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KINEMATICS

KINEMATICS. Chp 2 Pg 20-38. What movie is this scene taken from? Ans : X-Men First Class What is the model of the airplane? Ans : Lockheed SR-71 Blackbird What’s so special about the SR-71?. Contents. Displacement vs Distance Speed vs Velocity Acceleration Free Fall

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KINEMATICS

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  1. KINEMATICS Chp 2 Pg 20-38

  2. What movie is this scene taken from? • Ans: X-Men First Class • What is the model of the airplane? • Ans: Lockheed SR-71 Blackbird • What’s so special about the SR-71?

  3. Contents • Displacement vs Distance • Speed vs Velocity • Acceleration • Free Fall • Displacement-Time Graphs • Velocity-Time Graphs

  4. Recall from Previous Topic • Distance is a scalar • Displacement is a vector • Implications: • Distance only has magnitude, no direction • Displacement has both magnitude AND direction • Displacement needs to be measured from a reference point (i.e. a “zero point” or “origin”) • Direction of displacement needs to be specified or else it is ambigious

  5. Further Implications • Distance is always positive • Displacement can either be positive or negative • For O levels, we will only be studying one-dimensional displacement (i.e. straight line) • One direction will be positive displacement, while the opposite direction will be negative displacement. • E.g. if going to the right is positive, going to the left is negative • Some questions allow you to determine positive direction yourself. Other questions specify for you.

  6. Example 1 • Shifu started at displacement = 0. He walks 5 metres to the right, and then walks 10 metres to the left. • Qn: what is his distance travelled? • Ans: 15 m [note: ignore direction, just total everything] • Qn: what is the final displacement of Shifu? • Ans: 5 m or -5 m? • Up to you to specify!

  7. example 2 • Po started at displacement = 0. He travels X km north on the first day. One the second day, he travelled 10 km south. If his final displacement is 6 km, what was his displacement at the end of the first day, taking South to be the positive direction? • [Note: question has already fixed South to be positive, hence North must be taken to be negative] • Ans: -4 km

  8. Speed and Velocity • Also recall from previous topic: • Speed is a scalar • Velocity is a vector • In fact • Speed = Distance / Time • Velocity = Displacement / Time • Official Definitions: • Speed is the distance moved per unit Time • Velocity is the rate of change of displacement. • Unit = ms-1 • Average Speed = Total Distance / Time • Average Velocity = Final Displacement / Time

  9. Example 3 • Tai Lung ran 400 m East in 20 s, and then ran 1000 m West in 120 s. What was his (i) average speed, (ii) average velocity? • (i) Average speed = Total Distance / Time = 1400/140 = 10 ms-1 • (ii) Average velocity = Final Displacement / Time = 600 m/140 = 4.29 ms-1 (Can it be -4.29ms-1 ?)

  10. Some Hints • Average velocity is not necessarily the same as instantaneous velocity (similarly, average speed vs instantaneous speed) • Because velocity is a vector, once the direction has changed, the velocity has changed, even if the magnitude remains the same (for e.g. object travelling in a circle)

  11. Lame Joke Alert • How do you tell apart Speed from Velocity? • Speed is an action movie starring Keanu Reeves and Sandra Bullock • Velocity is a shopping mall in Novena

  12. Did You Know? • [Not in syllabus] • What the term “MACH” means? • Ans: It is a measure of how fast an object is compared to the speed of sound, i.e. 330 ms-1 • E.g. If an object is travelling at 660 ms-1, it is said to be travelling at “MACH 2”. • Commonly used to describe the speed of supersonic aircraft

  13. Speed Records [Not In Syllabus] • What is the speed of the fastest human? • Ans: 12.42 ms-1 • Usain Bolt

  14. Speed Records [Not In Syllabus] • Speed of fastest animal on land? • Ans: 29.0 ms-1 • Cheetah

  15. Speed Records [Not In Syllabus] • Speed of fastest animal in water • Ans: 30.33 ms-1 • Sailfish

  16. Speed Records [Not In Syllabus] • Speed of fastest animal in air • 108 ms-1 • Peregrine Falcon (while diving)

  17. Speed Records [Not In Syllabus] • Fastest car • 341 ms-1 • ThrustSSC • First car to beat the sound barrier

  18. Speed Records [Not In Syllabus] • Fastest Aeroplane SR-71 Blackbird X-15 MACH 3 MACH 6

  19. Speed Records [Not In Syllabus] • Fastest Manmade object anywhere • Helios 2 probe – launched in 1976 to orbit the sun and send data back • Speed = MACH 203

  20. Okay, back to your actual lesson….

  21. Acceleration • Acceleration is defined as the rate of change of velocity with respect to time. • Formula: a = (Vfinal – Vinitial)/t • a = (v – u)/t • Units: ms-2 • When the object’s velocity is changing with time, it is said to have an acceleration • When the object’s is changing it’s velocity at a constant rate, it is said to have uniform acceleration.

  22. Example 4 • A train initially has a velocity of 100 and it is moving towards the west. After braking for 20 s, it has a velocity of 50 . Calculate the acceleration of the train.

  23. Negative Acceleration • Just like velocity, acceleration is a vector • That means that there can be positive or negative acceleration • Usually, when an object is going faster and faster (increasing velocity), it is said to have positive acceleration. • When will an object have negative acceleration? • Ans: going slower and slower (decreasing velocity) • Is this the only situation of negative acceleration? • Going faster and faster in the negative direction is also negative acceleration

  24. Some Hints • Never use the term “accelerating” as a verb. Instead use the term “acceleration” as a noun. [Note: we never use velocity as a verb either] • Never use the words “deceleration” or “retardation”. Describe the motion in terms of negative acceleration, or decreasing velocity. • WHY? – in English, the word “accelerating” always refers to going faster and faster. But in Physics, this might not necessarily be the case. [When is it not the case?]

  25. Time for a Five Minute Test! =P

  26. Concept Cartoon! Tiger VS Mantis

  27. Free Fall (ignoring air resistance) • If I drop a metal coin and a feather in a vacuum, which would fall faster? • Watch video: http://www.youtube.com/watch?v=AV-qyDnZx0A • In a vacuum, the coin and the feather falls at the same rate! • When an object falls under the pull of gravity without any other forces acting on it, it is said to be in free fall. • Qn: what is the rate of falling when an object is in free fall?

  28. Free Fall (ignoring air resistance) • The rate of free fall (near the surface of the Earth) is around 9.81 ms-2. • For O levels, this is estimated to be 10 ms-2, unless otherwise stated by the question. • This is a constant, and is represented by the letter g, i.e. g = 10 ms-2 • “g” is sometimes referred to as the “acceleration due to free fall” • “g” is also referred to as “gravitational field strength near the surface of the Earth” • To find out what is “gravitational field strength”, you need to take Physics at A Levels.

  29. Example 5 • A ball is dropped at rest. Assuming that there is no air resistance, find the time taken for the ball to reach a velocity of 20 ms-1.

  30. Did You Know? • [not in syllabus] • Just like we use the speed of sound as a measure of speed (MACH), we also sometimes use the number of “g”sas a measure of acceleration. • An astronaut or pilot may withstand as much as 9 “g”s of acceleration in their training. [“G-Force simulator”]

  31. END OF PART 1

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