**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

**Okay, back to your actual lesson….**

**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.

**Time for a Five Minute Test!** =P