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Speed, velocity and acceleration

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

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Who is right? Me, as I spent less time on the trip. No, I travelled more distance every second I ran. Who runs faster?

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2Some kid is lost in the woods. (a)If he walks at a speed of 4 km/hr, try to locate his possible positions on the map.

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2Some kid is lost in the woods. (b)What else is important to find the kid? The direction in which he has been walking.

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Reference Points

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All Motion is relative All motion is relative to a reference. This means that we describe motion of an object relative to some other object

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All Motion is relative In our environment, the reference for motion is the earth’s surface, and speeds are measured relative to the earth The earth moves at 107,000 km/h relative to the sun

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Calculating Motion and Speed

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1Speed How can we describe how fast an object moves? E.g.A car on Beltway 8 travels 90 km in 1 hour. We say that the car travels at a speed of 90 km/hr.

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1Speed Speed is a measure of how fast something moves. Speed = distance travelled per unit of time SI unit: m/s or km/h (for long distances) How can we describe how fast an object moves?

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and speeds up again to 60 km/. aAverage speed Its average speed over the whole journey overall distance travelled total time of travel slows down to 0 km/h, A car travels at 50 km/h, 1Speed =

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Question 1 A bug crawls 10 cm in 5 sec. What was the bug’s approximate speed in cm/sec? 2 cm/sec

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

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bInstantaneous speed = speed at any instant Instantaneous speed 1Speed The word ‘speed’ alone instantaneous speed Instantaneous speed distance travelled in an extremely short time interval

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Speedometer tells the car’s speed at any instant! 1Speed bInstantaneous speed

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Question 2 ( ) Average speed = 10.49 s = 9.53 m/s 100 m The world record for women’s 100-m race is 10.49 s. What is the average speed?

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Velocity

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2Velocity rate of change of displacement. a speed in a given direction or velocity a vector quantity direction magnitude (speed) Velocity is...

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speed = 300 km/h direction = west Train engineers use speed only. aSpeed with direction 2Velocity Pilots use velocity (direction & speed). speed = 90 km/h

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bAverage velocity Average velocity = overall displacement total time of travel direction of velocity = direction of overall displacement 2Velocity

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Question 3 A student runs around a circular track that is 1000m in length in a time of 30 min. What is the average velocity of the student? 0 m/min If the student runs completely around the track, they are back where they began. So their overall displacement is zero.

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cInstantaneous velocity The 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

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Question 4 Which of these is a correct velocity? A.55 miles per hour B.15 kilometers per second C.30 meters per second east D.400,000,000 meters per hour

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Acceleration

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3Acceleration When an object moves faster and faster, its speed is increasing (velocity changed).

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3Acceleration When an object moves slower and slower, its speed is decreasing (velocity changed).

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When an object changes direction, 3Acceleration its velocity changes too.

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3Acceleration Acceleration measures the change in velocity Acceleration = velocity per unit time direction speed overall change in velocity total time taken = m/s 2 Unit: m/s/s vector quantity =

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If a car accelerates at 2 m s –2, what does that mean? 3Acceleration t = 1 s v = 2 m s –1, v = 2 m s –1 v = 0 t = 2 s v = 4 m s –1, v = 2 m s –1 v = 6 m s –1, v = 2 m s –1 t = 3 s 1 m t = 0 3 m 5 m

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Tommy accelerates from 20 m/s at 15 sec (t 1 ) to 40 m/s at 60 sec (t 2 ). Question 5 What is his avg. acceleration between t 1 and t 2 ? Avg. acceleration = 40 m/s – 20 m/s 60s – 15 s 20 m/s 45 s = =.44 m/s 2

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