Book 2 Section 1.3 Speed and velocity Is the typhoon coming? Speed Check-point 4 Velocity Check-point 5 1 2 Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Is the typhoon coming? The figure shows the positions of a severe tropical storm at different times. Can you find out if and when the storm will hit Hong Kong? Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Is the typhoon coming? The answer is NO. What other information do you need? Speed and direction of the storm Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Speed = distance travelled per unit time - Scalar (no direction included) SI unit of speed: metre per second (m s–1) Another common unit: kilometre per hour (km h–1) Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity a Average speed Normally, a car does not always move in the same speed. Finding the average speed may help us study the motion of the car. Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity a Average speed The speed at any instant varies from object to object. Examples of speed: Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity b Instantaneous speed Instantaneous speed - the speed at any instant - estimated by measuring the distance travelled in an extremely short time interval 1.3 Average speed and instantaneous speed Simulation Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity b Instantaneous speed Speedometer in a car Speed display unit of a minibus Show instantaneous speed Does the speed limit of a road refer to the average or the instantaneous speed? Example 5 Should the driver be charged? Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity b Instantaneous speed When speed changes over time interval, average speed instantaneous speed When an object moves with constant speed, average speed = instantaneous speed Example 6 Average speed of a return trip Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 4 Here is a map showing some of the MTR lines. Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 4 Complete the following table. Island Line Airport Express Disneyland Resort Line Distance / km 35.3 3.5 Time of travel / s 1560 210 Average speed / m s–1 8.53 25.6 13.3 1380 16.7 Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Velocity = change in displacement per unit time Velocity - how fast and in what direction it moves - vector: magnitude + direction - SI unit: m s–1 Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity a Average velocity Average velocity = Direction of total displacement: N60E Direction of average velocity: Direction: same as total displacement Example 7 Average velocity of a car Book 2 Section 1.3 Speed and velocity
b Instantaneous velocity Instantaneous velocity - the velocity at any instant Direction: direction of motion of an object (where the object heads for at that instant) Magnitude: always the same as the instantaneous speed Book 2 Section 1.3 Speed and velocity
c Velocity along a straight line under uniform motion Direction of velocity along a straight line can be represented by + and – signs. positive Going from O to B : velocity = +1 m s–1 Going from B to A : velocity = –2 m s–1 Book 2 Section 1.3 Speed and velocity
c Velocity along a straight line under uniform motion positive In each journey, the velocity is constant. This kind of motion is called uniform motion. Average velocity = instantaneous velocity Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 5 – Q1 In an orienteering event, Maria and Karen reach their control points at the same time. Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 5 – Q1 Who runs at a higher average speed? A Maria B Karen C Undetermined since their paths are unknown D Incomparable since they run along different directions Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 5 – Q2 Who runs at a greater magnitude of average velocity? A Maria B Karen C Undetermined since their paths are unknown D Incomparable since they run along different directions Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 5 – Q3 A butterfly flies along this semi-circular path in 30 s. What are its average speed and average velocity? Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity Check-point 5 – Q3 Average speed = distance travelled total time of travel 3.14 30 = 0.105 m s–1 Average velocity = total displacement total time of travel 2 30 = 0.0667 m s–1 (N32E) Book 2 Section 1.3 Speed and velocity
Book 2 Section 1.3 Speed and velocity The End Book 2 Section 1.3 Speed and velocity