Speed and velocity
How to measure distance and time, the speed formula with a worked example, average speed for a journey with a stop, and the difference between scalar speed and vector velocity. With Vietnamese subtitles. A “Check yourself” section with typed questions is below the video on the site.
Practice
- Check yourself: Speed and velocity – Type the missing words, put the steps in order and write a short answer. No multiple choice: you have to remember it.
Transcript
Speed tells us how fast an object moves. To find the speed, we need two measurements: the distance, and the time.
We measure distance in metres, with a ruler or a measuring tape. We measure time in seconds, with a stopwatch.
Speed is the distance travelled in each unit of time. The formula is: speed equals distance divided by time.
Here is an example. A runner covers one hundred metres in twenty seconds. Speed equals one hundred divided by twenty. The speed is five metres per second.
We can rearrange the formula. Distance equals speed times time. And time equals distance divided by speed.
Real journeys do not have a steady speed. So we use the average speed. Average speed is the total distance divided by the total time.
In this journey, a car travels three hundred metres in one hundred seconds. It stops for fifty seconds. Then it travels three hundred metres in fifty seconds. The total is six hundred metres in two hundred seconds. The average speed is three metres per second.
Speed is a scalar. It has size only. Distance and time are scalars too.
Velocity is a vector. It has size and direction. These two cars have the same speed, but they have different velocities, because they go in opposite directions.
Remember. Speed equals distance divided by time. Speed is measured in metres per second. Average speed is the total distance divided by the total time. Speed is a scalar. And velocity is a vector, with a direction.
More in this series
6:12P1 Motion: key vocabulary
Twelve key words for IGCSE Co-ordinated Sciences topic P1 (motion), with the Vietnamese meaning, how to say each word, a picture and an example. Ends with a test section, and a typing trainer on the site that also practises twelve general science words.
2:25Motion graphs
Reading distance-time graphs (gradient is speed, a flat line is stationary) and speed-time graphs (gradient is acceleration, area is distance), with the numbers worked out on screen. With Vietnamese subtitles. A “Check yourself” section with typed questions is below the video on the site.
5:41P1 quiz: motion
Ten think-time questions on speed, average speed, scalars and vectors, distance-time graphs and speed-time graphs.