Distance, time and speed To work out the speed of an object you need to know: the distance travelled how long it took to travel that distance.

Calculating average speed Average speed is calculated using this equation: d s t formula triangle total distance total time Average speed = 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. Speed Worksheet 1 accompanies this slide. It has been designed to be used at any point in the presentation to help to reinforce how speed is calculated. This worksheet involves an outdoor activity.

Words and units

Speed formula triangle

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 distance (in km) average speed (in km/h) time (in h) = 0.5 h = 10 km 20 km/h

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) time (in s) = distance (in m) 3600 s 10,000 m = 2.8 m/s

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

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

Car graphing activity – animation

Distance–time graph

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. It is difficult to calculate the gradient of ‘realistic’ graphs because the line is curved. Simple graphs use straight lines only, making it easy to calculate the gradient. time distance time distance

What’s the speed? What is the speed of the object between points A and B? time (s) distance (m) 2 1 3 4 5 6 7 8 9 10 20 30 40 50 60 70 A B the object has moved 60 m (70 – 10 ) it took 3 s to move this distance (6 – 3) speed = distance/time = 60/3 = 20 m/s

Speed–time graph Speed Worksheet 2 accompanies this slide.

What’s the speed? How can we work out the average speed of this object between points C and D? time (s) distance (m) 2 1 3 4 5 6 7 8 9 10 20 30 40 50 60 70 A B D the object has moved 70 m (70 – 0 ) it took 9 s to move this distance (9 – 0) speed = distance/time C = 70/9 = 7.8 m/s

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.

Calculating average speeds Speed Worksheet 3 accompanies this slide.

Glossary acceleration – An increase in speed. average speed – The total distance travelled divided by the total time taken for a whole journey. deceleration – A decrease in speed. distance/time graph – A way of showing how an object moves using a graph. metres per second – The unit of speed used in science. Distance must be measured in metres and time in seconds. speed – How quickly an object is moving. It equals the distance moved divided by the time taken.

Anagrams

Multiple-choice quiz