KINEMATICS (Part 1). Chp 2 Pg 19-26. 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
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KINEMATICS (Part 1) Chp 2 Pg 19-26
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 • Distance-Time Graphs • Speed-Time Graphs
Recall from Previous Topic • Distance is a scalar. • It is always positive in value. • Displacement is a vector • It can be either positive or negative in value • Displacement has both magnitude AND direction • The magntiude of displacement is measured from a reference point (i.e. a “zero point” or “origin”)
Further Implications • 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
Example 1 • Shifu started at displacement = 0. He walks 5 metres to the right, and then walks 10 metres to the left. Take left to be the positive direction. • 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
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? • Ans: -4 km
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
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 ?)
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
Speed Records [Not In Syllabus] • What is the speed of the fastest human? • Ans: 12.42 ms-1 • Usain Bolt
Speed Records [Not In Syllabus] • Speed of fastest animal on land? • Ans: 29.0 ms-1 • Cheetah
Speed Records [Not In Syllabus] • Speed of fastest animal in water • Ans: 30.33 ms-1 • Sailfish
Speed Records [Not In Syllabus] • Speed of fastest animal in air • 108 ms-1 • Peregrine Falcon (while diving)
Speed Records [Not In Syllabus] • Fastest car • 341 ms-1 • ThrustSSC • First car to beat the sound barrier
Speed Records [Not In Syllabus] • Fastest Aeroplane SR-71 Blackbird X-15 1000 ms-1 2000 ms-1
Acceleration • Acceleration is defined as the rate of change of velocity with respect to time. • Formula: a = (Final Velocity – Initial Velocity)/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.
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.