kinematics (part 1)

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-151000 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