1 of 24 Boardworks Ltd 2008 2 of
1 of 24 © Boardworks Ltd 2008
2 of 24 © Boardworks Ltd 2008
Distance, time and speed To work out the speed of an object you need to know: l the distance travelled l how long it took to travel that distance. 3 of 24 © Boardworks Ltd 2008
Calculating average speed Average speed is calculated using this equation: Average speed d total distance = total time s t formula triangle Speed can be measured in different units, e. g. m/s, km/h, km/s, miles per hour. The units of distance and time will determine the units to be used for speed. 4 of 24 © Boardworks Ltd 2008
Words and units 5 of 24 © Boardworks Ltd 2008
Speed formula triangle 6 of 24 © Boardworks Ltd 2008
Speed calculation example A girl takes 30 minutes to travel around a cross-country ski course: a distance of 10 km. Calculate her average speed in km/h. 10 km average speed = (in km/h) = 7 of 24 distance (in km) time (in h) 10 km 0. 5 h = 20 km/h © Boardworks Ltd 2008
Speed calculation example – units Sometimes the units have to be changed in a speed calculation. Calculate the average speed of the skier in m/s. 10 km average speed = (in m/s) distance (in m) time (in s) 10, 000 m = 3600 s 8 of 24 = 2. 8 m/s © Boardworks Ltd 2008
Distance calculation A boy sets off for a walk around his village at an average speed of 3. 6 km/h. How far will he travel in 30 minutes? Give your answer in km. distance (km) = speed (km/h) × time (h) = 3. 6 km/h × 0. 5 h = 1. 8 km 9 of 24 © Boardworks Ltd 2008
Time calculation – question 1 How long would it take the boy to walk 200 m to the church, if his average speed was 0. 8 m/s? time = = distance speed 200 m 0. 8 m/s = 250 seconds 10 of 24 © Boardworks Ltd 2008
11 of 24 © Boardworks Ltd 2008
Car graphing activity – animation 12 of 24 © Boardworks Ltd 2008
Distance–time graph 13 of 24 © Boardworks Ltd 2008
Calculating speed from the gradient The slope of a graph is called the gradient. The gradient of the line in a distance–time graph equals the speed. Simple graphs use straight lines only, making it easy to calculate the gradient. distance It is difficult to calculate the gradient of ‘realistic’ graphs because the line is curved. time 14 of 24 time © Boardworks Ltd 2008
What’s the speed? distance (m) What is the speed of the object between points A and B? 70 60 50 40 30 20 10 0 B l the object has moved 60 m (70 – 10 ) l it took 3 s to move this distance (6 – 3) A l speed = distance/time = 60/3 0 1 2 3 4 5 6 7 8 9 time (s) 15 of 24 = 20 m/s © Boardworks Ltd 2008
Speed–time graph 16 of 24 © Boardworks Ltd 2008
17 of 24 © Boardworks Ltd 2008
What’s the speed? distance (m) How can we work out the average speed of this object between points C and D? D B 70 l the object has moved 60 70 m (70 – 0 ) 50 l it took 9 s to move this 40 distance (9 – 0) 30 20 l speed = distance/time A 10 C = 70/9 0 0 1 2 3 4 5 6 7 8 9 = 7. 8 m/s time (s) 18 of 24 © Boardworks Ltd 2008
Average speed vs. instantaneous speed The speed of a race car at any point around a track is called the instantaneous speed. A car will have a different instantaneous speed at each of the points around the track. Average speed is the total distance travelled divided by the total time taken. It is never bigger than the instantaneous speed. 19 of 24 © Boardworks Ltd 2008
Calculating average speeds 20 of 24 © Boardworks Ltd 2008
21 of 24 © Boardworks Ltd 2008
Glossary 22 of 24 © Boardworks Ltd 2008
Anagrams 23 of 24 © Boardworks Ltd 2008
Multiple-choice quiz 24 of 24 © Boardworks Ltd 2008
- Slides: 24