his walk? (200 m)

  17. Velocity and Speed ARE YOU MOVING? CAN YOU FEEL MOVEMENT? HOW DO YOU KNOW IF YOU ARE MOVING?

  18. If you are standing on the equator, how fast are you moving? How fast is Earth moving? Work with two others to solve these problems. Ask me questions if you need certain facts.

  19. Velocity is defined as the change in position with respect to time. It is a vector quantity. a vector divided by a scalar yields a vector Speed is defined as distance traveled with respect to time. It is a scalar quantity.

  20. 1. Aaron runs 40 m [E] on a soccer field in 28 seconds as she hustles to get the ball. Calculate her average velocity. (1.428 m/s [E]) 2. Karan jogs at 52 m/s [NE] on his daily run. Today he ran 14.7 km [NE]. How long did he jog for? (282.7 s) 3. Mr. Moors runs at 1.8 m/s for 22 seconds while Stacy chases him uttering profanities. What distance did Mr. Moors travel during the chase? (39.6 m)

  21. 4. Jordyn walks 300 m [N] for 38 s then 178 m [S] for 62 s then 53 m [N] for 8 s in an effort to find Jackie. Her final position is 109 m [S] of the office. • What was Jordyn’s average velocity? (1.620 m/s [N]) • b) What was Jordyn’s average speed? (4.917 m/s) • c) What was Jordyn’s initial position? (284 m [S]) • d) What was Jordyn’s average velocity for the first two legs of her trip? (1.12 m/s [N])

  22. Acceleration Acceleration is defined as change in velocity with respect to time. In everyday usage acceleration often seems to mean speed up while decelerate means to slow down however technically this is not correct.

  23. Acceleration is a vector quantity therefore has magnitude and direction. In one-dimension acceleration can have a positive or negative value. The sign indicates the sign of the velocity change since the change in time is always positive.Physicists do not mention deceleration. Speeding up refers to increasing magnitude of velocity while slowing down means the opposite. LET’S DEMONSTRATE!!!

  24. A runner accelerates from rest at 1.8 m/s2 [W].The final velocity of the runner is 11.5 m/s [W].How much time elapsed during this acceleration?

  25. Billy travels at 14 m/s [NE] when hit by a truck, 0.145 s later Billy is travelling 52 m/s [SW]. What was B’s average acceleration during this time?(455.2m/s2[SW]) • Gina accelerates at 2.89 m/s2 [N] for 14 s, Gina’s initial velocity was 15 km/hr [N]. What is G’s final velocity?(44.63 m/s [N]) • Jen accelerates at 2.5 m/s2 [E] from 12.36 m/s [W] to 15.98 m/s [E]. How much time elapsed for this acceleration?(11.34 s)

  26. 4. A car accelerates at 2481 km/hr2 [S] for 10 min. Its final velocity is 26 m/s [S].What was the initial velocity of the car? (88.86 m/s [N])

  27. Constant Acceleration Kinematic Equations Motion with a constant acceleration is an important type of motion which bears further analysis. In this unit only constant acceleration problems will be studied. The kinematic equations about to be shown or developed are only for constant acceleration!

  28. The average velocity during a constant acceleration is found with this equation.

  29. So at this point when studying constant acceleration we have two equations. • Billy accelerates uniformly from 10 m/s [E] to 21.3 m/s [W] in 13.4 s. a)What was Billy’s displacement during this time? (75.7 m [W])b) What was Billy’s acceleration? (2.336 m/s2 [W])

  30. 2. A car travels 52 m [S] during 3.5 s of constant acceleration. The final velocity of the car was 77.76 km/hr [S] a) What was the car’s initial velocity? (8.114 m/s [S])b) What was the cars acceleration? (3.853 m/s2 [S])

  31. Constant Acceleration Kinematic Equations The two constant acceleration equations below lead to 3 other kinematic equations.

  32. -try and show how the last two are made

  33. A turtle attained a velocity of 20 cm/s [E] after accelerating at 5 cm/s2 [E] for 3.5 s. a) What was the displacement of the turtle during this acceleration? (39.38 cm [E]) b) What was the turtle’s initial velocity? (2.5 cm/s [E]) • 2. A bike accelerated from rest to a velocity of 42 km/hr [S] over a displacement of 87 m [S]. What was the acceleration of the bike? (0.7825 m/s2 [S])

  34. 3. A sportscar brakes hard and accelerates at 4.51m/s2 [S] over a displacement of 48.5 m [N] and a time of 2.31 s. What was the initial velocity of the car? (26.20 m/s [N]) 4. An object starts from rest and accelerates at 3.0 ms-2 for 4.0 s.Its velocity remains constant for 7.0 s and it finally comes to rest with uniform deceleration after another 5.0 s. a) What is the displacement of each stage. b) What is the average velocity of the whole trip?

  35. 5. A turtle is moving with a constant acceleration along a straight ditch. He starts his stopwatch as he passes a fence post and notes that it takes him 10 s to reach a pine tree 10 m farther along the ditch.As he passes the pine tree his speed is 1.2 ms-1. How far was he from the fence post when he started from rest?

  36. p. 28 3-8 10-12 extra (graphs) p. 36 1-9 p. 46 1-5,7

  37. ACCELERATION DUE TO GRAVITY Objects that have a net force of gravity acting upon them experience an accel-eration due to gravity of 9.81 m/s2 [down]. g = 9.81 m/s2 [down] In these situations the force of air resistance is ignored. (terminal velocity, coffee filter and motion sensor)This acceleration applies to objects going up or down (or both).

  38. An object is thrown upwards and returns to the same height. position time

  39. slope = - 9.81 m/s2 velocity time

  40. - 9.81 m/s2 acceleration time

  41. at max. height v = 0 the velocity at this time is negative of the initial velocity position time these two Dt’s are equal

  42. An object is thrown upwards with a velocity of 24.2 m/s. • What is the object’s maximum height? • How long does it take the object to return to its starting height?

  43. Kinematics Describing how things move is called kinematics and it has terms that are very specific. These terms include position, displacement, distance, velocity, speed and acceleration.Proper use of these terms first demands that you understand each term’s definition, symbol and any equations associated with it.

  44. Graphing Graphs are often used in kinematics to represent position,velocity or acceleration (all versus time). Other quantities can be calculated from analysis of these graphs.

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