speed, velocity, acceeration

Speed, velocity and acceleration

5.1 Speed, velocity and acceleration

1Both Mr Rabbit and Mr Tortoise took the same round trip, but Mr Rabbit slept & returned later.


Comment on their their argument.Me, as I spent less time on the trip.No, I travelled longer distance every minute.Who runs faster?


O2A boy has been missing in a forest for 2 hours.scale = 1 cm : 5 km(a)If he walks at a speed of 4 km h1,try to locate his possible positions on the map.


O2A boy has been missing in a forest for 2 hours.(b)What else is important to spot the boy?The direction in which he has been walking.scale = 1 cm : 5 km


1SpeedHow can we describe how fast an object moves?E.g.A car on Tolo Highway travels 90 km in 1 hour. We say that the car travels at a speed of 90 km h1.


1SpeedSpeed is a measure of how fast something moves.Speed = distance travelled per unit of time

SI unit: m s1 or km h1 (for long distances)How can we describe how fast an object moves?


aAverage speedand speeds up again to 60 km h1.Its average speed over the whole journeyoverall distance travelledtotal time of travelslows down to 0 km h1, A car travels at 50 km h1,1Speed=


aAverage speedAverage speed does not tell the variations during the journey. On most trips, the speed at any instant is often different from the average speed.1Speed


bInstantaneous speed= speed at any instantInstantaneous speed1SpeedThe word speed alone instantaneous speedInstantaneous speed distance travelled in an extremely short time intervalSimulation


bInstantaneous speedSpeedometer tells the cars speed at any instant!1Speed


2Velocity rate of change of displacement. a speed in a given direction orvelocitya vector quantityVelocity is...


aSpeed with directionspeed = 300 km h1direction = westMTR drivers concern speed only.2VelocityPilots concern velocity (direction & speed).speed = 90 km h1


bAverage velocityAverage velocity =overall displacementtotal time of traveldirection of velocity = direction of overall displacement2Velocity


cInstantaneous velocityThe velocity at any instant is called instantaneous velocity. If a car moves at a constant velocity... its average and instantaneous velocities have the same value.2Velocity


Q1The world record...= 9.53 m s1 or 34.3 km h1100The world record of women 100-m race is 10.49 s.What is the average speed?


Q2In an orienteering event...In an orienteering event, Maria and Karen reach their control points at the same time.start, 10:00 amMaria, 10:30 amKaren, 10:30 amWho runs in a higher average velocity?


Q2In an orienteering event...AMaria.BKaren.CUndetermined since their paths are unknown.DIncomparable since they run along different directions.Who runs in a higher average velocity?


Q3True or false:Note: The distance travelled is equal to magnitude of displacement only if it is a straight-line motion. Speed is usually larger than the magnitude of velocity.(T/F)Average speed of an object magnitude of its average velocity.


Q4True or false:A man takes a walk starting from rest and ending at rest. (T/F)It is possible for him to attain an average speed of 5 km h1 but he never goes faster than 5 km h1.


3AccelerationWhen a car moves faster and faster, its speed is increasing (velocity changed).


3AccelerationWhen a car moves slower and slower,its speed is decreasing (velocity changed).


3AccelerationWhen a car changes direction, its velocity changes too.


3AccelerationAcceleration measures the change in velocityAcceleration = velocity per unit time

overall change in velocitytotal time taken= m s2Unit: m s1 / svector quantity=


3AccelerationIf a car accelerates at 2 m s2, what does that mean? t = 1 sv = 2 m s1,v = 2 m s1v = 0t = 2 sv = 4 m s1, v = 2 m s1v = 6 m s1, v = 2 m s1 t = 3 s1 mt = 03 m5 m


Example 1Airport Express takes 0.35 h to go from HK station to Airport station (34 km). Ave. speed =34 km/0.35 hComplete table.= 97 km h1


Example 1(b)Kln Tsing Yi:Time = distance / ave. speed= 8.9 / 90= 0.0989 h= 356 s


Example 1(a)Tsing Yi Airport:Distance = ave. speed time= 105 12.7762 s = (762/3600) h= 12.7 h= 22.2 km


Example 1(c)HK Kln:Ave. speed = distance / time= 2.6 / 0.0425153 s = (153/3600) h= 0.0425 h= 61.2 km


Example 2A man walks from A to B at 1 km h1, AB1 km h1and returns at 2 km h1.Average speed for the whole trip = ?


Example 2= 1.33 km h1AB1 km h1Suppose AB = 1 kmTime for whole trip = = 1 h + 0.5 h = 1.5 h whole journey = 2 kmAve. speed = distance / time= 2/1.5


Example 3A car travels 7 km north and then 3 km west in 10 minutes. FindCBA3 km7 km(a) average speed, Ave. speed = distance travelledtime taken= 60 km h1


Example 3A car travels 7 km north and then 3 km west in 10 minutes. Find CBA3 km7 km(b) ave. velocity?AC = = 7.62 kmtan q = q =23.2o3/7


Example 3A car travels 7 km north and then 3 km west in 10 minutes. FindCBA3 km7 kmAC = 7.62 km, q =23.2oSize of ave. velocity == 45.7 km h1 displacementtime7.62 km(10/60) h=Ave. velocity is 45.7 km h1, 23.2 north of west.(b) ave. velocity?


Example 4The Ferrari 348 can go from rest to 100 km h1 in 5.6 s. What is its ave. acceleration (in m s2)?Ave. acceleration== 4.96 m s2


Q1A running student...A running student is slowing down in front of a teacher. With reference to the sign convention,Acceleration of student: positive / negativeVelocity of student: positive / negative


Q2When time is measured...Quantity Unit Scalar/VectorSpeed ______ _____Velocity ______ _____Change in velocity ______ _____Acceleration ______ _____Unit of time: hour (h) km h1km h1km h1km h2scalarvectorvectorvectorUnit of distance/displacement: kilometer (km)


Q3In 2.5 s, a car speeds up...In 2.5 s, a car speeds up from 60 km h1 to 65 km h1...while a bicycle goes from rest to 5 km h1. Which one has the greater acceleration?They have the same acceleration!


Q4A car is moving in positive...A car is moving in +ve direction. What happens if it moves under a ve acceleration? What happens if it moves under a ve deceleration?The car will slow down.The car will move in +ve direction with increasing speed.


speed, velocity, acceeration

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